Add availability check of pre-defined tools

see: https://git-scm.com/docs/git-difftool
see: https://git-scm.com/docs/git-mergetool

* now all available tools are printed with "--tool-help"
* if no diff.tool or merge.tool is defined the first available
pre-defined tool is used

TODO:
- add mergetools to difftools --> extra change or merge to this
- return the exit-code of the tool to jgit / java runtime

Bug: 356832
Change-Id: I20fb04e71ced981f5625020f461bbac24e6cec70
Signed-off-by: Andre Bossert <andre.bossert@siemens.com>

18 files changed, 357 insertions, 97 deletions

diff --git a/org.eclipse.jgit.pgm.test/tst/org/eclipse/jgit/pgm/DiffToolTest.java b/org.eclipse.jgit.pgm.test/tst/org/eclipse/jgit/pgm/DiffToolTest.java
index 8daaa6ad9e..a258821f0c 100644
--- a/org.eclipse.jgit.pgm.test/tst/org/eclipse/jgit/pgm/DiffToolTest.java
+++ b/org.eclipse.jgit.pgm.test/tst/org/eclipse/jgit/pgm/DiffToolTest.java
@@ -20,8 +20,10 @@ import java.io.InputStream;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.Map;
 
-import org.eclipse.jgit.internal.diffmergetool.CommandLineDiffTool;
+import org.eclipse.jgit.internal.diffmergetool.DiffTools;
+import org.eclipse.jgit.internal.diffmergetool.ExternalDiffTool;
 import org.eclipse.jgit.lib.StoredConfig;
 import org.junit.Before;
 import org.junit.Test;
@@ -149,20 +151,38 @@ public class DiffToolTest extends ToolTestCase {
 
 	@Test
 	public void testToolHelp() throws Exception {
-		CommandLineDiffTool[] defaultTools = CommandLineDiffTool.values();
 		List<String> expectedOutput = new ArrayList<>();
+
+		DiffTools diffTools = new DiffTools(db);
+		Map<String, ExternalDiffTool> predefinedTools = diffTools
+				.getPredefinedTools(true);
+		List<ExternalDiffTool> availableTools = new ArrayList<>();
+		List<ExternalDiffTool> notAvailableTools = new ArrayList<>();
+		for (ExternalDiffTool tool : predefinedTools.values()) {
+			if (tool.isAvailable()) {
+				availableTools.add(tool);
+			} else {
+				notAvailableTools.add(tool);
+			}
+		}
+
 		expectedOutput.add(
 				"'git difftool --tool=<tool>' may be set to one of the following:");
-		for (CommandLineDiffTool defaultTool : defaultTools) {
-			String toolName = defaultTool.name();
+		for (ExternalDiffTool tool : availableTools) {
+			String toolName = tool.getName();
 			expectedOutput.add(toolName);
 		}
 		String customToolHelpLine = TOOL_NAME + "." + CONFIG_KEY_CMD + " "
 				+ getEchoCommand();
 		expectedOutput.add("user-defined:");
 		expectedOutput.add(customToolHelpLine);
+		expectedOutput.add(
+				"The following tools are valid, but not currently available:");
+		for (ExternalDiffTool tool : notAvailableTools) {
+			String toolName = tool.getName();
+			expectedOutput.add(toolName);
+		}
 		String[] userDefinedToolsHelp = {
-				"The following tools are valid, but not currently available:",
 				"Some of the tools listed above only work in a windowed",
 				"environment. If run in a terminal-only session, they will fail.",
 		};
diff --git a/org.eclipse.jgit.pgm.test/tst/org/eclipse/jgit/pgm/MergeToolTest.java b/org.eclipse.jgit.pgm.test/tst/org/eclipse/jgit/pgm/MergeToolTest.java
index 2e50f09081..e9d559e70e 100644
--- a/org.eclipse.jgit.pgm.test/tst/org/eclipse/jgit/pgm/MergeToolTest.java
+++ b/org.eclipse.jgit.pgm.test/tst/org/eclipse/jgit/pgm/MergeToolTest.java
@@ -19,8 +19,10 @@ import java.io.InputStream;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.Map;
 
