Optionally measure filesystem timestamp resolution asynchronously

In order to avoid blocking on the main thread during measurement
interactive applications like EGit may want to measure the filesystem
timestamp resolution asynchronously.

In order to enable measurement in the background call
FileStoreAttributeCache.setAsyncfileStoreAttrCache(true)
before the first access to cached FileStore attributes.

Bug: 548188
Change-Id: I8c9a2dbfc3f1d33441edea18b90e36b1dc0156c7
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/.settings/.api_filters

@@ -115,6 +115,12 @@
                 <message_argument value="fileAttributes(File)"/>
             </message_arguments>
         </filter>
+        <filter id="1142947843">
+            <message_arguments>
+                <message_argument value="5.1.9"/>
+                <message_argument value="setAsyncfileStoreAttrCache(boolean)"/>
+            </message_arguments>
+        </filter>
         <filter id="1142947843">
             <message_arguments>
                 <message_argument value="5.2.3"/>

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

@@ -55,7 +55,6 @@ import java.io.InputStreamReader;
 import java.io.OutputStream;
 import java.io.PrintStream;
 import java.nio.charset.Charset;
-import java.nio.file.AccessDeniedException;
 import java.nio.file.FileStore;
 import java.nio.file.Files;
 import java.nio.file.Path;
@@ -71,12 +70,17 @@ import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
 import java.util.UUID;
+import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.stream.Collectors;
 
 import org.eclipse.jgit.annotations.NonNull;
@@ -197,74 +201,143 @@ public abstract class FS {
 
 		private static final Map<FileStore, FileStoreAttributeCache> attributeCache = new ConcurrentHashMap<>();
 
-		static Duration getFsTimestampResolution(Path file) {
+		private static AtomicBoolean background = new AtomicBoolean();
+
+		private static Map<FileStore, Lock> locks = new ConcurrentHashMap<>();
+
+		private static void setBackground(boolean async) {
+			background.set(async);
+		}
+
+		private static Duration getFsTimestampResolution(Path file) {
+			Path dir = Files.isDirectory(file) ? file : file.getParent();
+			FileStore s;
 			try {
-				Path dir = Files.isDirectory(file) ? file : file.getParent();
+				if (Files.exists(dir)) {
-				if (!dir.toFile().canWrite()) {
+					s = Files.getFileStore(dir);
-					// can not determine FileStore of an unborn directory or in
+					FileStoreAttributeCache c = attributeCache.get(s);
-					// a read-only directory
+					if (c != null) {
+						return c.getFsTimestampResolution();
+					}
+					if (!Files.isWritable(dir)) {
+						// cannot measure resolution in a read-only directory
+						return FALLBACK_TIMESTAMP_RESOLUTION;
+					}
+				} else {
+					// cannot determine FileStore of an unborn directory
 					return FALLBACK_TIMESTAMP_RESOLUTION;
 				}
-				FileStore s = Files.getFileStore(dir);
+				CompletableFuture<Optional<Duration>> f = CompletableFuture
-				FileStoreAttributeCache c = attributeCache.get(s);
+						.supplyAsync(() -> {
-				if (c == null) {
+							Lock lock = locks.computeIfAbsent(s,
-					c = new FileStoreAttributeCache(s, dir);
+									l -> new ReentrantLock());
-					attributeCache.put(s, c);
+							if (!lock.tryLock()) {
-					if (LOG.isDebugEnabled()) {
+								return Optional.empty();
-						LOG.debug(c.toString());
+							}
-					}
+							Optional<Duration> resolution;
+							try {
+								// Some earlier future might have set the value
+								// and removed itself since we checked for the
+								// value above. Hence check cache again.
+								FileStoreAttributeCache c = attributeCache
+										.get(s);
+								if (c != null) {
+									return Optional
+											.of(c.getFsTimestampResolution());
+								}
+								resolution = measureFsTimestampResolution(s,
+										dir);
+								if (resolution.isPresent()) {
+									FileStoreAttributeCache cache = new FileStoreAttributeCache(
+											resolution.get());
+									attributeCache.put(s, cache);
+									if (LOG.isDebugEnabled()) {
+										LOG.debug(cache.toString());
+									}
+								}
+							} finally {
+								lock.unlock();
+								locks.remove(s);
+							}
+							return resolution;
+						});
+				// even if measuring in background wait a little - if the result
+				// arrives, it's better than returning the large fallback
+				Optional<Duration> d = f.get(background.get() ? 50 : 2000,
+						TimeUnit.MILLISECONDS);
+				if (d.isPresent()) {
+					return d.get();
 				}
-				return c.getFsTimestampResolution();
+				// return fallback until measurement is finished
-
+			} catch (IOException | InterruptedException
-			} catch (IOException | InterruptedException e) {
+					| ExecutionException e) {
-				LOG.warn(e.getMessage(), e);
+				LOG.error(e.getMessage(), e);
-				return FALLBACK_TIMESTAMP_RESOLUTION;
+			} catch (TimeoutException e) {
+				// use fallback
 			}
+			return FALLBACK_TIMESTAMP_RESOLUTION;
 		}
 
