Merge branch 'master' into stable-6.2

* master:
  Adapt diff- and merge tool code for PGM and EGit usage
  Teach JGit to handle external diff/merge tools defined in
.gitattributes

Change-Id: I3aefc14160caaac859bd3548460dd755ebe42fc5

17 files changed, 907 insertions, 217 deletions

diff --git a/org.eclipse.jgit/META-INF/MANIFEST.MF b/org.eclipse.jgit/META-INF/MANIFEST.MF
index 242b3a4c92..fda154f93e 100644
--- a/org.eclipse.jgit/META-INF/MANIFEST.MF
+++ b/org.eclipse.jgit/META-INF/MANIFEST.MF
@@ -73,7 +73,8 @@ Export-Package: org.eclipse.jgit.annotations;version="6.2.0",
  org.eclipse.jgit.internal.diffmergetool;version="6.2.0";
   x-friends:="org.eclipse.jgit.test,
    org.eclipse.jgit.pgm.test,
-   org.eclipse.jgit.pgm",
+   org.eclipse.jgit.pgm,
+   org.eclipse.egit.ui",
  org.eclipse.jgit.internal.fsck;version="6.2.0";
   x-friends:="org.eclipse.jgit.test",
  org.eclipse.jgit.internal.revwalk;version="6.2.0";
@@ -133,7 +134,8 @@ Export-Package: org.eclipse.jgit.annotations;version="6.2.0",
    org.eclipse.jgit.util.time",
  org.eclipse.jgit.lib.internal;version="6.2.0";
   x-friends:="org.eclipse.jgit.test,
-   org.eclipse.jgit.pgm",
+   org.eclipse.jgit.pgm,
+   org.eclipse.egit.ui",
  org.eclipse.jgit.logging;version="6.2.0",
  org.eclipse.jgit.merge;version="6.2.0";
   uses:="org.eclipse.jgit.dircache,
diff --git a/org.eclipse.jgit/resources/org/eclipse/jgit/internal/JGitText.properties b/org.eclipse.jgit/resources/org/eclipse/jgit/internal/JGitText.properties
index 9f264fca34..f0bb6c6c99 100644
--- a/org.eclipse.jgit/resources/org/eclipse/jgit/internal/JGitText.properties
+++ b/org.eclipse.jgit/resources/org/eclipse/jgit/internal/JGitText.properties
@@ -237,6 +237,9 @@ deleteTagUnexpectedResult=Delete tag returned unexpected result {0}
 deletingNotSupported=Deleting {0} not supported.
 destinationIsNotAWildcard=Destination is not a wildcard.
 detachedHeadDetected=HEAD is detached
+diffToolNotGivenError=No diff tool provided and no defaults configured.
+diffToolNotSpecifiedInGitAttributesError=Diff tool specified in git attributes cannot be found.
+diffToolNullError=Parameter for diff tool cannot be null.
 dirCacheDoesNotHaveABackingFile=DirCache does not have a backing file
 dirCacheFileIsNotLocked=DirCache {0} not locked
 dirCacheIsNotLocked=DirCache is not locked
@@ -457,6 +460,8 @@ mergeStrategyDoesNotSupportHeads=merge strategy {0} does not support {1} heads t
 mergeUsingStrategyResultedInDescription=Merge of revisions {0} with base {1} using strategy {2} resulted in: {3}. {4}
 mergeRecursiveConflictsWhenMergingCommonAncestors=Multiple common ancestors were found and merging them resulted in a conflict: {0}, {1}
 mergeRecursiveTooManyMergeBasesFor = "More than {0} merge bases for:\n a {1}\n b {2} found:\n  count {3}"
+mergeToolNotGivenError=No merge tool provided and no defaults configured.
+mergeToolNullError=Parameter for merge tool cannot be null.
 messageAndTaggerNotAllowedInUnannotatedTags = Unannotated tags cannot have a message or tagger
 minutesAgo={0} minutes ago
 mismatchOffset=mismatch offset for object {0}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java
index b81e605c13..17e359de49 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java
@@ -265,6 +265,9 @@ public class JGitText extends TranslationBundle {
 	/***/ public String deletingNotSupported;
 	/***/ public String destinationIsNotAWildcard;
 	/***/ public String detachedHeadDetected;
+	/***/ public String diffToolNotGivenError;
+	/***/ public String diffToolNotSpecifiedInGitAttributesError;
+	/***/ public String diffToolNullError;
 	/***/ public String dirCacheDoesNotHaveABackingFile;
 	/***/ public String dirCacheFileIsNotLocked;
 	/***/ public String dirCacheIsNotLocked;
@@ -485,6 +488,8 @@ public class JGitText extends TranslationBundle {
 	/***/ public String mergeUsingStrategyResultedInDescription;
 	/***/ public String mergeRecursiveConflictsWhenMergingCommonAncestors;
 	/***/ public String mergeRecursiveTooManyMergeBasesFor;
+	/***/ public String mergeToolNotGivenError;
+	/***/ public String mergeToolNullError;
 	/***/ public String messageAndTaggerNotAllowedInUnannotatedTags;
 	/***/ public String minutesAgo;
 	/***/ public String mismatchOffset;
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/CommandExecutor.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/CommandExecutor.java
index ad79fe8fc6..668adeab65 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/CommandExecutor.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/CommandExecutor.java
@@ -14,13 +14,19 @@ import java.io.File;
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.OutputStream;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.util.Arrays;
 import java.util.Map;
+
+import org.eclipse.jgit.errors.NoWorkTreeException;
 import org.eclipse.jgit.util.FS;
 import org.eclipse.jgit.util.FS.ExecutionResult;
 import org.eclipse.jgit.util.FS_POSIX;
 import org.eclipse.jgit.util.FS_Win32;
 import org.eclipse.jgit.util.FS_Win32_Cygwin;
+import org.eclipse.jgit.util.StringUtils;
 
 /**
  * Runs a command with help of FS.
@@ -91,6 +97,49 @@ public class CommandExecutor {
 		}
 	}
 