-import org.eclipse.jgit.internal.diffmergetool.CommandLineMergeTool;
+import org.eclipse.jgit.internal.diffmergetool.ExternalMergeTool;
+import org.eclipse.jgit.internal.diffmergetool.MergeTools;
 import org.eclipse.jgit.lib.StoredConfig;
 import org.junit.Before;
 import org.junit.Test;
@@ -157,20 +159,38 @@ public class MergeToolTest extends ToolTestCase {
 
 	@Test
 	public void testToolHelp() throws Exception {
-		CommandLineMergeTool[] defaultTools = CommandLineMergeTool.values();
 		List<String> expectedOutput = new ArrayList<>();
+
+		MergeTools diffTools = new MergeTools(db);
+		Map<String, ExternalMergeTool> predefinedTools = diffTools
+				.getPredefinedTools(true);
+		List<ExternalMergeTool> availableTools = new ArrayList<>();
+		List<ExternalMergeTool> notAvailableTools = new ArrayList<>();
+		for (ExternalMergeTool tool : predefinedTools.values()) {
+			if (tool.isAvailable()) {
+				availableTools.add(tool);
+			} else {
+				notAvailableTools.add(tool);
+			}
+		}
+
 		expectedOutput.add(
 				"'git mergetool --tool=<tool>' may be set to one of the following:");
-		for (CommandLineMergeTool defaultTool : defaultTools) {
-			String toolName = defaultTool.name();
+		for (ExternalMergeTool tool : availableTools) {
+			String toolName = tool.getName();
 			expectedOutput.add(toolName);
 		}
 		String customToolHelpLine = TOOL_NAME + "." + CONFIG_KEY_CMD + " "
 				+ getEchoCommand();
 		expectedOutput.add("user-defined:");
 		expectedOutput.add(customToolHelpLine);
+		expectedOutput.add(
+				"The following tools are valid, but not currently available:");
+		for (ExternalMergeTool tool : notAvailableTools) {
+			String toolName = tool.getName();
+			expectedOutput.add(toolName);
+		}
 		String[] userDefinedToolsHelp = {
-				"The following tools are valid, but not currently available:",
 				"Some of the tools listed above only work in a windowed",
 				"environment. If run in a terminal-only session, they will fail.", };
 		expectedOutput.addAll(Arrays.asList(userDefinedToolsHelp));
diff --git a/org.eclipse.jgit.pgm/resources/org/eclipse/jgit/pgm/internal/CLIText.properties b/org.eclipse.jgit.pgm/resources/org/eclipse/jgit/pgm/internal/CLIText.properties
index 674185df2b..b14531a1bd 100644
--- a/org.eclipse.jgit.pgm/resources/org/eclipse/jgit/pgm/internal/CLIText.properties
+++ b/org.eclipse.jgit.pgm/resources/org/eclipse/jgit/pgm/internal/CLIText.properties
@@ -61,6 +61,8 @@ deletedRemoteBranch=Deleted remote branch {0}
 diffToolHelpSetToFollowing=''git difftool --tool=<tool>'' may be set to one of the following:\n{0}\n\tuser-defined:\n{1}\nThe following tools are valid, but not currently available:\n{2}\nSome of the tools listed above only work in a windowed\nenvironment. If run in a terminal-only session, they will fail.
 diffToolLaunch=Viewing ({0}/{1}): ''{2}''\nLaunch ''{3}'' [Y/n]?
 diffToolDied=external diff died, stopping at path ''{0}'' due to exception: {1}
+diffToolPromptToolName=This message is displayed because 'diff.tool' is not configured.\nSee 'git difftool --tool-help' or 'git help config' for more details.\n'git difftool' will now attempt to use one of the following tools:\n{0}\n
+diffToolUnknownToolName=Unknown diff tool '{0}'
 doesNotExist={0} does not exist
 dontOverwriteLocalChanges=error: Your local changes to the following file would be overwritten by merge:
 everythingUpToDate=Everything up-to-date
