Log if Repository.useCnt becomes negative

We observe in Gerrit 2.12 that useCnt can become negative in rare cases.
Log this to help finding the bug.

Change-Id: Ie91c7f9d190a5d7cf4733d4bf84124d119ca20f7
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

@@ -185,6 +185,7 @@ corruptObjectTruncatedInMode=truncated in mode
 corruptObjectTruncatedInName=truncated in name
 corruptObjectTruncatedInObjectId=truncated in object id
 corruptObjectZeroId=entry points to null SHA-1
+corruptUseCnt=close() called when useCnt is already zero
 couldNotCheckOutBecauseOfConflicts=Could not check out because of conflicts
 couldNotDeleteLockFileShouldNotHappen=Could not delete lock file. Should not happen
 couldNotDeleteTemporaryIndexFileShouldNotHappen=Could not delete temporary index file. Should not happen

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

@@ -245,6 +245,7 @@ public class JGitText extends TranslationBundle {
 	/***/ public String corruptObjectTruncatedInObjectId;
 	/***/ public String corruptObjectZeroId;
 	/***/ public String corruptPack;
+	/***/ public String corruptUseCnt;
 	/***/ public String couldNotCheckOutBecauseOfConflicts;
 	/***/ public String couldNotDeleteLockFileShouldNotHappen;
 	/***/ public String couldNotDeleteTemporaryIndexFileShouldNotHappen;

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

@@ -94,6 +94,8 @@ import org.eclipse.jgit.util.IO;
 import org.eclipse.jgit.util.RawParseUtils;
 import org.eclipse.jgit.util.SystemReader;
 import org.eclipse.jgit.util.io.SafeBufferedOutputStream;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /**
  * Represents a Git repository.
@@ -104,6 +106,8 @@ import org.eclipse.jgit.util.io.SafeBufferedOutputStream;
  * This class is thread-safe.
  */
 public abstract class Repository implements AutoCloseable {
+	private static Logger LOG = LoggerFactory.getLogger(Repository.class);
+
 	private static final ListenerList globalListeners = new ListenerList();
 
 	/** @return the global listener list observing all events in this JVM. */
@@ -866,12 +870,24 @@ public abstract class Repository implements AutoCloseable {
 
 	/** Decrement the use count, and maybe close resources. */
 	public void close() {
-		if (useCnt.decrementAndGet() == 0) {
+		int newCount = useCnt.decrementAndGet();
+		if (newCount == 0) {
 			if (RepositoryCache.isCached(this)) {
 				closedAt.set(System.currentTimeMillis());
 			} else {
 				doClose();
 			}
+		} else if (newCount == -1) {
+			// should not happen, only log when useCnt became negative to
+			// minimize number of log entries
+			LOG.warn(JGitText.get().corruptUseCnt);
+			if (LOG.isDebugEnabled()) {
+				IllegalStateException e = new IllegalStateException();
+				LOG.debug("", e); //$NON-NLS-1$
+			}
+			if (RepositoryCache.isCached(this)) {
+				closedAt.set(System.currentTimeMillis());
+			}
 		}
 	}