+	/**
+	 * @param path
+	 *            the executable path
+	 * @param workingDir
+	 *            the working directory
+	 * @param env
+	 *            the environment
+	 * @return the execution result
+	 * @throws ToolException
+	 * @throws InterruptedException
+	 * @throws IOException
+	 */
+	public boolean checkExecutable(String path, File workingDir,
+			Map<String, String> env)
+			throws ToolException, IOException, InterruptedException {
+		checkUseMsys2(path);
+		String command = null;
+		if (fs instanceof FS_Win32 && !useMsys2) {
+			Path p = Paths.get(path);
+			// Win32 (and not cygwin or MSYS2) where accepts only command / exe
+			// name as parameter
+			// so check if exists and executable in this case
+			if (p.isAbsolute() && Files.isExecutable(p)) {
+				return true;
+			}
+			// try where command for all other cases
+			command = "where " + ExternalToolUtils.quotePath(path); //$NON-NLS-1$
+		} else {
+			command = "which " + ExternalToolUtils.quotePath(path); //$NON-NLS-1$
+		}
+		boolean available = true;
+		try {
+			ExecutionResult rc = run(command, workingDir, env);
+			if (rc.getRc() != 0) {
+				available = false;
+			}
+		} catch (IOException | InterruptedException | NoWorkTreeException
+				| ToolException e) {
+			// no op: is true to not hide possible tools from user
+		}
+		return available;
+	}
+
 	private void deleteCommandArray() {
 		deleteCommandFile();
 	}
@@ -127,7 +176,7 @@ public class CommandExecutor {
 	private void checkUseMsys2(String command) {
 		useMsys2 = false;
 		String useMsys2Str = System.getProperty("jgit.usemsys2bash"); //$NON-NLS-1$
-		if (useMsys2Str != null && !useMsys2Str.isEmpty()) {
+		if (!StringUtils.isEmptyOrNull(useMsys2Str)) {
 			if (useMsys2Str.equalsIgnoreCase("auto")) { //$NON-NLS-1$
 				useMsys2 = command.contains(".sh"); //$NON-NLS-1$
 			} else {
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/CommandLineDiffTool.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/CommandLineDiffTool.java
index 509515c37a..00dec32718 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/CommandLineDiffTool.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/CommandLineDiffTool.java
@@ -111,7 +111,7 @@ public enum CommandLineDiffTool {
 	 * See: <a href=
 	 * "http://vimdoc.sourceforge.net/htmldoc/diff.html">http://vimdoc.sourceforge.net/htmldoc/diff.html</a>
 	 */
-	gvimdiff("gviewdiff", "\"$LOCAL\" \"$REMOTE\""),
+	gvimdiff("gvimdiff", "\"$LOCAL\" \"$REMOTE\""),
 	/**
 	 * See: <a href=
 	 * "http://vimdoc.sourceforge.net/htmldoc/diff.html">http://vimdoc.sourceforge.net/htmldoc/diff.html</a>
@@ -160,7 +160,7 @@ public enum CommandLineDiffTool {
 	 * See: <a href=
 	 * "http://vimdoc.sourceforge.net/htmldoc/diff.html">http://vimdoc.sourceforge.net/htmldoc/diff.html</a>
 	 */
-	vimdiff("viewdiff", gvimdiff),
+	vimdiff("vimdiff", gvimdiff),
 	/**
 	 * See: <a href=
 	 * "http://vimdoc.sourceforge.net/htmldoc/diff.html">http://vimdoc.sourceforge.net/htmldoc/diff.html</a>
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/DiffTools.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/DiffTools.java
index 1dcc523bf8..7cedd82995 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/DiffTools.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/DiffTools.java
@@ -1,5 +1,6 @@
 /*
  * Copyright (C) 2018-2022, Andre Bossert <andre.bossert@siemens.com>
+ * Copyright (C) 2019, Tim Neumann <tim.neumann@advantest.com>
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -10,14 +11,22 @@
 
 package org.eclipse.jgit.internal.diffmergetool;
 
-import java.util.TreeMap;
-import java.util.Collections;
+import java.io.File;
 import java.io.IOException;
+import java.util.Collections;
+import java.util.LinkedHashSet;
 import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Optional;
 import java.util.Set;
+import java.util.TreeMap;
 
+import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.lib.StoredConfig;
 import org.eclipse.jgit.lib.internal.BooleanTriState;
+import org.eclipse.jgit.treewalk.TreeWalk;
+import org.eclipse.jgit.util.FS;
 import org.eclipse.jgit.util.FS.ExecutionResult;
 import org.eclipse.jgit.util.StringUtils;
 
@@ -26,10 +35,16 @@ import org.eclipse.jgit.util.StringUtils;
  */
 public class DiffTools {
 
-	private final Repository repo;
+	private final FS fs;
+
+	private final File gitDir;
+
+	private final File workTree;
 
 	private final DiffToolConfig config;
 
+	private final Repository repo;
+
 	private final Map<String, ExternalDiffTool> predefinedTools;
 
 	private final Map<String, ExternalDiffTool> userDefinedTools;
@@ -41,69 +56,220 @@ public class DiffTools {
 	 *            the repository
 	 */
 	public DiffTools(Repository repo) {
+		this(repo, repo.getConfig());
+	}
+
+	/**
+	 * Creates the external merge-tools manager for given configuration.
+	 *
+	 * @param config
+	 *            the git configuration
+	 */
+	public DiffTools(StoredConfig config) {
+		this(null, config);
+	}
+
+	private DiffTools(Repository repo, StoredConfig config) {
 		this.repo = repo;
-		config = repo.getConfig().get(DiffToolConfig.KEY);
+		this.config = config.get(DiffToolConfig.KEY);
+		this.gitDir = repo == null ? null : repo.getDirectory();
+		this.fs = repo == null ? FS.DETECTED : repo.getFS();
+		this.workTree = repo == null ? null : repo.getWorkTree();
 		predefinedTools = setupPredefinedTools();
-		userDefinedTools = setupUserDefinedTools(config, predefinedTools);
+		userDefinedTools = setupUserDefinedTools(predefinedTools);
 	}
 
 	/**
 	 * Compare two versions of a file.
 	 *
 	 * @param localFile
-	 *            the local file element
+	 *            The local/left version of the file.
 	 * @param remoteFile
-	 *            the remote file element
-	 * @param mergedFile
-	 *            the merged file element, it's path equals local or remote
-	 *            element path
+	 *            The remote/right version of the file.
 	 * @param toolName
-	 *            the selected tool name (can be null)
+	 *            Optionally the name of the tool to use. If not given the
+	 *            default tool will be used.
 	 * @param prompt
-	 *            the prompt option
+	 *            Optionally a flag whether to prompt the user before compare.
+	 *            If not given the default will be used.
 	 * @param gui
-	 *            the GUI option
+	 *            A flag whether to prefer a gui tool.
+	 * @param trustExitCode
+	 *            Optionally a flag whether to trust the exit code of the tool.
+	 *            If not given the default will be used.
+	 * @param promptHandler
+	 *            The handler to use when needing to prompt the user if he wants
+	 *            to continue.
+	 * @param noToolHandler
+	 *            The handler to use when needing to inform the user, that no
+	 *            tool is configured.
+	 * @return the optioanl result of executing the tool if it was executed
+	 * @throws ToolException
+	 *             when the tool fails
+	 */
+	public Optional<ExecutionResult> compare(FileElement localFile,
+			FileElement remoteFile, Optional<String> toolName,
+			BooleanTriState prompt, boolean gui, BooleanTriState trustExitCode,
+			PromptContinueHandler promptHandler,
+			InformNoToolHandler noToolHandler) throws ToolException {
+
+		String toolNameToUse;
+
+		if (toolName == null) {
+			throw new ToolException(JGitText.get().diffToolNullError);
+		}
+
+		if (toolName.isPresent()) {
+			toolNameToUse = toolName.get();
+		} else {
+			toolNameToUse = getDefaultToolName(gui);
+		}
+
+		if (StringUtils.isEmptyOrNull(toolNameToUse)) {
+			throw new ToolException(JGitText.get().diffToolNotGivenError);
+		}
+
+		boolean doPrompt;
+		if (prompt != BooleanTriState.UNSET) {
+			doPrompt = prompt == BooleanTriState.TRUE;
+		} else {
+			doPrompt = isInteractive();
+		}
+
+		if (doPrompt) {
+			if (!promptHandler.prompt(toolNameToUse)) {
+				return Optional.empty();
+			}
+		}
+
+		boolean trust;
+		if (trustExitCode != BooleanTriState.UNSET) {
+			trust = trustExitCode == BooleanTriState.TRUE;
+		} else {
+			trust = config.isTrustExitCode();
+		}
+
+		ExternalDiffTool tool = getTool(toolNameToUse);
+		if (tool == null) {
+			throw new ToolException(
+					"External diff tool is not defined: " + toolNameToUse); //$NON-NLS-1$
+		}
+
+		return Optional.of(
+				compare(localFile, remoteFile, tool, trust));
+	}
+
+	/**
+	 * Compare two versions of a file.
+	 *
+	 * @param localFile
+	 *            the local file element
+	 * @param remoteFile
+	 *            the remote file element
+	 * @param tool
+	 *            the selected tool
 	 * @param trustExitCode
 	 *            the "trust exit code" option
 	 * @return the execution result from tool
 	 * @throws ToolException
 	 */
 	public ExecutionResult compare(FileElement localFile,
-			FileElement remoteFile, FileElement mergedFile, String toolName,
-			BooleanTriState prompt, BooleanTriState gui,
-			BooleanTriState trustExitCode) throws ToolException {
+			FileElement remoteFile, ExternalDiffTool tool,
+			boolean trustExitCode) throws ToolException {
 		try {
+			if (tool == null) {
+				throw new ToolException(JGitText
+						.get().diffToolNotSpecifiedInGitAttributesError);
+			}
 			// prepare the command (replace the file paths)
-			String command = ExternalToolUtils.prepareCommand(
-					guessTool(toolName, gui).getCommand(), localFile,
-					remoteFile, mergedFile, null);
+			String command = ExternalToolUtils.prepareCommand(tool.getCommand(),
+					localFile, remoteFile, null, null);
 			// prepare the environment
-			Map<String, String> env = ExternalToolUtils.prepareEnvironment(repo,
-					localFile, remoteFile, mergedFile, null);
-			boolean trust = config.isTrustExitCode();
-			if (trustExitCode != BooleanTriState.UNSET) {
-				trust = trustExitCode == BooleanTriState.TRUE;
-			}
+			Map<String, String> env = ExternalToolUtils.prepareEnvironment(
+					gitDir, localFile, remoteFile, null, null);
 			// execute the tool
-			CommandExecutor cmdExec = new CommandExecutor(repo.getFS(), trust);
-			return cmdExec.run(command, repo.getWorkTree(), env);
+			CommandExecutor cmdExec = new CommandExecutor(fs, trustExitCode);
+			return cmdExec.run(command, workTree, env);
 		} catch (IOException | InterruptedException e) {
 			throw new ToolException(e);
 		} finally {
 			localFile.cleanTemporaries();
 			remoteFile.cleanTemporaries();
-			mergedFile.cleanTemporaries();
 		}
 	}
 
