[JGIT PATCH 01/13] Fix invalid "double checked locking" in InflaterCache

From: Shawn O. Pearce <spearce@spearce.org>
Date: Dec 23, 2008, 00:27 UTC
Message-ID: <1229992043-1053-2-git-send-email-spearce@spearce.org>
In-Reply-To: <1229992043-1053-1-git-send-email-spearce@spearce.org>

We move the Inflater allocation out of the synchronized block as it requires JNI code to build the libz structure. This permits other threads to get into the Inflater allocation faster.

We also fix the release implementation to validate the array size only after we enter the lock, rather than before. This prevents us from releasing an inflater due to a stale copy of the open count being in our processor cache.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
---
 .../src/org/spearce/jgit/lib/InflaterCache.java    |   31 ++++++++++---------
 1 files changed, 16 insertions(+), 15 deletions(-)

diff --git a/org.spearce.jgit/src/org/spearce/jgit/lib/InflaterCache.java b/org.spearce.jgit/src/org/spearce/jgit/lib/InflaterCache.java
index f0f9139..578911c 100644
--- a/org.spearce.jgit/src/org/spearce/jgit/lib/InflaterCache.java
+++ b/org.spearce.jgit/src/org/spearce/jgit/lib/InflaterCache.java
@@ -59,13 +59,18 @@
 	 * 
 	 * @return an available inflater. Never null.
 	 */
-	public synchronized static Inflater get() {
+	public static Inflater get() {
+		final Inflater r = getImpl();
+		return r != null ? r : new Inflater(false);
+	}
+
+	private synchronized static Inflater getImpl() {
 		if (openInflaterCount > 0) {
 			final Inflater r = inflaterCache[--openInflaterCount];
 			inflaterCache[openInflaterCount] = null;
 			return r;
 		}
-		return new Inflater(false);
+		return null;
 	}
 
 	/**
@@ -76,23 +81,19 @@ public synchronized static Inflater get() {
 	 *            does nothing.
 	 */
 	public static void release(final Inflater i) {
-		if (i == null)
-			return;
-
-		if (openInflaterCount == SZ) {
-			i.end();
-			return;
+		if (i != null) {
+			i.reset();
+			if (releaseImpl(i))
+				i.end();
 		}
-
-		i.reset();
-		releaseImpl(i);
 	}
 
-	private static synchronized void releaseImpl(final Inflater i) {
-		if (openInflaterCount == SZ)
-			i.end();
-		else
+	private static synchronized boolean releaseImpl(final Inflater i) {
+		if (openInflaterCount < SZ) {
 			inflaterCache[openInflaterCount++] = i;
+			return false;
+		}
+		return true;
 	}
 
 	private InflaterCache() {

-- 
1.6.1.rc4.301.g5497a