include/Cytoplasm/Db.h

@@ -48,6 +48,17 @@
  */
 typedef struct Db Db;
 
+/**
+ * Some "hints" for the database backend for operations. 
+ * Hints are a way for the program to declare what to except of it
+ * (and the program MUST adhere to these hints, but the backend 
+ * MAY adhere).
+ */
+typedef enum DbHint {
+    DB_HINT_READONLY,   /* The database reference is treated as read-only */
+    DB_HINT_WRITE       /* The database reference is treated as read/write */
+} DbHint;
+
 /**
  * When an object is locked, a reference is returned. This reference
  * is owned by the current thread, and the database is inaccessible to
@@ -118,6 +129,15 @@ extern DbRef * DbCreate(Db *, size_t,...);
  */
 extern DbRef * DbLock(Db *, size_t,...);
 
+/**
+ * Behaves like
+ * .Fn DbLock ,
+ * but with hints on the reference itself, as 
+ * .Fn DbLock 
+ * itself is read/write.
+ */
+extern DbRef * DbLockIntent(Db *, DbHint, size_t,...);
+
 /**
  * Immediately and permanently remove an object from the database.
  * This function assumes the object is not locked, otherwise undefined

src/Db/Db.c

@@ -327,7 +327,30 @@ DbLock(Db * db, size_t nArgs,...)
         return NULL;
     }
 
-    ret = db->lockFunc(db, args);
+    ret = db->lockFunc(db, DB_HINT_WRITE, args);
+
+    ArrayFree(args);
+
+    return ret;
+}
+
+DbRef *
+DbLockIntent(Db * db, DbHint hint, size_t nArgs,...)
+{
+    va_list ap;
+    Array *args;
+    DbRef *ret;
+
+    va_start(ap, nArgs);
+    args = ArrayFromVarArgs(nArgs, ap);
+    va_end(ap);
+
+    if (!args || !db->lockFunc)
+    {
+        return NULL;
+    }
+
+    ret = db->lockFunc(db, hint, args);
 
     ArrayFree(args);
 

src/Db/Flat.c

@@ -101,7 +101,7 @@ DbFileName(FlatDb * db, Array * args)
 }
 
 static DbRef *
-FlatLock(Db *d, Array *dir)
+FlatLock(Db *d, DbHint hint, Array *dir)
 {
     FlatDb *db = (FlatDb *) d;
     FlatDbRef *ref = NULL;
@@ -146,6 +146,8 @@ FlatLock(Db *d, Array *dir)
 
         ref = Malloc(sizeof(*ref));
         DbRefInit(d, (DbRef *) ref);
+        /* TODO: Hints */
+        ref->base.hint = hint;
         ref->base.ts = UtilLastModified(path);
         ref->base.json = JsonDecode(stream);
         ref->stream = stream;
@@ -248,7 +250,7 @@ FlatCreate(Db *d, Array *dir)
 
     /* FlatLock() will lock again for us */
     pthread_mutex_unlock(&d->lock);
-    ret = FlatLock(d, dir);
+    ret = FlatLock(d, DB_HINT_WRITE, dir);
 
     return ret;
 }

src/Db/Internal.h

@@ -66,7 +66,7 @@ struct Db
 
     /* Functions for implementation-specific operations
      * (opening a ref, closing a db, removing an entry, ...) */
-    DbRef * (*lockFunc)(Db *, Array *);
+    DbRef * (*lockFunc)(Db *, DbHint, Array *);
     DbRef * (*create)(Db *, Array *);
     Array * (*list)(Db *, Array *);
     bool (*unlock)(Db *, DbRef *);
@@ -89,8 +89,7 @@ struct DbRef
     DbRef *prev;
     DbRef *next;
 
-    /* TODO: Functions for implementation-specific operations */
+    DbHint hint;
-
     /* Implementation-specific constructs */
 };
 

src/Db/LMDB.c

@@ -135,14 +135,14 @@ LMDBKeyHead(MDB_val key)
 }
 
 static DbRef *
-LMDBLock(Db *d, Array *k)
+LMDBLock(Db *d, DbHint hint, Array *k)
 {
     LMDB *db = (LMDB *) d;
     MDB_txn *transaction;
     LMDBRef *ret = NULL;
     MDB_val key, empty_json;
     MDB_dbi dbi;
-    int code;
+    int code, flags;
     if (!d || !k)
     {
         return NULL;
@@ -152,7 +152,8 @@ LMDBLock(Db *d, Array *k)
     key = LMDBTranslateKey(k);
 
     /* create a txn */
-    if ((code = mdb_txn_begin(db->environ, NULL, 0, &transaction)) != 0)
+    flags = hint == DB_HINT_READONLY ? MDB_RDONLY : 0;
+    if ((code = mdb_txn_begin(db->environ, NULL, flags, &transaction)) != 0)
     {
         /* Very bad! */
         Log(LOG_ERR,
@@ -206,10 +207,21 @@ LMDBLock(Db *d, Array *k)
         }
     }
     ret->base.json = LMDBDecode(empty_json);
+    ret->base.hint = hint;
+    ret->transaction = NULL;
+
+    if (hint == DB_HINT_WRITE)
+    {
         ret->transaction = transaction;
         ret->dbi = dbi;
+    }
+    else
+    {
+        mdb_txn_abort(transaction);
+        mdb_dbi_close(db->environ, dbi);
+    }
 end:
-    if (!ret)
+    if (!ret || hint == DB_HINT_READONLY)
     {
         pthread_mutex_unlock(&d->lock);
     }
@@ -319,7 +331,7 @@ LMDBUnlock(Db *d, DbRef *r)
     FILE *fakestream;
     Stream *stream;
     MDB_val key, val;
-    bool ret;
+    bool ret = true;
 
     if (!d || !r)
     {
@@ -328,6 +340,8 @@ LMDBUnlock(Db *d, DbRef *r)
 
     val.mv_data = NULL;
     val.mv_size = 0;
+    if (ref->transaction && r->hint == DB_HINT_WRITE)
+    {
         key = LMDBTranslateKey(r->name);
 
         fakestream = open_memstream((char **) &val.mv_data, &val.mv_size);
@@ -340,16 +354,17 @@ LMDBUnlock(Db *d, DbRef *r)
 
         mdb_txn_commit(ref->transaction);
         mdb_dbi_close(db->environ, ref->dbi);
+        LMDBKillKey(key);
+    }
     StringArrayFree(ref->base.name);
     JsonFree(ref->base.json);
     Free(ref);
 
-    LMDBKillKey(key);
     if (val.mv_data)
     {
         free(val.mv_data);
     }
-    if (ret)
+    if (ret && r->hint == DB_HINT_WRITE)
     {
         pthread_mutex_unlock(&d->lock);
     }
@@ -430,6 +445,7 @@ LMDBCreate(Db *d, Array *k)
             StringArrayAppend(ret->base.name, ent);
         }
     }
+    ret->base.hint = DB_HINT_WRITE;
     ret->base.json = HashMapCreate();
     ret->transaction = transaction;
     ret->dbi = dbi;
@@ -464,6 +480,8 @@ LMDBList(Db *d, Array *k)
 
     pthread_mutex_lock(&d->lock);
 
+    /* Marked as read-only, as we just don't need to write anything 
+     * when listing */
     if ((code = mdb_txn_begin(db->environ, NULL, 0, &txn)) != 0)
     {
         /* Very bad! */