@@ -107,6 +109,8 @@ mergeToolDeletedConflictByThem=  {local}: modified file\n  {remote}: deleted
 mergeToolContinueUnresolvedPaths=\nContinue merging other unresolved paths [y/n]?
 mergeToolWasMergeSuccessfull=Was the merge successful [y/n]?
 mergeToolDeletedMergeDecision=Use (m)odified or (d)eleted file, or (a)bort?
+mergeToolPromptToolName=This message is displayed because 'merge.tool' is not configured.\nSee 'git mergetool --tool-help' or 'git help config' for more details.\n'git mergetool' will now attempt to use one of the following tools:\n{0}\n
+mergeToolUnknownToolName=Unknown merge tool '{0}'
 mergeFailed=Automatic merge failed; fix conflicts and then commit the result
 mergeCheckoutFailed=Please, commit your changes or stash them before you can merge.
 mergeMadeBy=Merge made by the ''{0}'' strategy.
diff --git a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/DiffTool.java b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/DiffTool.java
index 74d91cd3d7..e5a3c53e3f 100644
--- a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/DiffTool.java
+++ b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/DiffTool.java
@@ -23,29 +23,30 @@ import java.text.MessageFormat;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.TimeUnit;
+
 import org.eclipse.jgit.diff.ContentSource;
 import org.eclipse.jgit.diff.ContentSource.Pair;
 import org.eclipse.jgit.diff.DiffEntry;
 import org.eclipse.jgit.diff.DiffEntry.Side;
-import org.eclipse.jgit.internal.diffmergetool.ToolException;
-import org.eclipse.jgit.internal.diffmergetool.DiffTools;
-import org.eclipse.jgit.internal.diffmergetool.FileElement;
-import org.eclipse.jgit.internal.diffmergetool.ExternalDiffTool;
 import org.eclipse.jgit.diff.DiffFormatter;
 import org.eclipse.jgit.dircache.DirCacheCheckout;
-import org.eclipse.jgit.dircache.DirCacheIterator;
 import org.eclipse.jgit.dircache.DirCacheCheckout.CheckoutMetadata;
+import org.eclipse.jgit.dircache.DirCacheIterator;
 import org.eclipse.jgit.errors.AmbiguousObjectException;
 import org.eclipse.jgit.errors.CorruptObjectException;
 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.NoWorkTreeException;
 import org.eclipse.jgit.errors.RevisionSyntaxException;
+import org.eclipse.jgit.internal.diffmergetool.DiffTools;
+import org.eclipse.jgit.internal.diffmergetool.ExternalDiffTool;
+import org.eclipse.jgit.internal.diffmergetool.FileElement;
+import org.eclipse.jgit.internal.diffmergetool.ToolException;
 import org.eclipse.jgit.lib.Constants;
+import org.eclipse.jgit.lib.CoreConfig.EolStreamType;
 import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ObjectReader;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.lib.TextProgressMonitor;
-import org.eclipse.jgit.lib.CoreConfig.EolStreamType;
 import org.eclipse.jgit.lib.internal.BooleanTriState;
 import org.eclipse.jgit.pgm.internal.CLIText;
 import org.eclipse.jgit.pgm.opt.PathTreeFilterHandler;
@@ -58,8 +59,8 @@ import org.eclipse.jgit.treewalk.WorkingTreeIterator;
 import org.eclipse.jgit.treewalk.WorkingTreeOptions;
 import org.eclipse.jgit.treewalk.filter.PathFilterGroup;
 import org.eclipse.jgit.treewalk.filter.TreeFilter;
-import org.eclipse.jgit.util.StringUtils;
 import org.eclipse.jgit.util.FS.ExecutionResult;
+import org.eclipse.jgit.util.StringUtils;
 import org.kohsuke.args4j.Argument;
 import org.kohsuke.args4j.Option;
 
