Fix race conditions that are possible in Db.

This is accomplished by locking the entire database, and keeping it locked
until the last reference is unlocked. We get rid of per-reference locks,
because those are what cause race conditions.

src/Db.c

@@ -75,7 +75,6 @@ struct Db
 struct DbRef
 {
     HashMap *json;
-    pthread_mutex_t lock;
 
     unsigned long ts;
     size_t size;
@@ -290,16 +289,7 @@ DbCacheEvict(Db * db)
     {
         char *hash;
 
-        if (pthread_mutex_trylock(&ref->lock) != 0)
-        {
-            /* This ref is locked by another thread, don't evict it. */
-            ref = ref->next;
-            continue;
-        }
-
         JsonFree(ref->json);
-        pthread_mutex_unlock(&ref->lock);
-        pthread_mutex_destroy(&ref->lock);
 
         hash = DbHashKey(ref->name);
         HashMapDelete(db->cache, hash);
@@ -338,6 +328,7 @@ Db *
 DbOpen(char *dir, size_t cache)
 {
     Db *db;
+    pthread_mutexattr_t attr;
 
     if (!dir)
     {
@@ -353,7 +344,10 @@ DbOpen(char *dir, size_t cache)
     db->dir = dir;
     db->maxCache = cache;
 
-    pthread_mutex_init(&db->lock, NULL);
+    pthread_mutexattr_init(&attr);
+    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
+    pthread_mutex_init(&db->lock, &attr);
+    pthread_mutexattr_destroy(&attr);
 
     db->mostRecent = NULL;
     db->leastRecent = NULL;
@@ -383,6 +377,8 @@ DbMaxCacheSet(Db * db, size_t cache)
         return;
     }
 
+    pthread_mutex_lock(&db->lock);
+
     db->maxCache = cache;
     if (db->maxCache && !db->cache)
     {
@@ -391,6 +387,8 @@ DbMaxCacheSet(Db * db, size_t cache)
     }
 
     DbCacheEvict(db);
+
+    pthread_mutex_unlock(&db->lock);
 }
 
 void
@@ -401,10 +399,13 @@ DbClose(Db * db)
         return;
     }
 
+    pthread_mutex_lock(&db->lock);
+
     DbMaxCacheSet(db, 0);
     DbCacheEvict(db);
     HashMapFree(db->cache);
 
+    pthread_mutex_unlock(&db->lock);
     pthread_mutex_destroy(&db->lock);
 
     Free(db);
@@ -441,8 +442,6 @@ DbLockFromArr(Db * db, Array * args)
     {
         if (ref)
         {
-            pthread_mutex_lock(&ref->lock);
-
             HashMapDelete(db->cache, hash);
             JsonFree(ref->json);
             StringArrayFree(ref->name);
@@ -472,8 +471,6 @@ DbLockFromArr(Db * db, Array * args)
                 db->leastRecent = db->mostRecent;
             }
 
-            pthread_mutex_unlock(&ref->lock);
-            pthread_mutex_destroy(&ref->lock);
             Free(ref);
         }
         ref = NULL;
@@ -499,8 +496,6 @@ DbLockFromArr(Db * db, Array * args)
     {
         unsigned long diskTs = UtilLastModified(file);
 
-        pthread_mutex_lock(&ref->lock);
-
         ref->fd = fd;
         ref->stream = stream;
 
@@ -511,7 +506,6 @@ DbLockFromArr(Db * db, Array * args)
 
             if (!json)
             {
-                pthread_mutex_unlock(&ref->lock);
                 StreamClose(ref->stream);
                 ref = NULL;
                 goto finish;
@@ -585,9 +579,6 @@ DbLockFromArr(Db * db, Array * args)
         ref->fd = fd;
         ref->stream = stream;
 
