Persist filesystem timestamp resolution and allow manual configuration

To enable persisting filesystem timestamp resolution per FileStore add a
new config section to the user global git configuration:

- Config section is "filesystem"
- Config subsection is concatenation of
  - Java vendor (system property "java.vm.vendor")
  - runtime version (system property "java.vm.version")
  - FileStore's name
  - separated by '|'
  e.g.
    "AdoptOpenJDK|1.8.0_212-b03|/dev/disk1s1"
  The prefix is needed since some Java versions do not expose the full
  timestamp resolution of the underlying filesystem. This may also
  depend on the underlying operating system hence concrete key values
  may not be portable.
- Config key for timestamp resolution is "timestampResolution" as a time
  value, supported time units are those supported by
  DefaultTypedConfigGetter#getTimeUnit

If timestamp resolution is already configured for a given FileStore
the configured value is used instead of measuring the resolution.

When timestamp resolution was measured it is persisted in the user
global git configuration.

Example:

[filesystem "AdoptOpenJDK|1.8.0_212-b03|/dev/disk1s1"]
	timestampResolution = 1 seconds

If locking the git config file fails retry saving the resolution up to 5
times in order to workaround races with another thread.

In order to avoid stack overflow use the fallback filesystem timestamp
resolution when loading FileBasedConfig which creates itself a
FileSnapshot to help checking if the config changed.

Note:
- on some OSes Java 8,9 truncate to milliseconds or seconds, see
https://bugs.openjdk.java.net/browse/JDK-8177809, fixed in Java 10
- UnixFileAttributes up to Java 12 truncates timestamp resolution to
microseconds when converting the internal representation to FileTime
exposed in the API, see https://bugs.openjdk.java.net/browse/JDK-8181493
- WindowsFileAttributes also provides only microsecond resolution up to
Java 12

Hence do not attempt to manually configure a higher timestamp resolution
than supported by the Java version being used at runtime.

Bug: 546891
Bug: 548188
Change-Id: Iff91b8f9e6e5e2295e1463f87c8e95edf4abbcf8
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/.settings/.api_filters

@@ -22,6 +22,28 @@
             </message_arguments>
         </filter>
     </resource>
+    <resource path="src/org/eclipse/jgit/lib/ConfigConstants.java" type="org.eclipse.jgit.lib.ConfigConstants">
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="5.1.9"/>
+                <message_argument value="CONFIG_FILESYSTEM_SECTION"/>
+            </message_arguments>
+        </filter>
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="5.1.9"/>
+                <message_argument value="CONFIG_KEY_TIMESTAMP_RESOLUTION"/>
+            </message_arguments>
+        </filter>
+    </resource>
+    <resource path="src/org/eclipse/jgit/lib/Constants.java" type="org.eclipse.jgit.lib.Constants">
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="5.1.9"/>
+                <message_argument value="FALLBACK_TIMESTAMP_RESOLUTION"/>
+            </message_arguments>
+        </filter>
+    </resource>
     <resource path="src/org/eclipse/jgit/lib/GitmoduleEntry.java" type="org.eclipse.jgit.lib.GitmoduleEntry">
         <filter id="1109393411">
             <message_arguments>

org.eclipse.jgit/resources/org/eclipse/jgit/internal/JGitText.properties

@@ -104,6 +104,7 @@ cannotReadObjectsPath=Cannot read {0}/{1}: {2}
 cannotReadTree=Cannot read tree {0}
 cannotRebaseWithoutCurrentHead=Can not rebase without a current HEAD
 cannotResolveLocalTrackingRefForUpdating=Cannot resolve local tracking ref {0} for updating.
+cannotSaveConfig=Cannot save config file ''{0}''
 cannotSquashFixupWithoutPreviousCommit=Cannot {0} without previous commit.
 cannotStoreObjects=cannot store objects
 cannotResolveUniquelyAbbrevObjectId=Could not resolve uniquely the abbreviated object ID
@@ -557,6 +558,7 @@ pushIsNotSupportedForBundleTransport=Push is not supported for bundle transport
 pushNotPermitted=push not permitted
 pushOptionsNotSupported=Push options not supported; received {0}
 rawLogMessageDoesNotParseAsLogEntry=Raw log message does not parse as log entry
+readConfigFailed=Reading config file ''{0}'' failed
 readerIsRequired=Reader is required
 readingObjectsFromLocalRepositoryFailed=reading objects from local repository failed: {0}
 readTimedOut=Read timed out after {0} ms