@@ -144,19 +145,14 @@ class DiffTool extends TextBuiltin {
 				if (prompt != BooleanTriState.UNSET) {
 					showPrompt = prompt == BooleanTriState.TRUE;
 				}
-				String toolNamePrompt = toolName;
-				if (showPrompt) {
-					if (StringUtils.isEmptyOrNull(toolNamePrompt)) {
-						toolNamePrompt = diffTools.getDefaultToolName(gui);
-					}
-				}
+				// get passed or default tool name
+				String toolNameToUse = promptToolName();
 				// get the changed files
 				List<DiffEntry> files = getFiles();
 				if (files.size() > 0) {
-					compare(files, showPrompt, toolNamePrompt);
+					compare(files, showPrompt, toolNameToUse);
 				}
 			}
-			outw.flush();
 		} catch (RevisionSyntaxException | IOException e) {
 			throw die(e.getMessage(), e);
 		} finally {
@@ -164,8 +160,32 @@ class DiffTool extends TextBuiltin {
 		}
 	}
 
+	private String promptToolName() throws IOException {
+		String toolNameToUse = toolName;
+		if (StringUtils.isEmptyOrNull(toolNameToUse)) {
+			toolNameToUse = diffTools.getDefaultToolName(gui);
+		}
+		if (StringUtils.isEmptyOrNull(toolNameToUse)) {
+			Map<String, ExternalDiffTool> predefTools = diffTools
+					.getPredefinedTools(false);
+			StringBuilder toolNames = new StringBuilder();
+			for (String name : predefTools.keySet()) {
+				toolNames.append(name + " "); //$NON-NLS-1$
+			}
+			outw.println(MessageFormat.format(
+					CLIText.get().diffToolPromptToolName, toolNames));
+			outw.flush();
+			toolNameToUse = diffTools.getFirstAvailableTool();
+		}
+		if (StringUtils.isEmptyOrNull(toolNameToUse)) {
+			throw new IOException(MessageFormat
+					.format(CLIText.get().diffToolUnknownToolName, toolName));
+		}
+		return toolNameToUse;
+	}
+
 	private void compare(List<DiffEntry> files, boolean showPrompt,
-			String toolNamePrompt) throws IOException {
+			String toolNameToUse) throws IOException {
 		ContentSource.Pair sourcePair = new ContentSource.Pair(source(oldTree),
 				source(newTree));
 		try {
@@ -179,7 +199,7 @@ class DiffTool extends TextBuiltin {
 				boolean launchCompare = true;
 				if (showPrompt) {
 					launchCompare = isLaunchCompare(fileIndex + 1, files.size(),
-							mergedFilePath, toolNamePrompt);
+							mergedFilePath, toolNameToUse);
 				}
 				if (launchCompare) {
 					try {
@@ -195,17 +215,21 @@ class DiffTool extends TextBuiltin {
 						// to jgit / java runtime ?
 						// int rc =...
 						ExecutionResult result = diffTools.compare(local,
-								remote, merged, toolName, prompt, gui,
+								remote, merged, toolNameToUse, prompt, gui,
 								trustExitCode);
 						outw.println(new String(result.getStdout().toByteArray()));
+						outw.flush();
 						errw.println(
 								new String(result.getStderr().toByteArray()));
+						errw.flush();
 					} catch (ToolException e) {
 						outw.println(e.getResultStdout());
 						outw.flush();
 						errw.println(e.getMessage());
+						errw.flush();
 						throw die(MessageFormat.format(
-								CLIText.get().diffToolDied, mergedFilePath), e);
+								CLIText.get().diffToolDied, mergedFilePath, e),
+								e);
 					}
 				} else {
 					break;
@@ -232,16 +256,17 @@ class DiffTool extends TextBuiltin {
 		}
 		return launchCompare;
 	}
-
 	private void showToolHelp() throws IOException {
+		Map<String, ExternalDiffTool> predefTools = diffTools
+				.getPredefinedTools(true);
 		StringBuilder availableToolNames = new StringBuilder();
-		for (String name : diffTools.getAvailableTools().keySet()) {
-			availableToolNames.append(MessageFormat.format("\t\t{0}\n", name)); //$NON-NLS-1$
-		}
 		StringBuilder notAvailableToolNames = new StringBuilder();
-		for (String name : diffTools.getNotAvailableTools().keySet()) {
-			notAvailableToolNames
-					.append(MessageFormat.format("\t\t{0}\n", name)); //$NON-NLS-1$
+		for (String name : predefTools.keySet()) {
+			if (predefTools.get(name).isAvailable()) {
+				availableToolNames.append(MessageFormat.format("\t\t{0}\n", name)); //$NON-NLS-1$
+			} else {
+				notAvailableToolNames.append(MessageFormat.format("\t\t{0}\n", name)); //$NON-NLS-1$
+			}
 		}
 		StringBuilder userToolNames = new StringBuilder();
 		Map<String, ExternalDiffTool> userTools = diffTools
diff --git a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/MergeTool.java b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/MergeTool.java
index 9712770758..f5884c44da 100644
--- a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/MergeTool.java
+++ b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/MergeTool.java
@@ -48,6 +48,7 @@ import org.eclipse.jgit.treewalk.WorkingTreeOptions;
 import org.eclipse.jgit.treewalk.filter.PathFilterGroup;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.util.StringUtils;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.lib.internal.BooleanTriState;
 import org.eclipse.jgit.lib.CoreConfig.EolStreamType;
@@ -121,14 +122,11 @@ class MergeTool extends TextBuiltin {
 					showPrompt = prompt == BooleanTriState.TRUE;
 				}
 				// get passed or default tool name
-				String toolNameSelected = toolName;
-				if ((toolNameSelected == null) || toolNameSelected.isEmpty()) {
-					toolNameSelected = mergeTools.getDefaultToolName(gui);
-				}
+				String toolNameToUse = promptToolName();
 				// get the changed files
 				Map<String, StageState> files = getFiles();
 				if (files.size() > 0) {
-					merge(files, showPrompt, toolNameSelected);
+					merge(files, showPrompt, toolNameToUse);
 				} else {
 					outw.println(CLIText.get().mergeToolNoFiles);
 				}
@@ -139,6 +137,30 @@ class MergeTool extends TextBuiltin {
 		}
 	}
 