 	/**
-	 * @return the tool names
+	 * Get user defined tool names.
+	 *
+	 * @return the user defined tool names
+	 */
+	public Set<String> getUserDefinedToolNames() {
+		return userDefinedTools.keySet();
+	}
+
+	/**
+	 * Get predefined tool names.
+	 *
+	 * @return the predefined tool names
+	 */
+	public Set<String> getPredefinedToolNames() {
+		return predefinedTools.keySet();
+	}
+
+	/**
+	 * Get all tool names.
+	 *
+	 * @return the all tool names (default or available tool name is the first
+	 *         in the set)
+	 */
+	public Set<String> getAllToolNames() {
+		String defaultName = getDefaultToolName(false);
+		if (defaultName == null) {
+			defaultName = getFirstAvailableTool();
+		}
+		return ExternalToolUtils.createSortedToolSet(defaultName,
+				getUserDefinedToolNames(), getPredefinedToolNames());
+	}
+
+	/**
+	 * Provides {@link Optional} with the name of an external diff tool if
+	 * specified in git configuration for a path.
+	 *
+	 * The formed git configuration results from global rules as well as merged
+	 * rules from info and worktree attributes.
+	 *
+	 * Triggers {@link TreeWalk} until specified path found in the tree.
+	 *
+	 * @param path
+	 *            path to the node in repository to parse git attributes for
+	 * @return name of the difftool if set
+	 * @throws ToolException
+	 */
+	public Optional<String> getExternalToolFromAttributes(final String path)
+			throws ToolException {
+		return ExternalToolUtils.getExternalToolFromAttributes(repo, path,
+				ExternalToolUtils.KEY_DIFF_TOOL);
+	}
+
+	/**
+	 * Checks the availability of the predefined tools in the system.
+	 *
+	 * @return set of predefined available tools
 	 */
-	public Set<String> getToolNames() {
-		return config.getToolNames();
+	public Set<String> getPredefinedAvailableTools() {
+		Map<String, ExternalDiffTool> defTools = getPredefinedTools(true);
+		Set<String> availableTools = new LinkedHashSet<>();
+		for (Entry<String, ExternalDiffTool> elem : defTools.entrySet()) {
+			if (elem.getValue().isAvailable()) {
+				availableTools.add(elem.getKey());
+			}
+		}
+		return availableTools;
 	}
 
 	/**
+	 * Get user defined tools map.
+	 *
 	 * @return the user defined tools
 	 */
 	public Map<String, ExternalDiffTool> getUserDefinedTools() {
@@ -111,48 +277,70 @@ public class DiffTools {
 	}
 
 	/**
-	 * @return the available predefined tools
+	 * Get predefined tools map.
+	 *
+	 * @param checkAvailability
+	 *            true: for checking if tools can be executed; ATTENTION: this
+	 *            check took some time, do not execute often (store the map for
+	 *            other actions); false: availability is NOT checked:
+	 *            isAvailable() returns default false is this case!
+	 * @return the predefined tools with optionally checked availability (long
+	 *         running operation)
 	 */
-	public Map<String, ExternalDiffTool> getAvailableTools() {
+	public Map<String, ExternalDiffTool> getPredefinedTools(
+			boolean checkAvailability) {
+		if (checkAvailability) {
+			for (ExternalDiffTool tool : predefinedTools.values()) {
+				PreDefinedDiffTool predefTool = (PreDefinedDiffTool) tool;
+				predefTool.setAvailable(ExternalToolUtils.isToolAvailable(fs,
+						gitDir, workTree, predefTool.getPath()));
+			}
+		}
 		return Collections.unmodifiableMap(predefinedTools);
 	}
 
 	/**
-	 * @return the NOT available predefined tools
+	 * Get first available tool name.
+	 *
+	 * @return the name of first available predefined tool or null
 	 */
-	public Map<String, ExternalDiffTool> getNotAvailableTools() {
-		return Collections.unmodifiableMap(new TreeMap<>());
+	public String getFirstAvailableTool() {
+		for (ExternalDiffTool tool : predefinedTools.values()) {
+			if (ExternalToolUtils.isToolAvailable(fs, gitDir, workTree,
+					tool.getPath())) {
+				return tool.getName();
+			}
+		}
+		return null;
 	}
 
 	/**
+	 * Get default (gui-)tool name.
+	 *
 	 * @param gui
 	 *            use the diff.guitool setting ?
 	 * @return the default tool name
 	 */
-	public String getDefaultToolName(BooleanTriState gui) {
-		return gui != BooleanTriState.UNSET ? "my_gui_tool" //$NON-NLS-1$
-				: config.getDefaultToolName();
+	public String getDefaultToolName(boolean gui) {
+		String guiToolName;
+		if (gui) {
+			guiToolName = config.getDefaultGuiToolName();
+			if (guiToolName != null) {
+				return guiToolName;
+			}
+		}
+		return config.getDefaultToolName();
 	}
 
 	/**
+	 * Is interactive diff (prompt enabled) ?
+	 *
 	 * @return is interactive (config prompt enabled) ?
 	 */
 	public boolean isInteractive() {
 		return config.isPrompt();
 	}
 
-	private ExternalDiffTool guessTool(String toolName, BooleanTriState gui)
-			throws ToolException {
-		if (StringUtils.isEmptyOrNull(toolName)) {
-			toolName = getDefaultToolName(gui);
-		}
-		ExternalDiffTool tool = getTool(toolName);
-		if (tool == null) {
-			throw new ToolException("Unknown diff tool " + toolName); //$NON-NLS-1$
-		}
-		return tool;
-	}
-
 	private ExternalDiffTool getTool(final String name) {
 		ExternalDiffTool tool = userDefinedTools.get(name);
 		if (tool == null) {
@@ -169,10 +357,10 @@ public class DiffTools {
 		return tools;
 	}
 