org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java

@@ -165,6 +165,7 @@ public static JGitText get() {
 	/***/ public String cannotReadTree;
 	/***/ public String cannotRebaseWithoutCurrentHead;
 	/***/ public String cannotResolveLocalTrackingRefForUpdating;
+	/***/ public String cannotSaveConfig;
 	/***/ public String cannotSquashFixupWithoutPreviousCommit;
 	/***/ public String cannotStoreObjects;
 	/***/ public String cannotResolveUniquelyAbbrevObjectId;
@@ -618,6 +619,7 @@ public static JGitText get() {
 	/***/ public String pushNotPermitted;
 	/***/ public String pushOptionsNotSupported;
 	/***/ public String rawLogMessageDoesNotParseAsLogEntry;
+	/***/ public String readConfigFailed;
 	/***/ public String readerIsRequired;
 	/***/ public String readingObjectsFromLocalRepositoryFailed;
 	/***/ public String readTimedOut;

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileSnapshot.java

@@ -43,6 +43,7 @@
 
 package org.eclipse.jgit.internal.storage.file;
 
+import static org.eclipse.jgit.lib.Constants.FALLBACK_TIMESTAMP_RESOLUTION;
 import java.io.File;
 import java.io.IOException;
 import java.nio.file.attribute.BasicFileAttributes;
@@ -122,6 +123,22 @@ public static FileSnapshot save(File path) {
 		return new FileSnapshot(path);
 	}
 
+	/**
+	 * Record a snapshot for a specific file path without using config file to
+	 * get filesystem timestamp resolution.
+	 * <p>
+	 * This method should be invoked before the file is accessed. It is used by
+	 * FileBasedConfig to avoid endless recursion.
+	 *
+	 * @param path
+	 *            the path to later remember. The path's current status
+	 *            information is saved.
+	 * @return the snapshot.
+	 */
+	public static FileSnapshot saveNoConfig(File path) {
+		return new FileSnapshot(path);
+	}
+
 	private static Object getFileKey(BasicFileAttributes fileAttributes) {
 		Object fileKey = fileAttributes.fileKey();
 		return fileKey == null ? MISSING_FILEKEY : fileKey;
@@ -177,13 +194,30 @@ public static FileSnapshot save(long modified) {
 	 * This method should be invoked before the file is accessed.
 	 *
 	 * @param path
-	 *            the path to later remember. The path's current status
+	 *            the path to remember meta data for. The path's current status
 	 *            information is saved.
 	 */
 	protected FileSnapshot(File path) {
+		this(path, true);
+	}
+
+	/**
+	 * Record a snapshot for a specific file path.
+	 * <p>
+	 * This method should be invoked before the file is accessed.
+	 *
+	 * @param path
+	 *            the path to remember meta data for. The path's current status
+	 *            information is saved.
+	 * @param useConfig
+	 *            if {@code true} read filesystem time resolution from
+	 *            configuration file otherwise use fallback resolution
+	 */
+	protected FileSnapshot(File path, boolean useConfig) {
 		this.lastRead = System.currentTimeMillis();
-		this.fsTimestampResolution = FS
-				.getFsTimerResolution(path.toPath().getParent());
+		this.fsTimestampResolution = useConfig
+				? FS.getFsTimerResolution(path.toPath().getParent())
+				: FALLBACK_TIMESTAMP_RESOLUTION;
 		BasicFileAttributes fileAttributes = null;
 		try {
 			fileAttributes = FS.DETECTED.fileAttributes(path);

org.eclipse.jgit/src/org/eclipse/jgit/lib/ConfigConstants.java

@@ -432,4 +432,16 @@ public final class ConfigConstants {
 	 * @since 4.11
 	 */
 	public static final String CONFIG_SECTION_LFS = "lfs";
+
+	/**
+	 * The "filesystem" section
+	 * @since 5.1.9
+	 */
+	public static final String CONFIG_FILESYSTEM_SECTION = "filesystem";
+
+	/**
+	 * The "timestampResolution" key
+	 * @since 5.1.9
+	 */
+	public static final String CONFIG_KEY_TIMESTAMP_RESOLUTION = "timestampResolution";
 }