+	private String promptToolName() throws IOException {
+		String toolNameToUse = toolName;
+		if (StringUtils.isEmptyOrNull(toolNameToUse)) {
+			toolNameToUse = mergeTools.getDefaultToolName(gui);
+		}
+		if (StringUtils.isEmptyOrNull(toolNameToUse)) {
+			Map<String, ExternalMergeTool> predefTools = mergeTools
+					.getPredefinedTools(false);
+			StringBuilder toolNames = new StringBuilder();
+			for (String name : predefTools.keySet()) {
+				toolNames.append(name + " "); //$NON-NLS-1$
+			}
+			outw.println(MessageFormat
+					.format(CLIText.get().mergeToolPromptToolName, toolNames));
+			outw.flush();
+			toolNameToUse = mergeTools.getFirstAvailableTool();
+		}
+		if (StringUtils.isEmptyOrNull(toolNameToUse)) {
+			throw new IOException(MessageFormat
+					.format(CLIText.get().mergeToolUnknownToolName, toolName));
+		}
+		return toolNameToUse;
+	}
+
 	private void merge(Map<String, StageState> files, boolean showPrompt,
 			String toolNamePrompt) throws Exception {
 		// sort file names
@@ -420,14 +442,16 @@ class MergeTool extends TextBuiltin {
 	}
 