-	private static Map<String, ExternalDiffTool> setupUserDefinedTools(
-			DiffToolConfig cfg, Map<String, ExternalDiffTool> predefTools) {
+	private Map<String, ExternalDiffTool> setupUserDefinedTools(
+			Map<String, ExternalDiffTool> predefTools) {
 		Map<String, ExternalDiffTool> tools = new TreeMap<>();
-		Map<String, ExternalDiffTool> userTools = cfg.getTools();
+		Map<String, ExternalDiffTool> userTools = config.getTools();
 		for (String name : userTools.keySet()) {
 			ExternalDiffTool userTool = userTools.get(name);
 			// if difftool.<name>.cmd is defined we have user defined tool
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/ExternalDiffTool.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/ExternalDiffTool.java
index f2d7e828cb..e01b892a53 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/ExternalDiffTool.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/ExternalDiffTool.java
@@ -30,4 +30,10 @@ public interface ExternalDiffTool {
 	 */
 	String getCommand();
 
+	/**
+	 * @return availability of the tool: true if tool can be executed and false
+	 *         if not
+	 */
+	boolean isAvailable();
+
 }
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/ExternalToolUtils.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/ExternalToolUtils.java
index 3efb90c490..b2dd846d70 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/ExternalToolUtils.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/ExternalToolUtils.java
@@ -10,10 +10,22 @@
 package org.eclipse.jgit.internal.diffmergetool;
 
 import java.util.TreeMap;
+import java.io.File;
 import java.io.IOException;
+import java.util.LinkedHashSet;
 import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+
+import org.eclipse.jgit.attributes.Attributes;
+import org.eclipse.jgit.errors.RevisionSyntaxException;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.treewalk.FileTreeIterator;
+import org.eclipse.jgit.treewalk.TreeWalk;
+import org.eclipse.jgit.treewalk.WorkingTreeIterator;
+import org.eclipse.jgit.treewalk.filter.NotIgnoredFilter;
+import org.eclipse.jgit.util.FS;
 
 /**
  * Utilities for diff- and merge-tools.
@@ -21,6 +33,16 @@ import org.eclipse.jgit.lib.Repository;
 public class ExternalToolUtils {
 
 	/**
+	 * Key for merge tool git configuration section
+	 */
+	public static final String KEY_MERGE_TOOL = "mergetool"; //$NON-NLS-1$
+
+	/**
+	 * Key for diff tool git configuration section
+	 */
+	public static final String KEY_DIFF_TOOL = "difftool"; //$NON-NLS-1$
+
+	/**
 	 * Prepare command for execution.
 	 *
 	 * @param command
@@ -39,9 +61,15 @@ public class ExternalToolUtils {
 	public static String prepareCommand(String command, FileElement localFile,
 			FileElement remoteFile, FileElement mergedFile,
 			FileElement baseFile) throws IOException {
-		command = localFile.replaceVariable(command);
-		command = remoteFile.replaceVariable(command);
-		command = mergedFile.replaceVariable(command);
+		if (localFile != null) {
+			command = localFile.replaceVariable(command);
+		}
+		if (remoteFile != null) {
+			command = remoteFile.replaceVariable(command);
+		}
+		if (mergedFile != null) {
+			command = mergedFile.replaceVariable(command);
+		}
 		if (baseFile != null) {
 			command = baseFile.replaceVariable(command);
 		}
@@ -51,8 +79,8 @@ public class ExternalToolUtils {
 	/**
 	 * Prepare environment needed for execution.
 	 *
-	 * @param repo
-	 *            the repository
+	 * @param gitDir
+	 *            the .git directory
 	 * @param localFile
 	 *            the local file (ours)
 	 * @param remoteFile
@@ -64,18 +92,151 @@ public class ExternalToolUtils {
 	 * @return the environment map with variables and values (file paths)
 	 * @throws IOException
 	 */
-	public static Map<String, String> prepareEnvironment(Repository repo,
+	public static Map<String, String> prepareEnvironment(File gitDir,
 			FileElement localFile, FileElement remoteFile,
 			FileElement mergedFile, FileElement baseFile) throws IOException {
 		Map<String, String> env = new TreeMap<>();
-		env.put(Constants.GIT_DIR_KEY, repo.getDirectory().getAbsolutePath());
-		localFile.addToEnv(env);
-		remoteFile.addToEnv(env);
-		mergedFile.addToEnv(env);
+		if (gitDir != null) {
+			env.put(Constants.GIT_DIR_KEY, gitDir.getAbsolutePath());
+		}
+		if (localFile != null) {
+			localFile.addToEnv(env);
+		}
+		if (remoteFile != null) {
+			remoteFile.addToEnv(env);
+		}
+		if (mergedFile != null) {
+			mergedFile.addToEnv(env);
+		}
 		if (baseFile != null) {
 			baseFile.addToEnv(env);
 		}
 		return env;
 	}
 
+	/**
+	 * @param path
+	 *            the path to be quoted
+	 * @return quoted path if it contains spaces
+	 */
+	@SuppressWarnings("nls")
+	public static String quotePath(String path) {
+		// handling of spaces in path
+		if (path.contains(" ")) {
+			// add quotes before if needed
+			if (!path.startsWith("\"")) {
+				path = "\"" + path;
+			}
+			// add quotes after if needed
+			if (!path.endsWith("\"")) {
+				path = path + "\"";
+			}
+		}
+		return path;
+	}
+
+	/**
+	 * @param fs
+	 *            the file system abstraction
+	 * @param gitDir
+	 *            the .git directory
+	 * @param directory
+	 *            the working directory
+	 * @param path
+	 *            the tool path
+	 * @return true if tool available and false otherwise
+	 */
+	public static boolean isToolAvailable(FS fs, File gitDir, File directory,
+			String path) {
+		boolean available = true;
+		try {
+			CommandExecutor cmdExec = new CommandExecutor(fs, false);
+			available = cmdExec.checkExecutable(path, directory,
+					prepareEnvironment(gitDir, null, null, null, null));
+		} catch (Exception e) {
+			available = false;
+		}
+		return available;
+	}
+
+	/**
+	 * @param defaultName
+	 *            the default tool name
+	 * @param userDefinedNames
+	 *            the user defined tool names
+	 * @param preDefinedNames
+	 *            the pre defined tool names
+	 * @return the sorted tool names set: first element is default tool name if
+	 *         valid, then user defined tool names and then pre defined tool
+	 *         names
+	 */
+	public static Set<String> createSortedToolSet(String defaultName,
+			Set<String> userDefinedNames, Set<String> preDefinedNames) {
+		Set<String> names = new LinkedHashSet<>();
+		if (defaultName != null) {
+			// remove defaultName from both sets
+			Set<String> namesPredef = new LinkedHashSet<>();
+			Set<String> namesUser = new LinkedHashSet<>();
+			namesUser.addAll(userDefinedNames);
+			namesUser.remove(defaultName);
+			namesPredef.addAll(preDefinedNames);
+			namesPredef.remove(defaultName);
+			// add defaultName as first in set
+			names.add(defaultName);
+			names.addAll(namesUser);
+			names.addAll(namesPredef);
+		} else {
+			names.addAll(userDefinedNames);
+			names.addAll(preDefinedNames);
+		}
+		return names;
+	}
+
+	/**
+	 * Provides {@link Optional} with the name of an external tool if specified
+	 * in git configuration for a path.
+	 *
+	 * The formed git configuration results from global rules as well as merged
+	 * rules from info and worktree attributes.
+	 *
+	 * Triggers {@link TreeWalk} until specified path found in the tree.
+	 *
+	 * @param repository
+	 *            target repository to traverse into
+	 * @param path
+	 *            path to the node in repository to parse git attributes for
+	 * @param toolKey
+	 *            config key name for the tool
+	 * @return attribute value for the given tool key if set
+	 * @throws ToolException
+	 */
+	public static Optional<String> getExternalToolFromAttributes(
+			final Repository repository, final String path,
+			final String toolKey) throws ToolException {
+		try {
+			WorkingTreeIterator treeIterator = new FileTreeIterator(repository);
+			try (TreeWalk walk = new TreeWalk(repository)) {
+				walk.addTree(treeIterator);
+				walk.setFilter(new NotIgnoredFilter(0));
+				while (walk.next()) {
+					String treePath = walk.getPathString();
+					if (treePath.equals(path)) {
+						Attributes attrs = walk.getAttributes();
+						if (attrs.containsKey(toolKey)) {
+							return Optional.of(attrs.getValue(toolKey));
+						}
+					}
+					if (walk.isSubtree()) {
+						walk.enterSubtree();
+					}
+				}
+				// no external tool specified
+				return Optional.empty();
+			}
+
+		} catch (RevisionSyntaxException | IOException e) {
+			throw new ToolException(e);
+		}
+	}
+
 }
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/FileElement.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/FileElement.java
index 5902c1e1b8..ba8ca54c58 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/FileElement.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/FileElement.java
@@ -57,6 +57,8 @@ public class FileElement {
 
