Fix eviction of repositories with negative usage count

If the repository close method was called twice (or more) for one open,
the usage count became negative and the repository was never be evicted
from the cache because the method checking if repository is expired was
not considering negative usage count.

Change-Id: I18a80c415c54c37d1b9def2b311ff2d0afa455ca
Signed-off-by: Hugo Arès <hugo.ares@ericsson.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/RepositoryCacheTest.java

@@ -209,6 +209,27 @@ public class RepositoryCacheTest extends RepositoryTestCase {
 		assertTrue(RepositoryCache.isCached(d2));
 	}
 
+	@Test
+	public void testRepositoryUnregisteringWhenExpiredAndUsageCountNegative()
+			throws Exception {
+		Repository repoA = createBareRepository();
+		RepositoryCache.register(repoA);
+
+		assertEquals(1, RepositoryCache.getRegisteredKeys().size());
+		assertTrue(RepositoryCache.isCached(repoA));
+
+		// close the repo twice to make usage count negative
+		repoA.close();
+		repoA.close();
+		// fake that repoA was closed more than 1 hour ago (default expiration
+		// time)
+		repoA.closedAt.set(System.currentTimeMillis() - 65 * 60 * 1000);
+
+		RepositoryCache.clearExpired();
+
+		assertEquals(0, RepositoryCache.getRegisteredKeys().size());
+	}
+
 	@Test
 	public void testRepositoryUnregisteringWhenExpired() throws Exception {
 		Repository repoA = createBareRepository();

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

@@ -298,7 +298,7 @@ public class RepositoryCache {
 	}
 
 	private boolean isExpired(Repository db) {
-		return db != null && db.useCnt.get() == 0
+		return db != null && db.useCnt.get() <= 0
 			&& (System.currentTimeMillis() - db.closedAt.get() > expireAfter);
 	}