 	private void showToolHelp() throws IOException {
+		Map<String, ExternalMergeTool> predefTools = mergeTools
+				.getPredefinedTools(true);
 		StringBuilder availableToolNames = new StringBuilder();
-		for (String name : mergeTools.getAvailableTools().keySet()) {
-			availableToolNames.append(MessageFormat.format("\t\t{0}\n", name)); //$NON-NLS-1$
-		}
 		StringBuilder notAvailableToolNames = new StringBuilder();
-		for (String name : mergeTools.getNotAvailableTools().keySet()) {
-			notAvailableToolNames
-					.append(MessageFormat.format("\t\t{0}\n", name)); //$NON-NLS-1$
+		for (String name : predefTools.keySet()) {
+			if (predefTools.get(name).isAvailable()) {
+				availableToolNames.append(MessageFormat.format("\t\t{0}\n", name)); //$NON-NLS-1$
+			} else {
+				notAvailableToolNames.append(MessageFormat.format("\t\t{0}\n", name)); //$NON-NLS-1$
+			}
 		}
 		StringBuilder userToolNames = new StringBuilder();
 		Map<String, ExternalMergeTool> userTools = mergeTools
diff --git a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/internal/CLIText.java b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/internal/CLIText.java
index 989e649b72..e06f150e51 100644
--- a/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/internal/CLIText.java
+++ b/org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/internal/CLIText.java
@@ -139,6 +139,8 @@ public class CLIText extends TranslationBundle {
 	/***/ public String diffToolHelpSetToFollowing;
 	/***/ public String diffToolLaunch;
 	/***/ public String diffToolDied;
+	/***/ public String diffToolPromptToolName;
+	/***/ public String diffToolUnknownToolName;
 	/***/ public String doesNotExist;
 	/***/ public String dontOverwriteLocalChanges;
 	/***/ public String everythingUpToDate;
@@ -185,6 +187,8 @@ public class CLIText extends TranslationBundle {
 	/***/ public String mergeToolContinueUnresolvedPaths;
 	/***/ public String mergeToolWasMergeSuccessfull;
 	/***/ public String mergeToolDeletedMergeDecision;
+	/***/ public String mergeToolPromptToolName;
+	/***/ public String mergeToolUnknownToolName;
 	/***/ public String mergeFailed;
 	/***/ public String mergeCheckoutFailed;
 	/***/ public String mergeMadeBy;
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/diffmergetool/ExternalDiffToolTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/diffmergetool/ExternalDiffToolTest.java
index 4fd55c6cad..1b501c25b0 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/diffmergetool/ExternalDiffToolTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/diffmergetool/ExternalDiffToolTest.java
@@ -97,7 +97,7 @@ public class ExternalDiffToolTest extends ExternalToolTestCase {
 	@Test
 	public void testAllTools() {
 		DiffTools manager = new DiffTools(db);
-		Set<String> actualToolNames = manager.getAvailableTools().keySet();
+		Set<String> actualToolNames = manager.getPredefinedTools(true).keySet();
 		Set<String> expectedToolNames = new LinkedHashSet<>();
 		CommandLineDiffTool[] defaultTools = CommandLineDiffTool.values();
 		for (CommandLineDiffTool defaultTool : defaultTools) {
@@ -153,15 +153,6 @@ public class ExternalDiffToolTest extends ExternalToolTestCase {
 	}
 
 	@Test
-	public void testNotAvailableTools() {
-		DiffTools manager = new DiffTools(db);
-		Set<String> actualToolNames = manager.getNotAvailableTools().keySet();
-		Set<String> expectedToolNames = Collections.emptySet();
-		assertEquals("Incorrect set of not available external diff tools",
-				expectedToolNames, actualToolNames);
-	}
-
-	@Test
 	public void testCompare() throws ToolException {
 		String toolName = "customTool";
 
@@ -239,7 +230,7 @@ public class ExternalDiffToolTest extends ExternalToolTestCase {
 