 	private final Type type;
 
+	private final File workDir;
+
 	private InputStream stream;
 
 	private File tempFile;
@@ -70,7 +72,7 @@ public class FileElement {
 	 *            the element type
 	 */
 	public FileElement(String path, Type type) {
-		this(path, type, null, null);
+		this(path, type, null);
 	}
 
 	/**
@@ -80,17 +82,31 @@ public class FileElement {
 	 *            the file path
 	 * @param type
 	 *            the element type
-	 * @param tempFile
-	 *            the temporary file to be used (can be null and will be created
-	 *            then)
+	 * @param workDir
+	 *            the working directory of the path (can be null, then current
+	 *            working dir is used)
+	 */
+	public FileElement(String path, Type type, File workDir) {
+		this(path, type, workDir, null);
+	}
+
+	/**
+	 * @param path
+	 *            the file path
+	 * @param type
+	 *            the element type
+	 * @param workDir
+	 *            the working directory of the path (can be null, then current
+	 *            working dir is used)
 	 * @param stream
-	 *            the object stream to load instead of file
+	 *            the object stream to load and write on demand, @see getFile(),
+	 *            to tempFile once (can be null)
 	 */
-	public FileElement(String path, Type type, File tempFile,
+	public FileElement(String path, Type type, File workDir,
 			InputStream stream) {
 		this.path = path;
 		this.type = type;
-		this.tempFile = tempFile;
+		this.workDir = workDir;
 		this.stream = stream;
 	}
 
@@ -109,41 +125,39 @@ public class FileElement {
 	}
 
 	/**
-	 * Return a temporary file within passed directory and fills it with stream
-	 * if valid.
-	 *
-	 * @param directory
-	 *            the directory where the temporary file is created
-	 * @param midName
-	 *            name added in the middle of generated temporary file name
-	 * @return the object stream
-	 * @throws IOException
-	 */
-	public File getFile(File directory, String midName) throws IOException {
-		if ((tempFile != null) && (stream == null)) {
-			return tempFile;
-		}
-		tempFile = getTempFile(path, directory, midName);
-		return copyFromStream(tempFile, stream);
-	}
-
-	/**
-	 * Return a real file from work tree or a temporary file with content if
-	 * stream is valid or if path is "/dev/null"
+	 * Return
+	 * <ul>
+	 * <li>a temporary file if already created and stream is not valid</li>
+	 * <li>OR a real file from work tree: if no temp file was created (@see
+	 * createTempFile()) and if no stream was set</li>
+	 * <li>OR an empty temporary file if path is "/dev/null"</li>
+	 * <li>OR a temporary file with stream content if stream is valid (not
+	 * null); stream is closed and invalidated (set to null) after write to temp
+	 * file, so stream is used only once during first call!</li>
+	 * </ul>
 	 *
 	 * @return the object stream
 	 * @throws IOException
 	 */
 	public File getFile() throws IOException {
+		// if we have already temp file and no stream
+		// then just return this temp file (it was filled from outside)
 		if ((tempFile != null) && (stream == null)) {
 			return tempFile;
 		}
-		File file = new File(path);
-		// if we have a stream or file is missing ("/dev/null") then create
-		// temporary file
+		File file = new File(workDir, path);
+		// if we have a stream or file is missing (path is "/dev/null")
+		// then optionally create temporary file and fill it with stream content
 		if ((stream != null) || isNullPath()) {
-			tempFile = getTempFile(file);
-			return copyFromStream(tempFile, stream);
+			if (tempFile == null) {
+				tempFile = getTempFile(file, type.name(), null);
+			}
+			if (stream != null) {
+				copyFromStream(tempFile, stream);
+			}
+			// invalidate the stream, because it is used once
+			stream = null;
+			return tempFile;
 		}
 		return file;
 	}
@@ -158,7 +172,7 @@ public class FileElement {
 	}
 
 	/**
-	 * Create temporary file in given or system temporary directory
+	 * Create temporary file in given or system temporary directory.
 	 *
 	 * @param directory
 	 *            the directory for the file (can be null); if null system
@@ -168,75 +182,23 @@ public class FileElement {
 	 */
 	public File createTempFile(File directory) throws IOException {
 		if (tempFile == null) {
-			File file = new File(path);
-			if (directory != null) {
-				tempFile = getTempFile(file, directory, type.name());
-			} else {
-				tempFile = getTempFile(file);
-			}
+			tempFile = getTempFile(new File(path), type.name(), directory);
 		}
 		return tempFile;
 	}
 
-	private static File getTempFile(File file) throws IOException {
-		return File.createTempFile(".__", "__" + file.getName()); //$NON-NLS-1$ //$NON-NLS-2$
-	}
-
-	private static File getTempFile(File file, File directory, String midName)
-			throws IOException {
-		String[] fileNameAndExtension = splitBaseFileNameAndExtension(file);
-		return File.createTempFile(
-				fileNameAndExtension[0] + "_" + midName + "_", //$NON-NLS-1$ //$NON-NLS-2$
-				fileNameAndExtension[1], directory);
-	}
-
-	private static File getTempFile(String path, File directory, String midName)
-			throws IOException {
-		return getTempFile(new File(path), directory, midName);
-	}
-
 	/**
 	 * Delete and invalidate temporary file if necessary.
 	 */
 	public void cleanTemporaries() {
-		if (tempFile != null && tempFile.exists())
-		tempFile.delete();
-		tempFile = null;
-	}
-
-	private static File copyFromStream(File file, final InputStream stream)
-			throws IOException, FileNotFoundException {
-		if (stream != null) {
-			try (OutputStream outStream = new FileOutputStream(file)) {
-				int read = 0;
-				byte[] bytes = new byte[8 * 1024];
-				while ((read = stream.read(bytes)) != -1) {
-					outStream.write(bytes, 0, read);
-				}
-			} finally {
-				// stream can only be consumed once --> close it
-				stream.close();
-			}
-		}
-		return file;
-	}
-
-	private static String[] splitBaseFileNameAndExtension(File file) {
-		String[] result = new String[2];
-		result[0] = file.getName();
-		result[1] = ""; //$NON-NLS-1$
-		int idx = result[0].lastIndexOf("."); //$NON-NLS-1$
-		// if "." was found (>-1) and last-index is not first char (>0), then
-		// split (same behavior like cgit)
-		if (idx > 0) {
-			result[1] = result[0].substring(idx, result[0].length());
-			result[0] = result[0].substring(0, idx);
+		if (tempFile != null && tempFile.exists()) {
+			tempFile.delete();
 		}
-		return result;
+		tempFile = null;
 	}
 
