Add debug trace for FileSnapshot

Checking lastModified is time critical hence debug trace is the only way
to analyze issues since debugging is impractical.

Also add configuration for buffering of log4j output to reduce runtime
impact when debug trace is on. Limit buffer to 1MiB and comment this
configuration out since we may not always want to use buffering.

Change-Id: Ib1a0537b67c8dc3fac994a77b42badd974ce6c97
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.test/tst-rsrc/log4j.properties

@@ -7,3 +7,5 @@ log4j.appender.stdout=org.apache.log4j.ConsoleAppender
 log4j.appender.stdout.Target=System.out
 log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
 log4j.appender.stdout.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n
+#log4j.appender.fileLogger.bufferedIO = true
+#log4j.appender.fileLogger.bufferSize = 1024

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

@@ -44,6 +44,7 @@
 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;
@@ -57,6 +58,8 @@ import java.util.concurrent.TimeUnit;
 
 import org.eclipse.jgit.annotations.NonNull;
 import org.eclipse.jgit.util.FS;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Caches when a file was last read, making it possible to detect future edits.
@@ -75,6 +78,8 @@ import org.eclipse.jgit.util.FS;
  * file is less than 3 seconds ago.
  */
 public class FileSnapshot {
+	private static final Logger LOG = LoggerFactory
+			.getLogger(FileSnapshot.class);
 	/**
 	 * An unknown file size.
 	 *
@@ -188,17 +193,19 @@ public class FileSnapshot {
 	 */
 	private final Object fileKey;
 
+	private final File file;
+
 	/**
 	 * Record a snapshot for a specific file path.
 	 * <p>
 	 * This method should be invoked before the file is accessed.
 	 *
-	 * @param path
+	 * @param file
 	 *            the path to remember meta data for. The path's current status
 	 *            information is saved.
 	 */
-	protected FileSnapshot(File path) {
-		this(path, true);
+	protected FileSnapshot(File file) {
+		this(file, true);
 	}
 
 	/**
@@ -206,30 +213,35 @@ public class FileSnapshot {
 	 * <p>
 	 * This method should be invoked before the file is accessed.
 	 *
-	 * @param path
+	 * @param file
 	 *            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) {
+	protected FileSnapshot(File file, boolean useConfig) {
+		this.file = file;
 		this.lastRead = System.currentTimeMillis();
 		this.fsTimestampResolution = useConfig
-				? FS.getFsTimerResolution(path.toPath().getParent())
+				? FS.getFsTimerResolution(file.toPath().getParent())
 				: FALLBACK_TIMESTAMP_RESOLUTION;
 		BasicFileAttributes fileAttributes = null;
 		try {
-			fileAttributes = FS.DETECTED.fileAttributes(path);
+			fileAttributes = FS.DETECTED.fileAttributes(file);
 		} catch (IOException e) {
-			this.lastModified = path.lastModified();
-			this.size = path.length();
+			this.lastModified = file.lastModified();
+			this.size = file.length();
 			this.fileKey = MISSING_FILEKEY;
 			return;
 		}
 		this.lastModified = fileAttributes.lastModifiedTime().toMillis();
 		this.size = fileAttributes.size();
 		this.fileKey = getFileKey(fileAttributes);
+		LOG.debug("file={}, create new FileSnapshot: lastRead={} ms" //$NON-NLS-1$
+				+ ", lastModified={} ms, size={}, fileKey={}", //$NON-NLS-1$
+				file, Long.valueOf(lastRead), Long.valueOf(lastModified),
+				Long.valueOf(size), fileKey);
 	}
 
 	private boolean sizeChanged;
@@ -242,6 +254,7 @@ public class FileSnapshot {
 
 	private FileSnapshot(long read, long modified, long size,
 			@NonNull Duration fsTimestampResolution, @NonNull Object fileKey) {
+		this.file = null;
 		this.lastRead = read;
 		this.lastModified = modified;
 		this.fsTimestampResolution = fsTimestampResolution;
@@ -432,7 +445,14 @@ public class FileSnapshot {
 	private boolean isRacyClean(long read) {
 		// add a 10% safety margin
 		long racyNanos = (fsTimestampResolution.toNanos() + 1) * 11 / 10;
-		return wasRacyClean = (read - lastModified) * 1_000_000 <= racyNanos;
+		long delta = (read - lastModified) * 1_000_000;
+		wasRacyClean = delta <= racyNanos;
+		LOG.debug("file={}, isRacyClean={}, read={} ms, lastModified={} ms," //$NON-NLS-1$
+				+ " delta={} ns, racy<={} ns", //$NON-NLS-1$
+				file, Boolean.valueOf(wasRacyClean), Long.valueOf(read),
+				Long.valueOf(lastModified), Long.valueOf(delta),
+				Long.valueOf(racyNanos));
+		return wasRacyClean;
 	}
 
 	private boolean isModified(long currLastModified) {
@@ -440,6 +460,9 @@ public class FileSnapshot {
 
 		lastModifiedChanged = lastModified != currLastModified;
 		if (lastModifiedChanged) {
+			LOG.debug("file={}, lastModified changed from {} to {}", //$NON-NLS-1$
+					file, Long.valueOf(lastModified),
+					Long.valueOf(currLastModified));
 			return true;
 		}
 
@@ -447,26 +470,40 @@ public class FileSnapshot {
 		// after the last modification that any new modifications
 		// are certain to change the last modified time.
 		if (cannotBeRacilyClean) {
+			LOG.debug("file={}, cannot be racily clean", file); //$NON-NLS-1$
 			return false;
 		}
 		if (!isRacyClean(lastRead)) {
 			// Our last read should have marked cannotBeRacilyClean,
 			// but this thread may not have seen the change. The read
 			// of the volatile field lastRead should have fixed that.
+			LOG.debug("file={}, is unmodified", file); //$NON-NLS-1$
 			return false;
 		}
 
 		// We last read this path too close to its last observed
 		// modification time. We may have missed a modification.
 		// Scan again, to ensure we still see the same state.
+		LOG.debug("file={}, is racily clean", file); //$NON-NLS-1$
 		return true;
 	}
 
 	private boolean isFileKeyChanged(Object currFileKey) {
-		return currFileKey != MISSING_FILEKEY && !currFileKey.equals(fileKey);
+		boolean changed = currFileKey != MISSING_FILEKEY
+				&& !currFileKey.equals(fileKey);
+		if (changed) {
+			LOG.debug("file={}, FileKey changed from {} to {}", //$NON-NLS-1$
+					file, fileKey, currFileKey);
+		}
+		return changed;
 	}
 
 	private boolean isSizeChanged(long currSize) {
-		return currSize != UNKNOWN_SIZE && currSize != size;
+		boolean changed = (currSize != UNKNOWN_SIZE) && (currSize != size);
+		if (changed) {
+			LOG.debug("file={}, size changed from {} to {} bytes", //$NON-NLS-1$
+					file, Long.valueOf(size), Long.valueOf(currSize));
+		}
+		return changed;
 	}
 }