 		DiffTools manager = new DiffTools(db);
 		Map<String, ExternalDiffTool> availableTools = manager
-				.getAvailableTools();
+				.getPredefinedTools(true);
 		ExternalDiffTool externalDiffTool = availableTools
 				.get(overridenToolName);
 		String actualDiffToolPath = externalDiffTool.getPath();
diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/diffmergetool/ExternalMergeToolTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/diffmergetool/ExternalMergeToolTest.java
index 50576682eb..305f2b470e 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/diffmergetool/ExternalMergeToolTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/diffmergetool/ExternalMergeToolTest.java
@@ -9,14 +9,14 @@
  */
 package org.eclipse.jgit.internal.diffmergetool;
 
-import static org.eclipse.jgit.lib.ConfigConstants.CONFIG_MERGETOOL_SECTION;
-import static org.eclipse.jgit.lib.ConfigConstants.CONFIG_MERGE_SECTION;
 import static org.eclipse.jgit.lib.ConfigConstants.CONFIG_KEY_CMD;
 import static org.eclipse.jgit.lib.ConfigConstants.CONFIG_KEY_GUITOOL;
 import static org.eclipse.jgit.lib.ConfigConstants.CONFIG_KEY_PATH;
 import static org.eclipse.jgit.lib.ConfigConstants.CONFIG_KEY_PROMPT;
 import static org.eclipse.jgit.lib.ConfigConstants.CONFIG_KEY_TOOL;
 import static org.eclipse.jgit.lib.ConfigConstants.CONFIG_KEY_TRUST_EXIT_CODE;
+import static org.eclipse.jgit.lib.ConfigConstants.CONFIG_MERGETOOL_SECTION;
+import static org.eclipse.jgit.lib.ConfigConstants.CONFIG_MERGE_SECTION;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertTrue;
@@ -95,7 +95,7 @@ public class ExternalMergeToolTest extends ExternalToolTestCase {
 	@Test
 	public void testAllTools() {
 		MergeTools manager = new MergeTools(db);
-		Set<String> actualToolNames = manager.getAvailableTools().keySet();
+		Set<String> actualToolNames = manager.getPredefinedTools(true).keySet();
 		Set<String> expectedToolNames = new LinkedHashSet<>();
 		CommandLineMergeTool[] defaultTools = CommandLineMergeTool.values();
 		for (CommandLineMergeTool defaultTool : defaultTools) {
@@ -151,15 +151,6 @@ public class ExternalMergeToolTest extends ExternalToolTestCase {
 	}
 
 	@Test
-	public void testNotAvailableTools() {
-		MergeTools manager = new MergeTools(db);
-		Set<String> actualToolNames = manager.getNotAvailableTools().keySet();
-		Set<String> expectedToolNames = Collections.emptySet();
-		assertEquals("Incorrect set of not available external merge tools",
-				expectedToolNames, actualToolNames);
-	}
-
-	@Test
 	public void testCompare() throws ToolException {
 		String toolName = "customTool";
 
@@ -236,7 +227,7 @@ public class ExternalMergeToolTest extends ExternalToolTestCase {
 
 		MergeTools manager = new MergeTools(db);
 		Map<String, ExternalMergeTool> availableTools = manager
-				.getAvailableTools();
+				.getPredefinedTools(true);
 		ExternalMergeTool externalMergeTool = availableTools
 				.get(overridenToolName);
 		String actualMergeToolPath = externalMergeTool.getPath();
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..6c1a780a35 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
@@ -111,17 +111,38 @@ public class DiffTools {
 	}
 
 	/**
-	 * @return the available predefined tools
+	 * @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(repo,
+						predefTool.getPath()));
+			}
+		}
 		return Collections.unmodifiableMap(predefinedTools);
 	}
 
 	/**
-	 * @return the NOT available predefined tools
+	 * @return the name of first available predefined tool or null
 	 */
-	public Map<String, ExternalDiffTool> getNotAvailableTools() {
-		return Collections.unmodifiableMap(new TreeMap<>());
+	public String getFirstAvailableTool() {
+		String name = null;
+		for (ExternalDiffTool tool : predefinedTools.values()) {
+			if (ExternalToolUtils.isToolAvailable(repo, tool.getPath())) {
+				name = tool.getName();
+				break;
+			}
+		}
+		return name;
 	}
 