org.eclipse.jgit/src/org/eclipse/jgit/lib/Constants.java

@@ -52,6 +52,7 @@
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 import java.text.MessageFormat;
+import java.time.Duration;
 
 import org.eclipse.jgit.errors.CorruptObjectException;
 import org.eclipse.jgit.internal.JGitText;
@@ -722,6 +723,16 @@ public static byte[] encode(String str) {
 	 */
 	public static final String LOCK_SUFFIX = ".lock"; //$NON-NLS-1$
 
+	/**
+	 * Fallback filesystem timestamp resolution used when we can't measure the
+	 * resolution. The last modified time granularity of FAT filesystems is 2
+	 * seconds.
+	 *
+	 * @since 5.1.9
+	 */
+	public static final Duration FALLBACK_TIMESTAMP_RESOLUTION = Duration
+			.ofMillis(2000);
+
 	private Constants() {
 		// Hide the default constructor
 	}

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/FileBasedConfig.java

@@ -153,7 +153,9 @@ public void load() throws IOException, ConfigInvalidException {
 		int retries = 0;
 		while (true) {
 			final FileSnapshot oldSnapshot = snapshot;
-			final FileSnapshot newSnapshot = FileSnapshot.save(getFile());
+			// don't use config in this snapshot to avoid endless recursion
+			final FileSnapshot newSnapshot = FileSnapshot
+					.saveNoConfig(getFile());
 			try {
 				final byte[] in = IO.readFully(getFile());
 				final ObjectId newHash = hash(in);

org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java

@@ -44,6 +44,7 @@
 package org.eclipse.jgit.util;
 
 import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.eclipse.jgit.lib.Constants.FALLBACK_TIMESTAMP_RESOLUTION;
 
 import java.io.BufferedReader;
 import java.io.ByteArrayInputStream;
@@ -87,9 +88,13 @@
 import org.eclipse.jgit.annotations.Nullable;
 import org.eclipse.jgit.api.errors.JGitInternalException;
 import org.eclipse.jgit.errors.CommandFailedException;
+import org.eclipse.jgit.errors.ConfigInvalidException;
+import org.eclipse.jgit.errors.LockFailedException;
 import org.eclipse.jgit.internal.JGitText;
+import org.eclipse.jgit.lib.ConfigConstants;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.storage.file.FileBasedConfig;
 import org.eclipse.jgit.treewalk.FileTreeIterator.FileEntry;
 import org.eclipse.jgit.treewalk.FileTreeIterator.FileModeStrategy;
 import org.eclipse.jgit.treewalk.WorkingTreeIterator.Entry;
@@ -193,11 +198,9 @@ public int getRc() {
 	}
 
 	private static final class FileStoreAttributeCache {
-		/**
-		 * The last modified time granularity of FAT filesystems is 2 seconds.
-		 */
-		private static final Duration FALLBACK_TIMESTAMP_RESOLUTION = Duration
-				.ofMillis(2000);
+
+		private static final Duration UNDEFINED_RESOLUTION = Duration
+				.ofNanos(Long.MAX_VALUE);
 
 		private static final Map<FileStore, FileStoreAttributeCache> attributeCache = new ConcurrentHashMap<>();
 
@@ -209,6 +212,10 @@ private static void setBackground(boolean async) {
 			background.set(async);
 		}
 