 	/**
-	 * Replace variable in input
+	 * Replace variable in input.
 	 *
 	 * @param input
 	 *            the input string
@@ -258,4 +220,43 @@ public class FileElement {
 		env.put(type.name(), getFile().getPath());
 	}
 
+	private static File getTempFile(final File file, final String midName,
+			final File workingDir) throws IOException {
+		String[] fileNameAndExtension = splitBaseFileNameAndExtension(file);
+		// TODO: avoid long random file name (number generated by
+		// createTempFile)
+		return File.createTempFile(
+				fileNameAndExtension[0] + "_" + midName + "_", //$NON-NLS-1$ //$NON-NLS-2$
+				fileNameAndExtension[1], workingDir);
+	}
+
+	private static void copyFromStream(final File file,
+			final InputStream stream)
+			throws IOException, FileNotFoundException {
+		try (OutputStream outStream = new FileOutputStream(file)) {
+			int read = 0;
+			byte[] bytes = new byte[8 * 1024];
+			while ((read = stream.read(bytes)) != -1) {
+				outStream.write(bytes, 0, read);
+			}
+		} finally {
+			// stream can only be consumed once --> close it and invalidate
+			stream.close();
+		}
+	}
+
+	private static String[] splitBaseFileNameAndExtension(File file) {
+		String[] result = new String[2];
+		result[0] = file.getName();
+		result[1] = ""; //$NON-NLS-1$
+		int idx = result[0].lastIndexOf("."); //$NON-NLS-1$
+		// if "." was found (>-1) and last-index is not first char (>0), then
+		// split (same behavior like cgit)
+		if (idx > 0) {
+			result[1] = result[0].substring(idx, result[0].length());
+			result[0] = result[0].substring(0, idx);
+		}
+		return result;
+	}
+
 }
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/InformNoToolHandler.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/InformNoToolHandler.java
new file mode 100644
index 0000000000..36b290d37d
--- /dev/null
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/InformNoToolHandler.java
@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2018-2019, Tim Neumann <Tim.Neumann@advantest.com>
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+package org.eclipse.jgit.internal.diffmergetool;
+
+import java.util.List;
+
+/**
+ * A handler for when the diff/merge tool manager wants to inform the user that
+ * no tool has been configured and one of the default tools will be used.
+ */
+public interface InformNoToolHandler {
+	/**
+	 * Inform the user, that no tool is configured and that one of the given
+	 * tools is used.
+	 *
+	 * @param toolNames
+	 *            The tools which are tried
+	 */
+	void inform(List<String> toolNames);
+}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/MergeToolConfig.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/MergeToolConfig.java
index 9be20b75ad..9625d5f101 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/MergeToolConfig.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/MergeToolConfig.java
@@ -31,7 +31,7 @@ import org.eclipse.jgit.lib.Config.SectionParser;
 import org.eclipse.jgit.lib.internal.BooleanTriState;
 
 /**
- * Keeps track of difftool related configuration options.
+ * Keeps track of merge tool related configuration options.
  */
 public class MergeToolConfig {
 
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/MergeTools.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/MergeTools.java
index 9a2a8304eb..b903201264 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/MergeTools.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/MergeTools.java
@@ -1,5 +1,6 @@
 /*
  * Copyright (C) 2018-2022, Andre Bossert <andre.bossert@siemens.com>
+ * Copyright (C) 2019, Tim Neumann <tim.neumann@advantest.com>
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
@@ -15,13 +16,23 @@ import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.nio.file.StandardCopyOption;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.LinkedHashSet;
 import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Optional;
 import java.util.Set;
 import java.util.TreeMap;
 
+import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.internal.diffmergetool.FileElement.Type;
 import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.lib.StoredConfig;
 import org.eclipse.jgit.lib.internal.BooleanTriState;
+import org.eclipse.jgit.treewalk.TreeWalk;
+import org.eclipse.jgit.util.FS;
+import org.eclipse.jgit.util.StringUtils;
 import org.eclipse.jgit.util.FS.ExecutionResult;
 
 /**
@@ -29,26 +40,135 @@ import org.eclipse.jgit.util.FS.ExecutionResult;
  */
 public class MergeTools {
 
-	Repository repo;
+	private final FS fs;
+
+	private final File gitDir;
+
+	private final File workTree;
 
 	private final MergeToolConfig config;
 
+	private final Repository repo;
+
 	private final Map<String, ExternalMergeTool> predefinedTools;
 
 	private final Map<String, ExternalMergeTool> userDefinedTools;
 
 	/**
+	 * Creates the external merge-tools manager for given repository.
+	 *
 	 * @param repo
 	 *            the repository
 	 */
 	public MergeTools(Repository repo) {
+		this(repo, repo.getConfig());
+	}
+
+	/**
+	 * Creates the external diff-tools manager for given configuration.
+	 *
+	 * @param config
+	 *            the git configuration
+	 */
+	public MergeTools(StoredConfig config) {
+		this(null, config);
+	}
+
+	private MergeTools(Repository repo, StoredConfig config) {
 		this.repo = repo;
-		config = repo.getConfig().get(MergeToolConfig.KEY);
+		this.config = config.get(MergeToolConfig.KEY);
+		this.gitDir = repo == null ? null : repo.getDirectory();
+		this.fs = repo == null ? FS.DETECTED : repo.getFS();
+		this.workTree = repo == null ? null : repo.getWorkTree();
 		predefinedTools = setupPredefinedTools();
-		userDefinedTools = setupUserDefinedTools(config, predefinedTools);
+		userDefinedTools = setupUserDefinedTools(predefinedTools);
 	}
 