-        pthread_mutex_init(&ref->lock, NULL);
-        pthread_mutex_lock(&ref->lock);
-
         name = ArrayCreate();
         for (i = 0; i < ArraySize(args); i++)
         {
@@ -622,7 +613,10 @@ DbLockFromArr(Db * db, Array * args)
     }
 
 finish:
+    if (!ref)
+    {
         pthread_mutex_unlock(&db->lock);
+    }
 
     Free(file);
     Free(hash);
@@ -654,12 +648,15 @@ DbCreate(Db * db, size_t nArgs,...)
         return NULL;
     }
 
+    pthread_mutex_lock(&db->lock);
+
     file = DbFileName(db, args);
 
     if (UtilLastModified(file))
     {
         Free(file);
         ArrayFree(args);
+        pthread_mutex_unlock(&db->lock);
         return NULL;
     }
 
@@ -669,6 +666,7 @@ DbCreate(Db * db, size_t nArgs,...)
         Free(file);
         ArrayFree(args);
         Free(dir);
+        pthread_mutex_unlock(&db->lock);
         return NULL;
     }
 
@@ -679,12 +677,16 @@ DbCreate(Db * db, size_t nArgs,...)
     if (!fp)
     {
         ArrayFree(args);
+        pthread_mutex_unlock(&db->lock);
         return NULL;
     }
 
     StreamPuts(fp, "{}");
     StreamClose(fp);
 
+    /* DbLockFromArr() will lock again for us */
+    pthread_mutex_unlock(&db->lock);
+
     ret = DbLockFromArr(db, args);
 
     ArrayFree(args);
@@ -719,8 +721,6 @@ DbDelete(Db * db, size_t nArgs,...)
     ref = HashMapGet(db->cache, hash);
     if (ref)
     {
-        pthread_mutex_lock(&ref->lock);
-
         HashMapDelete(db->cache, hash);
         JsonFree(ref->json);
         StringArrayFree(ref->name);
@@ -750,8 +750,6 @@ DbDelete(Db * db, size_t nArgs,...)
             db->leastRecent = db->mostRecent;
         }
 
-        pthread_mutex_unlock(&ref->lock);
-        pthread_mutex_destroy(&ref->lock);
         Free(ref);
     }
 
@@ -802,8 +800,6 @@ DbUnlock(Db * db, DbRef * ref)
         return 0;
     }
 
-    pthread_mutex_lock(&db->lock);
-
     lseek(ref->fd, 0L, SEEK_SET);
     if (ftruncate(ref->fd, 0) < 0)
     {
@@ -841,15 +837,11 @@ DbUnlock(Db * db, DbRef * ref)
         destroy = 1;
     }
 
-    pthread_mutex_unlock(&ref->lock);
-
     if (destroy)
     {
         JsonFree(ref->json);
         StringArrayFree(ref->name);
 
-        pthread_mutex_destroy(&ref->lock);
-
         Free(ref);
     }
 
@@ -874,11 +866,6 @@ DbExists(Db * db, size_t nArgs,...)
         return 0;
     }
 
-    /*
-     * Though it's not explicitly required, we lock the
-     * database before checking that an object exists to
-     * prevent any potential race conditions.
-     */
     pthread_mutex_lock(&db->lock);
 
     file = DbFileName(db, args);
@@ -917,12 +904,15 @@ DbList(Db * db, size_t nArgs,...)
     path = ArrayFromVarArgs(nArgs, ap);
     dir = DbDirName(db, path, 0);
 
+    pthread_mutex_lock(&db->lock);
+
     files = opendir(dir);
     if (!files)
     {
         ArrayFree(path);
         ArrayFree(result);
         Free(dir);
+        pthread_mutex_unlock(&db->lock);
         return NULL;
     }
     while ((file = readdir(files)))
@@ -944,6 +934,7 @@ DbList(Db * db, size_t nArgs,...)
 
     ArrayFree(path);
     Free(dir);
+    pthread_mutex_unlock(&db->lock);
 
     return result;
 }