+		private static final String javaVersionPrefix = System
+				.getProperty("java.vm.vendor") + '|' //$NON-NLS-1$
+				+ System.getProperty("java.vm.version") + '|'; //$NON-NLS-1$
+
 		private static Duration getFsTimestampResolution(Path file) {
 			Path dir = Files.isDirectory(file) ? file : file.getParent();
 			FileStore s;
@@ -280,6 +287,10 @@ private static Duration getFsTimestampResolution(Path file) {
 
 		private static Optional<Duration> measureFsTimestampResolution(
 			FileStore s, Path dir) {
+			Duration configured = readFileTimeResolution(s);
+			if (!UNDEFINED_RESOLUTION.equals(configured)) {
+				return Optional.of(configured);
+			}
 			Path probe = dir.resolve(".probe-" + UUID.randomUUID()); //$NON-NLS-1$
 			try {
 				Files.createFile(probe);
@@ -296,8 +307,9 @@ private static Optional<Duration> measureFsTimestampResolution(
 						wait = wait * 2;
 					}
 				}
-				return Optional
-						.of(Duration.between(t1.toInstant(), t2.toInstant()));
+				Duration resolution = Duration.between(t1.toInstant(), t2.toInstant());
+				saveFileTimeResolution(s, resolution);
+				return Optional.of(resolution);
 			} catch (IOException | TimeoutException e) {
 				LOG.error(e.getLocalizedMessage(), e);
 			} catch (InterruptedException e) {
@@ -328,6 +340,81 @@ private static void deleteProbe(Path probe) {
 			}
 		}
 
+		private static Duration readFileTimeResolution(FileStore s) {
+			FileBasedConfig userConfig = SystemReader.getInstance()
+					.openUserConfig(null, FS.DETECTED);
+			try {
+				userConfig.load();
+			} catch (IOException e) {
+				LOG.error(MessageFormat.format(JGitText.get().readConfigFailed,
+						userConfig.getFile().getAbsolutePath()), e);
+			} catch (ConfigInvalidException e) {
+				LOG.error(MessageFormat.format(
+						JGitText.get().repositoryConfigFileInvalid,
+						userConfig.getFile().getAbsolutePath(),
+						e.getMessage()));
+			}
+			Duration configured = Duration
+					.ofNanos(userConfig.getTimeUnit(
+							ConfigConstants.CONFIG_FILESYSTEM_SECTION,
+							javaVersionPrefix + s.name(),
+							ConfigConstants.CONFIG_KEY_TIMESTAMP_RESOLUTION,
+							UNDEFINED_RESOLUTION.toNanos(),
+							TimeUnit.NANOSECONDS));
+			return configured;
+		}
+
+		private static void saveFileTimeResolution(FileStore s,
+				Duration resolution) {
+			FileBasedConfig userConfig = SystemReader.getInstance()
+					.openUserConfig(null, FS.DETECTED);
+			long nanos = resolution.toNanos();
+			TimeUnit unit;
+			if (nanos < 200_000L) {
+				unit = TimeUnit.NANOSECONDS;
+			} else if (nanos < 200_000_000L) {
+				unit = TimeUnit.MICROSECONDS;
+			} else {
+				unit = TimeUnit.MILLISECONDS;
+			}
+
+			final int max_retries = 5;
+			int retries = 0;
+			boolean succeeded = false;
+			long value = unit.convert(nanos, TimeUnit.NANOSECONDS);
+			while (!succeeded && retries < max_retries) {
+				try {
+					userConfig.load();
+					userConfig.setString(
+							ConfigConstants.CONFIG_FILESYSTEM_SECTION,
+							javaVersionPrefix + s.name(),
+							ConfigConstants.CONFIG_KEY_TIMESTAMP_RESOLUTION,
+							String.format("%d %s", //$NON-NLS-1$
+									Long.valueOf(value),
+									unit.name().toLowerCase()));
+					userConfig.save();
+					succeeded = true;
+				} catch (LockFailedException e) {
+					// race with another thread, wait a bit and try again
+					try {
+						retries++;
+						Thread.sleep(20);
+					} catch (InterruptedException e1) {
+						Thread.interrupted();
+					}
+				} catch (IOException e) {
+					LOG.error(MessageFormat.format(
+							JGitText.get().cannotSaveConfig,
+							userConfig.getFile().getAbsolutePath()), e);
+				} catch (ConfigInvalidException e) {
+					LOG.error(MessageFormat.format(
+							JGitText.get().repositoryConfigFileInvalid,
+							userConfig.getFile().getAbsolutePath(),
+							e.getMessage()));
+				}
+			}
+		}
+
 		private final @NonNull Duration fsTimestampResolution;
 
 		@NonNull