-		private Duration fsTimestampResolution;
+		private static Optional<Duration> measureFsTimestampResolution(
-
+			FileStore s, Path dir) {
-		Duration getFsTimestampResolution() {
-			return fsTimestampResolution;
-		}
-
-		private FileStoreAttributeCache(FileStore s, Path dir)
-				throws IOException, InterruptedException {
 			Path probe = dir.resolve(".probe-" + UUID.randomUUID()); //$NON-NLS-1$
-			Files.createFile(probe);
 			try {
+				Files.createFile(probe);
+				long wait = 512;
 				long start = System.nanoTime();
-				FileTime startTime = Files.getLastModifiedTime(probe);
+				FileTime t1 = Files.getLastModifiedTime(probe);
-				FileTime actTime = startTime;
+				FileTime t2 = t1;
-				long sleepTime = 512;
+				while (t2.compareTo(t1) <= 0) {
-				while (actTime.compareTo(startTime) <= 0) {
+					TimeUnit.NANOSECONDS.sleep(wait);
-					TimeUnit.NANOSECONDS.sleep(sleepTime);
+					checkTimeout(s, start);
-					if (timeout(start)) {
-						LOG.warn(MessageFormat.format(JGitText
-								.get().timeoutMeasureFsTimestampResolution,
-								s.toString()));
-						fsTimestampResolution = FALLBACK_TIMESTAMP_RESOLUTION;
-						return;
-					}
 					FileUtils.touch(probe);
-					actTime = Files.getLastModifiedTime(probe);
+					t2 = Files.getLastModifiedTime(probe);
-					// limit sleep time to max. 100ms
+					if (wait < 100_000_000L) {
-					if (sleepTime < 100_000_000L) {
+						wait = wait * 2;
-						sleepTime = sleepTime * 2;
 					}
 				}
-				fsTimestampResolution = Duration.between(startTime.toInstant(),
+				return Optional
-						actTime.toInstant());
+						.of(Duration.between(t1.toInstant(), t2.toInstant()));
-			} catch (AccessDeniedException e) {
+			} catch (IOException | TimeoutException e) {
+				LOG.error(e.getLocalizedMessage(), e);
+			} catch (InterruptedException e) {
 				LOG.error(e.getLocalizedMessage(), e);
+				Thread.currentThread().interrupt();
 			} finally {
-				Files.delete(probe);
+				deleteProbe(probe);
 			}
+			return Optional.empty();
 		}
 
-		private static boolean timeout(long start) {
+		private static void checkTimeout(FileStore s, long start)
-			return System.nanoTime() - start >= FALLBACK_TIMESTAMP_RESOLUTION
+				throws TimeoutException {
-					.toNanos();
+			if (System.nanoTime() - start >= FALLBACK_TIMESTAMP_RESOLUTION
+					.toNanos()) {
+				throw new TimeoutException(MessageFormat.format(JGitText
+						.get().timeoutMeasureFsTimestampResolution,
+						s.toString()));
+			}
+		}
+		private static void deleteProbe(Path probe) {
+			if (Files.exists(probe)) {
+				try {
+					Files.delete(probe);
+				} catch (IOException e) {
+					LOG.error(e.getLocalizedMessage(), e);
+				}
+			}
+		}
+
+		private final @NonNull Duration fsTimestampResolution;
+
+		@NonNull
+		Duration getFsTimestampResolution() {
+			return fsTimestampResolution;
+		}
+
+		private FileStoreAttributeCache(
+				@NonNull Duration fsTimestampResolution) {
+			this.fsTimestampResolution = fsTimestampResolution;
 		}
 
 		@SuppressWarnings("nls")
@@ -292,6 +365,20 @@ public abstract class FS {
 		return detect(null);
 	}
 
+	/**
+	 * Whether FileStore attribute cache entries should be determined
+	 * asynchronously
+	 *
+	 * @param asynch
+	 *            whether FileStore attribute cache entries should be determined
+	 *            asynchronously. If false access to cached attributes may block
+	 *            for some seconds for the first call per FileStore
+	 * @since 5.1.9
+	 */
+	public static void setAsyncfileStoreAttrCache(boolean asynch) {
+		FileStoreAttributeCache.setBackground(asynch);
+	}
+
 	/**
 	 * Auto-detect the appropriate file system abstraction, taking into account
 	 * the presence of a Cygwin installation on the system. Using jgit in