String value unsharing refactored into proper function.

All the Redis functions that need to modify the string value of a key in
a destructive way (APPEND, SETBIT, SETRANGE, ...) require to make the
object unshared (if refcount > 1) and encoded in raw format (if encoding
is not already REDIS_ENCODING_RAW).

This was cut & pasted many times in multiple places of the code. This
commit puts the small logic needed into a function called
dbUnshareStringValue().

src/bitops.c

@@ -223,14 +223,7 @@ void setbitCommand(redisClient *c) {
         dbAdd(c->db,c->argv[1],o);
     } else {
         if (checkType(c,o,REDIS_STRING)) return;
-
-        /* Create a copy when the object is shared or encoded. */
-        if (o->refcount != 1 || o->encoding != REDIS_ENCODING_RAW) {
-            robj *decoded = getDecodedObject(o);
-            o = createRawStringObject(decoded->ptr, sdslen(decoded->ptr));
-            decrRefCount(decoded);
-            dbOverwrite(c->db,c->argv[1],o);
-        }
+        o = dbUnshareStringValue(c->db,c->argv[1],o);
     }
 
     /* Grow sds value to the right length if necessary */

src/db.c

@@ -170,6 +170,44 @@ int dbDelete(redisDb *db, robj *key) {
     }
 }
 
+/* Prepare the string object stored at 'key' to be modified destructively
+ * to implement commands like SETBIT or APPEND.
+ *
+ * An object is usually ready to be modified unless one of the two conditions
+ * are true:
+ *
+ * 1) The object 'o' is shared (refcount > 1), we don't want to affect
+ *    other users.
+ * 2) The object encoding is not "RAW".
+ *
+ * If the object is found in one of the above conditions (or both) by the
+ * function, an unshared / not-encoded copy of the string object is stored
+ * at 'key' in the specified 'db'. Otherwise the object 'o' itself is
+ * returned.
+ *
+ * USAGE:
+ *
+ * The object 'o' is what the caller already obtained by looking up 'key'
+ * in 'db', the usage pattern looks like this:
+ *
+ * o = lookupKeyWrite(db,key);
+ * if (checkType(c,o,REDIS_STRING)) return;
+ * o = dbUnshareStringValue(db,key,o);
+ *
+ * At this point the caller is ready to modify the object, for example
+ * using an sdscat() call to append some data, or anything else.
+ */
+robj *dbUnshareStringValue(redisDb *db, robj *key, robj *o) {
+    redisAssert(o->type == REDIS_STRING);
+    if (o->refcount != 1 || o->encoding != REDIS_ENCODING_RAW) {
+        robj *decoded = getDecodedObject(o);
+        o = createRawStringObject(decoded->ptr, sdslen(decoded->ptr));
+        decrRefCount(decoded);
+        dbOverwrite(db,key,o);
+    }
+    return o;
+}
+
 long long emptyDb(void(callback)(void*)) {
     int j;
     long long removed = 0;

src/hyperloglog.c

@@ -424,14 +424,7 @@ void hllAddCommand(redisClient *c) {
                 REDIS_HLL_SIZE);
             return;
         }
-
-        /* If the object is shared or encoded, we have to make a copy. */
-        if (o->refcount != 1 || o->encoding != REDIS_ENCODING_RAW) {
-            robj *decoded = getDecodedObject(o);
-            o = createRawStringObject(decoded->ptr, sdslen(decoded->ptr));
-            decrRefCount(decoded);
-            dbOverwrite(c->db,c->argv[1],o);
-        }
+        o = dbUnshareStringValue(c->db,c->argv[1],o);
     }
     /* Perform the low level ADD operation for every element. */
     registers = o->ptr;

src/object.c

@@ -660,7 +660,7 @@ unsigned long long estimateObjectIdleTime(robj *o) {
     }
 }
 
-/* This is a helper function for the DEBUG command. We need to lookup keys
+/* This is a helper function for the OBJECT command. We need to lookup keys
  * without any modification of LRU or other parameters. */
 robj *objectCommandLookup(redisClient *c, robj *key) {
     dictEntry *de;

src/redis.h

@@ -1252,6 +1252,7 @@ void setKey(redisDb *db, robj *key, robj *val);
 int dbExists(redisDb *db, robj *key);
 robj *dbRandomKey(redisDb *db);
 int dbDelete(redisDb *db, robj *key);
+robj *dbUnshareStringValue(redisDb *db, robj *key, robj *o);
 long long emptyDb(void(callback)(void*));
 int selectDb(redisClient *c, int id);
 void signalModifiedKey(redisDb *db, robj *key);

src/t_string.c

@@ -215,12 +215,7 @@ void setrangeCommand(redisClient *c) {
             return;
 
         /* Create a copy when the object is shared or encoded. */
-        if (o->refcount != 1 || o->encoding != REDIS_ENCODING_RAW) {
-            robj *decoded = getDecodedObject(o);
-            o = createRawStringObject(decoded->ptr, sdslen(decoded->ptr));
-            decrRefCount(decoded);
-            dbOverwrite(c->db,c->argv[1],o);
-        }
+        o = dbUnshareStringValue(c->db,c->argv[1],o);
     }
 
     if (sdslen(value) > 0) {
@@ -433,15 +428,8 @@ void appendCommand(redisClient *c) {
         if (checkStringLength(c,totlen) != REDIS_OK)
             return;
 
-        /* If the object is shared or encoded, we have to make a copy */
-        if (o->refcount != 1 || o->encoding != REDIS_ENCODING_RAW) {
-            robj *decoded = getDecodedObject(o);
-            o = createRawStringObject(decoded->ptr, sdslen(decoded->ptr));
-            decrRefCount(decoded);
-            dbOverwrite(c->db,c->argv[1],o);
-        }
-
         /* Append the value */
+        o = dbUnshareStringValue(c->db,c->argv[1],o);
         o->ptr = sdscatlen(o->ptr,append->ptr,sdslen(append->ptr));
         totlen = sdslen(o->ptr);
     }