sshd: don't lock the known_hosts files on reading

Similar to git config file reading lock the file only when writing.
There may still be lock conflicts on writing, but those in the worst
case result in an entry not being added and thus being asked for later
again.

Because the OpenSshServerkeyDatabase and its HostKeyFiles may be (and
usually are) shared between different SSH sessions, we still need to
ensure in-process mutual exclusion.

Bug: 559548
Change-Id: I4af97628deff9eaac2520576917c856949f2680d
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

org.eclipse.jgit.ssh.apache/resources/org/eclipse/jgit/internal/transport/sshd/SshdText.properties

@@ -24,7 +24,6 @@ keyEncryptedPrompt=Passphrase
 keyEncryptedRetry=Encrypted key ''{0}'' could not be decrypted. Enter the passphrase again.
 keyLoadFailed=Could not load key ''{0}''
 knownHostsCouldNotUpdate=Could not update known hosts file {0}
-knownHostsFileLockedRead=Could not read known hosts file (locked) {0}
 knownHostsFileLockedUpdate=Could not update known hosts file (locked) {0}
 knownHostsFileReadFailed=Failed to read known hosts file {0}
 knownHostsInvalidLine=Known hosts file {0} contains invalid line {1}

org.eclipse.jgit.ssh.apache/src/org/eclipse/jgit/internal/transport/sshd/OpenSshServerKeyDatabase.java

@@ -21,6 +21,7 @@ import java.net.InetSocketAddress;
 import java.net.SocketAddress;
 import java.nio.file.Files;
 import java.nio.file.InvalidPathException;
+import java.nio.file.NoSuchFileException;
 import java.nio.file.Path;
 import java.nio.file.Paths;
 import java.security.GeneralSecurityException;
@@ -561,29 +562,17 @@ public class OpenSshServerKeyDatabase
 		@Override
 		public List<HostEntryPair> get() {
 			Path path = getPath();
-			try {
-				if (checkReloadRequired()) {
-					if (!Files.exists(path)) {
-						// Has disappeared.
-						resetReloadAttributes();
-						return Collections.emptyList();
-					}
-					LockFile lock = new LockFile(path.toFile());
-					if (lock.lock()) {
-						try {
-							entries = reload(getPath());
-						} finally {
-							lock.unlock();
-						}
-					} else {
-						LOG.warn(format(SshdText.get().knownHostsFileLockedRead,
-								path));
+			synchronized (this) {
+				try {
+					if (checkReloadRequired()) {
+						entries = reload(getPath());
 					}
+				} catch (IOException e) {
+					LOG.warn(format(SshdText.get().knownHostsFileReadFailed,
+							path));
 				}
-			} catch (IOException e) {
-				LOG.warn(format(SshdText.get().knownHostsFileReadFailed, path));
+				return Collections.unmodifiableList(entries);
 			}
-			return Collections.unmodifiableList(entries);
 		}
 
 		private List<HostEntryPair> reload(Path path) throws IOException {
@@ -616,7 +605,7 @@ public class OpenSshServerKeyDatabase
 					}
 				}
 				return newEntries;
-			} catch (FileNotFoundException e) {
+			} catch (FileNotFoundException | NoSuchFileException e) {
 				resetReloadAttributes();
 				return Collections.emptyList();
 			}

org.eclipse.jgit.ssh.apache/src/org/eclipse/jgit/internal/transport/sshd/SshdText.java

@@ -44,7 +44,6 @@ public final class SshdText extends TranslationBundle {
 	/***/ public String keyEncryptedRetry;
 	/***/ public String keyLoadFailed;
 	/***/ public String knownHostsCouldNotUpdate;
-	/***/ public String knownHostsFileLockedRead;
 	/***/ public String knownHostsFileLockedUpdate;
 	/***/ public String knownHostsFileReadFailed;
 	/***/ public String knownHostsInvalidLine;