Always use try/finally around DfsBlockCache.clockLock

Any RuntimeException or Error in this block will leave the lock
held by the caller thread, which can later result in deadlock or
just cache requests hanging forever because they cannot get to
the lock object.

Wrap everything in try/finally to prevent the lock from hanging,
even though a RuntimeException or Error should never happen in
any of these code paths.

Change-Id: Ibb3467f7ee4c06f617b737858b4be17b10d936e0

org.eclipse.jgit/src/org/eclipse/jgit/storage/dfs/DfsBlockCache.java

@@ -389,6 +389,7 @@ public final class DfsBlockCache {
 	@SuppressWarnings("unchecked")
 	private void reserveSpace(int reserve) {
 		clockLock.lock();
+		try {
 			long live = liveBytes + reserve;
 			if (maxBytes < live) {
 				Ref prev = clockHand;
@@ -418,8 +419,10 @@ public final class DfsBlockCache {
 				clockHand = prev;
 			}
 			liveBytes = live;
+		} finally {
 			clockLock.unlock();
 		}
+	}
 
 	private void creditSpace(int credit) {
 		clockLock.lock();
@@ -429,14 +432,17 @@ public final class DfsBlockCache {
 
 	private void addToClock(Ref ref, int credit) {
 		clockLock.lock();
+		try {
 			if (credit != 0)
 				liveBytes -= credit;
 			Ref ptr = clockHand;
 			ref.next = ptr.next;
 			ptr.next = ref;
 			clockHand = ref;
+		} finally {
 			clockLock.unlock();
 		}
+	}
 
 	void put(DfsBlock v) {
 		put(v.pack, v.start, v.size(), v);