 	/**
@@ -146,9 +167,12 @@ public class DiffTools {
 		if (StringUtils.isEmptyOrNull(toolName)) {
 			toolName = getDefaultToolName(gui);
 		}
-		ExternalDiffTool tool = getTool(toolName);
+		ExternalDiffTool tool = null;
+		if (!StringUtils.isEmptyOrNull(toolName)) {
+			tool = getTool(toolName);
+		}
 		if (tool == null) {
-			throw new ToolException("Unknown diff tool " + toolName); //$NON-NLS-1$
+			throw new ToolException("Unknown diff tool '" + toolName + "'"); //$NON-NLS-1$ //$NON-NLS-2$
 		}
 		return 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..e10607d2fd 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
@@ -39,9 +39,15 @@ 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);
 		}
@@ -69,13 +75,59 @@ public class ExternalToolUtils {
 			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 (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 repo
+	 *            the repository
+	 * @param path
+	 *            the tool path
+	 * @return true if tool available and false otherwise
+	 */
+	public static boolean isToolAvailable(Repository repo, String path) {
+		boolean available = true;
+		try {
+			CommandExecutor cmdExec = new CommandExecutor(repo.getFS(), false);
+			available = cmdExec.checkExecutable(path, repo.getWorkTree(),
+					prepareEnvironment(repo, null, null, null, null));
+		} catch (Exception e) {
+			available = false;
+		}
+		return available;
+	}
+
 }
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..d91d57f1a8 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
@@ -23,6 +23,7 @@ import org.eclipse.jgit.internal.diffmergetool.FileElement.Type;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.lib.internal.BooleanTriState;
 import org.eclipse.jgit.util.FS.ExecutionResult;
+import org.eclipse.jgit.util.StringUtils;
 
 /**
  * Manages merge tools.
@@ -161,17 +162,38 @@ public class MergeTools {
 	}
 
 	/**
-	 * @return the available predefined tools
+	 * @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> getAvailableTools() {
+	public Map<String, ExternalMergeTool> getPredefinedTools(
+			boolean checkAvailability) {
+		if (checkAvailability) {
+			for (ExternalMergeTool tool : predefinedTools.values()) {
+				PreDefinedMergeTool predefTool = (PreDefinedMergeTool) tool;
+				predefTool.setAvailable(ExternalToolUtils.isToolAvailable(repo,
+						predefTool.getPath()));
+			}
+		}
 		return predefinedTools;
 	}
 
 	/**
-	 * @return the NOT available predefined tools
+	 * @return the name of first available predefined tool or null
 	 */
-	public Map<String, ExternalMergeTool> getNotAvailableTools() {
-		return new TreeMap<>();
+	public String getFirstAvailableTool() {
+		String name = null;
+		for (ExternalMergeTool tool : predefinedTools.values()) {
+			if (ExternalToolUtils.isToolAvailable(repo, tool.getPath())) {
+				name = tool.getName();
+				break;
+			}
+		}
+		return name;
 	}
 
 	/**
@@ -193,12 +215,15 @@ public class MergeTools {
 
 	private ExternalMergeTool guessTool(String toolName, BooleanTriState gui)
 			throws ToolException {
-		if ((toolName == null) || toolName.isEmpty()) {
+		if (StringUtils.isEmptyOrNull(toolName)) {
 			toolName = getDefaultToolName(gui);
 		}
-		ExternalMergeTool tool = getTool(toolName);
+		ExternalMergeTool tool = null;
+		if (!StringUtils.isEmptyOrNull(toolName)) {
+			tool = getTool(toolName);
+		}
 		if (tool == null) {
-			throw new ToolException("Unknown diff tool " + toolName); //$NON-NLS-1$
+			throw new ToolException("Unknown merge tool '" + toolName + "'"); //$NON-NLS-1$ //$NON-NLS-2$
 		}
 		return 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/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}.
 	 *