srm: Fix race in user persistence

Motivation:

A lazily allocated lock may get garbage collected even while somebody
is holding the lock.

Modification:

Keep a hard reference to the lock while holding it.

Result:

Race is gone.

Target: trunk
Require-notes: no
Require-book: no
Request: 2.14
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/8730/

modules/dcache-srm/src/main/java/diskCacheV111/srm/dcache/CanonicalizingByteArrayStore.java

@@ -154,11 +154,12 @@ public CanonicalizingByteArrayStore(
     @Nullable
     public Token toToken(long id)
     {
-        locks.get(id).lock();
+        Lock lock = locks.get(id);
+        lock.lock();
         try {
             return (load(id) != null) ? makeToken(id) : null;
         } finally {
-            locks.get(id).unlock();
+            lock.unlock();
         }
     }
 
@@ -181,7 +182,8 @@ public Token toToken(byte[] bytes)
         do {
             HashCode hash = Hashing.sipHash24(K0, k1++).hashBytes(bytes);
             long id = hash.asLong();
-            locks.get(id).lock();
+            Lock lock = locks.get(id);
+            lock.lock();
             try {
                 byte[] canonical = load(id);
                 if (canonical == null) {
@@ -191,7 +193,7 @@ public Token toToken(byte[] bytes)
                     token = makeToken(id);
                 }
             } finally {
-                locks.get(id).unlock();
+                lock.unlock();
             }
         } while (token == null);
         return token;
@@ -207,15 +209,16 @@ public Token toToken(byte[] bytes)
     public byte[] readBytes(Token token)
     {
         long id = token.getId();
-        locks.get(id).lock();
+        Lock lock = locks.get(id);
+        lock.lock();
         try {
             byte[] bytes = load(id);
             if (bytes == null) {
                 throw new IncorrectResultSizeDataAccessException(1, 0);
             }
             return bytes;
         } finally {
-            locks.get(id).unlock();
+            lock.unlock();
         }
     }
 
@@ -262,14 +265,15 @@ public void gc(List<Long> ids)
         canonicalizationCache.cleanUp();
 
         for (Long id : ids) {
-            locks.get(id).lock();
+            Lock lock = locks.get(id);
+            lock.lock();
             try {
                 if (canonicalizationCache.getIfPresent(id) == null) {
                     cache.invalidate(id);
                     delete.accept(id);
                 }
             } finally {
-                locks.get(id).unlock();
+                lock.unlock();
             }
         }
     }