 	/**
+	 * Merge two versions of a file with optional base file.
+	 *
+	 * @param localFile
+	 *            The local/left version of the file.
+	 * @param remoteFile
+	 *            The remote/right version of the file.
+	 * @param mergedFile
+	 *            The file for the result.
+	 * @param baseFile
+	 *            The base version of the file. May be null.
+	 * @param tempDir
+	 *            The tmepDir used for the files. May be null.
+	 * @param toolName
+	 *            Optionally the name of the tool to use. If not given the
+	 *            default tool will be used.
+	 * @param prompt
+	 *            Optionally a flag whether to prompt the user before compare.
+	 *            If not given the default will be used.
+	 * @param gui
+	 *            A flag whether to prefer a gui tool.
+	 * @param promptHandler
+	 *            The handler to use when needing to prompt the user if he wants
+	 *            to continue.
+	 * @param noToolHandler
+	 *            The handler to use when needing to inform the user, that no
+	 *            tool is configured.
+	 * @return the optional result of executing the tool if it was executed
+	 * @throws ToolException
+	 *             when the tool fails
+	 */
+	public Optional<ExecutionResult> merge(FileElement localFile,
+			FileElement remoteFile, FileElement mergedFile,
+			FileElement baseFile, File tempDir, Optional<String> toolName,
+			BooleanTriState prompt, boolean gui,
+			PromptContinueHandler promptHandler,
+			InformNoToolHandler noToolHandler) throws ToolException {
+
+		String toolNameToUse;
+
+		if (toolName == null) {
+			throw new ToolException(JGitText.get().diffToolNullError);
+		}
+
+		if (toolName.isPresent()) {
+			toolNameToUse = toolName.get();
+		} else {
+			toolNameToUse = getDefaultToolName(gui);
+
+			if (StringUtils.isEmptyOrNull(toolNameToUse)) {
+				noToolHandler.inform(new ArrayList<>(predefinedTools.keySet()));
+				toolNameToUse = getFirstAvailableTool();
+			}
+		}
+
+		if (StringUtils.isEmptyOrNull(toolNameToUse)) {
+			throw new ToolException(JGitText.get().diffToolNotGivenError);
+		}
+
+		boolean doPrompt;
+		if (prompt != BooleanTriState.UNSET) {
+			doPrompt = prompt == BooleanTriState.TRUE;
+		} else {
+			doPrompt = isInteractive();
+		}
+
+		if (doPrompt) {
+			if (!promptHandler.prompt(toolNameToUse)) {
+				return Optional.empty();
+			}
+		}
+
+		ExternalMergeTool tool = getTool(toolNameToUse);
+		if (tool == null) {
+			throw new ToolException(
+					"External merge tool is not defined: " + toolNameToUse); //$NON-NLS-1$
+		}
+
+		return Optional.of(merge(localFile, remoteFile, mergedFile, baseFile,
+				tempDir, tool));
+	}
+
+	/**
+	 * Merge two versions of a file with optional base file.
+	 *
 	 * @param localFile
 	 *            the local file element
 	 * @param remoteFile
@@ -60,38 +180,31 @@ public class MergeTools {
 	 * @param tempDir
 	 *            the temporary directory (needed for backup and auto-remove,
 	 *            can be null)
-	 * @param toolName
-	 *            the selected tool name (can be null)
-	 * @param prompt
-	 *            the prompt option
-	 * @param gui
-	 *            the GUI option
+	 * @param tool
+	 *            the selected tool
 	 * @return the execution result from tool
 	 * @throws ToolException
 	 */
 	public ExecutionResult merge(FileElement localFile, FileElement remoteFile,
 			FileElement mergedFile, FileElement baseFile, File tempDir,
-			String toolName, BooleanTriState prompt, BooleanTriState gui)
-			throws ToolException {
-		ExternalMergeTool tool = guessTool(toolName, gui);
+			ExternalMergeTool tool) throws ToolException {
 		FileElement backup = null;
 		ExecutionResult result = null;
 		try {
-			File workingDir = repo.getWorkTree();
 			// create additional backup file (copy worktree file)
-			backup = createBackupFile(mergedFile.getPath(),
-					tempDir != null ? tempDir : workingDir);
+			backup = createBackupFile(mergedFile,
+					tempDir != null ? tempDir : workTree);
 			// prepare the command (replace the file paths)
-			boolean trust = tool.getTrustExitCode() == BooleanTriState.TRUE;
 			String command = ExternalToolUtils.prepareCommand(
 					tool.getCommand(baseFile != null), localFile, remoteFile,
 					mergedFile, baseFile);
 			// prepare the environment
-			Map<String, String> env = ExternalToolUtils.prepareEnvironment(repo,
-					localFile, remoteFile, mergedFile, baseFile);
+			Map<String, String> env = ExternalToolUtils.prepareEnvironment(
+					gitDir, localFile, remoteFile, mergedFile, baseFile);
+			boolean trust = tool.getTrustExitCode() == BooleanTriState.TRUE;
 			// execute the tool
-			CommandExecutor cmdExec = new CommandExecutor(repo.getFS(), trust);
-			result = cmdExec.run(command, workingDir, env);
+			CommandExecutor cmdExec = new CommandExecutor(fs, trust);
+			result = cmdExec.run(command, workTree, env);
 			// keep backup as .orig file
 			if (backup != null) {
 				keepBackupFile(mergedFile.getPath(), backup);
@@ -123,19 +236,21 @@ public class MergeTools {
 		}
 	}
 
-	private FileElement createBackupFile(String filePath, File parentDir)
+	private FileElement createBackupFile(FileElement from, File toParentDir)
 			throws IOException {
 		FileElement backup = null;
-		Path path = Paths.get(filePath);
+		Path path = Paths.get(from.getPath());
 		if (Files.exists(path)) {
-			backup = new FileElement(filePath, Type.BACKUP);
-			Files.copy(path, backup.createTempFile(parentDir).toPath(),
+			backup = new FileElement(from.getPath(), Type.BACKUP);
+			Files.copy(path, backup.createTempFile(toParentDir).toPath(),
 					StandardCopyOption.REPLACE_EXISTING);
 		}
 		return backup;
 	}
 
 	/**
+	 * Create temporary directory.
+	 *
 	 * @return the created temporary directory if (mergetol.writeToTemp == true)
 	 *         or null if not configured or false.
 	 * @throws IOException
@@ -147,60 +262,138 @@ public class MergeTools {
 	}
 
 	/**
-	 * @return the tool names
+	 * Get user defined tool names.
+	 *
+	 * @return the user defined tool names
 	 */
-	public Set<String> getToolNames() {
-		return config.getToolNames();
+	public Set<String> getUserDefinedToolNames() {
+		return userDefinedTools.keySet();
 	}
 
 	/**
-	 * @return the user defined tools
+	 * @return the predefined tool names
 	 */
-	public Map<String, ExternalMergeTool> getUserDefinedTools() {
-		return userDefinedTools;
+	public Set<String> getPredefinedToolNames() {
+		return predefinedTools.keySet();
 	}
 
 	/**
-	 * @return the available predefined tools
+	 * Get all tool names.
+	 *
+	 * @return the all tool names (default or available tool name is the first
+	 *         in the set)
 	 */
-	public Map<String, ExternalMergeTool> getAvailableTools() {
-		return predefinedTools;
+	public Set<String> getAllToolNames() {
+		String defaultName = getDefaultToolName(false);
+		if (defaultName == null) {
+			defaultName = getFirstAvailableTool();
+		}
+		return ExternalToolUtils.createSortedToolSet(defaultName,
+				getUserDefinedToolNames(), getPredefinedToolNames());
 	}
 
 	/**
-	 * @return the NOT available predefined tools
+	 * Provides {@link Optional} with the name of an external merge tool if
+	 * specified in git configuration for a path.
+	 *
+	 * The formed git configuration results from global rules as well as merged
+	 * rules from info and worktree attributes.
+	 *
+	 * Triggers {@link TreeWalk} until specified path found in the tree.
+	 *
+	 * @param path
+	 *            path to the node in repository to parse git attributes for
+	 * @return name of the difftool if set
+	 * @throws ToolException
 	 */
-	public Map<String, ExternalMergeTool> getNotAvailableTools() {
-		return new TreeMap<>();
+	public Optional<String> getExternalToolFromAttributes(final String path)
+			throws ToolException {
+		return ExternalToolUtils.getExternalToolFromAttributes(repo, path,
+				ExternalToolUtils.KEY_MERGE_TOOL);
 	}
 
 	/**
-	 * @param gui
-	 *            use the diff.guitool setting ?
-	 * @return the default tool name
+	 * Checks the availability of the predefined tools in the system.
+	 *
+	 * @return set of predefined available tools
 	 */
-	public String getDefaultToolName(BooleanTriState gui) {
-		return gui != BooleanTriState.UNSET ? "my_gui_tool" //$NON-NLS-1$
-				: config.getDefaultToolName();
+	public Set<String> getPredefinedAvailableTools() {
+		Map<String, ExternalMergeTool> defTools = getPredefinedTools(true);
+		Set<String> availableTools = new LinkedHashSet<>();
+		for (Entry<String, ExternalMergeTool> elem : defTools.entrySet()) {
+			if (elem.getValue().isAvailable()) {
+				availableTools.add(elem.getKey());
+			}
+		}
+		return availableTools;
 	}
 
 	/**
+	 * @return the user defined tools
+	 */
+	public Map<String, ExternalMergeTool> getUserDefinedTools() {
+		return Collections.unmodifiableMap(userDefinedTools);
+	}
+
+	/**
+	 * Get predefined tools map.
+	 *
+	 * @param checkAvailability
+	 *            true: for checking if tools can be executed; ATTENTION: this
+	 *            check took some time, do not execute often (store the map for
+	 *            other actions); false: availability is NOT checked:
+	 *            isAvailable() returns default false is this case!
+	 * @return the predefined tools with optionally checked availability (long
+	 *         running operation)
+	 */
+	public Map<String, ExternalMergeTool> getPredefinedTools(
+			boolean checkAvailability) {
+		if (checkAvailability) {
+			for (ExternalMergeTool tool : predefinedTools.values()) {
+				PreDefinedMergeTool predefTool = (PreDefinedMergeTool) tool;
+				predefTool.setAvailable(ExternalToolUtils.isToolAvailable(fs,
+						gitDir, workTree, predefTool.getPath()));
+			}
+		}
+		return Collections.unmodifiableMap(predefinedTools);
+	}
+
+	/**
+	 * Get first available tool name.
+	 *
+	 * @return the name of first available predefined tool or null
+	 */
+	public String getFirstAvailableTool() {
+		String name = null;
+		for (ExternalMergeTool tool : predefinedTools.values()) {
+			if (ExternalToolUtils.isToolAvailable(fs, gitDir, workTree,
+					tool.getPath())) {
+				name = tool.getName();
+				break;
+			}
+		}
+		return name;
+	}
+
+	/**
+	 * Is interactive merge (prompt enabled) ?
+	 *
 	 * @return is interactive (config prompt enabled) ?
 	 */
 	public boolean isInteractive() {
 		return config.isPrompt();
 	}
 
-	private ExternalMergeTool guessTool(String toolName, BooleanTriState gui)
-			throws ToolException {
-		if ((toolName == null) || toolName.isEmpty()) {
-			toolName = getDefaultToolName(gui);
-		}
-		ExternalMergeTool tool = getTool(toolName);
-		if (tool == null) {
-			throw new ToolException("Unknown diff tool " + toolName); //$NON-NLS-1$
-		}
-		return tool;
+	/**
+	 * Get the default (gui-)tool name.
+	 *
+	 * @param gui
+	 *            use the diff.guitool setting ?
+	 * @return the default tool name
+	 */
+	public String getDefaultToolName(boolean gui) {
+		return gui ? config.getDefaultGuiToolName()
+				: config.getDefaultToolName();
 	}
 
 	private ExternalMergeTool getTool(final String name) {
@@ -231,9 +424,9 @@ public class MergeTools {
 	}
 
 	private Map<String, ExternalMergeTool> setupUserDefinedTools(
-			MergeToolConfig cfg, Map<String, ExternalMergeTool> predefTools) {
+			Map<String, ExternalMergeTool> predefTools) {
 		Map<String, ExternalMergeTool> tools = new TreeMap<>();
-		Map<String, ExternalMergeTool> userTools = cfg.getTools();
+		Map<String, ExternalMergeTool> userTools = config.getTools();
 		for (String name : userTools.keySet()) {
 			ExternalMergeTool userTool = userTools.get(name);
 			// if mergetool.<name>.cmd is defined we have user defined tool
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/PreDefinedDiffTool.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/PreDefinedDiffTool.java
index 092cb605be..e1169a2d60 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/PreDefinedDiffTool.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/PreDefinedDiffTool.java
@@ -56,7 +56,7 @@ public class PreDefinedDiffTool extends UserDefinedDiffTool {
 	 */
 	@Override
 	public String getCommand() {
-		return getPath() + " " + super.getCommand(); //$NON-NLS-1$
+		return ExternalToolUtils.quotePath(getPath()) + " " + super.getCommand(); //$NON-NLS-1$
 	}
 
 }
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/PreDefinedMergeTool.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/PreDefinedMergeTool.java
index 2c64c16667..7b28d32820 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/PreDefinedMergeTool.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/PreDefinedMergeTool.java
@@ -84,7 +84,7 @@ public class PreDefinedMergeTool extends UserDefinedMergeTool {
 	 */
 	@Override
 	public String getCommand(boolean withBase) {
-		return getPath() + " " //$NON-NLS-1$
+		return ExternalToolUtils.quotePath(getPath()) + " " //$NON-NLS-1$
 				+ (withBase ? super.getCommand() : parametersWithoutBase);
 	}
 
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/PromptContinueHandler.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/PromptContinueHandler.java
new file mode 100644
index 0000000000..6ad33df2a0
--- /dev/null
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/PromptContinueHandler.java
@@ -0,0 +1,27 @@
+/*
+ * Copyright (C) 2018-2019, Tim Neumann <Tim.Neumann@advantest.com>
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+package org.eclipse.jgit.internal.diffmergetool;
+
+/**
+ * A handler for when the diff/merge tool manager wants to prompt the user
+ * whether to continue
+ */
+public interface PromptContinueHandler {
+	/**
+	 * Prompt the user whether to continue with the next file by opening a given
+	 * tool.
+	 *
+	 * @param toolName
+	 *            The name of the tool to open
+	 * @return Whether the user wants to continue
+	 */
+	boolean prompt(String toolName);
+}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/ToolException.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/ToolException.java
index 27f7d12e66..7cc5bb50d9 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/ToolException.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/ToolException.java
@@ -110,6 +110,9 @@ public class ToolException extends Exception {
 	 * @return the result Stderr
 	 */
 	public String getResultStderr() {
+		if (result == null) {
+			return ""; //$NON-NLS-1$
+		}
 		try {
 			return new String(result.getStderr().toByteArray());
 		} catch (Exception e) {
@@ -122,6 +125,9 @@ public class ToolException extends Exception {
 	 * @return the result Stdout
 	 */
 	public String getResultStdout() {
+		if (result == null) {
+			return ""; //$NON-NLS-1$
+		}
 		try {
 			return new String(result.getStdout().toByteArray());
 		} catch (Exception e) {
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/UserDefinedDiffTool.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/UserDefinedDiffTool.java
index 012296eb35..eb72d01cdb 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/UserDefinedDiffTool.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/diffmergetool/UserDefinedDiffTool.java
@@ -15,6 +15,8 @@ package org.eclipse.jgit.internal.diffmergetool;
  */
 public class UserDefinedDiffTool implements ExternalDiffTool {
 
+	private boolean available;
+
 	/**
 	 * the diff tool name
 	 */
@@ -99,6 +101,23 @@ public class UserDefinedDiffTool implements ExternalDiffTool {
 	}
 
 	/**
+	 * @return availability of the tool: true if tool can be executed and false
+	 *         if not
+	 */
+	@Override
+	public boolean isAvailable() {
+		return available;
+	}
+
+	/**
+	 * @param available
+	 *            true if tool can be found and false if not
+	 */
+	public void setAvailable(boolean available) {
+		this.available = available;
+	}
+
+	/**
 	 * Overrides the path for the given tool. Equivalent to setting
 	 * {@code difftool.<tool>.path}.
 	 *