Trim SDS free space of retained module strings

In some cases processMultibulkBuffer uses sdsMakeRoomFor to
expand the querybuf, but later in some cases it uses that query
buffer as is for an argv element (see "Optimization"), which means
that the sds in argv may have a lot of wasted space, and then in case
modules keep that argv RedisString inside their data structure, this
space waste will remain for long (until restarted from rdb).

src/module.c

@@ -509,6 +509,17 @@ void RedisModuleCommandDispatcher(client *c) {
     cp->func(&ctx,(void**)c->argv,c->argc);
     moduleHandlePropagationAfterCommandCallback(&ctx);
     moduleFreeContext(&ctx);
+
+    /* In some cases processMultibulkBuffer uses sdsMakeRoomFor to
+     * expand the querybuf, but later in some cases it uses that query
+     * buffer as is for an argv element (see "Optimization"), which means
+     * that the sds in argv may have a lot of wasted space, and then in case
+     * modules keep that argv RedisString inside their data structure, this
+     * space waste will remain for long (until restarted from rdb). */
+    for (int i = 0; i < c->argc; i++) {
+        if (c->argv[i]->refcount > 1)
+            trimStringObjectIfNeeded(c->argv[i]);
+    }
 }
 
 /* This function returns the list of keys, with the same interface as the

src/object.c

@@ -415,6 +415,17 @@ int isObjectRepresentableAsLongLong(robj *o, long long *llval) {
     }
 }
 
+void trimStringObjectIfNeeded(robj *o) {
+    /* Optimize the SDS string inside the string object to require
+     * little space, in case there is more than 10% of free space
+     * at the end of the SDS string. */
+    if (o->encoding == OBJ_ENCODING_RAW &&
+        sdsavail(o->ptr) > sdslen(o->ptr)/10)
+    {
+        o->ptr = sdsRemoveFreeSpace(o->ptr);
+    }
+}
+
 /* Try to encode a string object in order to save space */
 robj *tryObjectEncoding(robj *o) {
     long value;
@@ -484,11 +495,7 @@ robj *tryObjectEncoding(robj *o) {
      * We do that only for relatively large strings as this branch
      * is only entered if the length of the string is greater than
      * OBJ_ENCODING_EMBSTR_SIZE_LIMIT. */
-    if (o->encoding == OBJ_ENCODING_RAW &&
-        sdsavail(s) > len/10)
-    {
-        o->ptr = sdsRemoveFreeSpace(o->ptr);
-    }
+    trimStringObjectIfNeeded(o);
 
     /* Return the original object. */
     return o;

src/sds.c

@@ -257,8 +257,12 @@ sds sdsRemoveFreeSpace(sds s) {
     char type, oldtype = s[-1] & SDS_TYPE_MASK;
     int hdrlen, oldhdrlen = sdsHdrSize(oldtype);
     size_t len = sdslen(s);
+    size_t avail = sdsavail(s);
     sh = (char*)s-oldhdrlen;
 
+    /* Return ASAP if there is no space left. */
+    if (avail == 0) return s;
+
     /* Check what would be the minimum SDS header that is just good enough to
      * fit this string. */
     type = sdsReqType(len);

src/server.h

@@ -1656,6 +1656,7 @@ int compareStringObjects(robj *a, robj *b);
 int collateStringObjects(robj *a, robj *b);
 int equalStringObjects(robj *a, robj *b);
 unsigned long long estimateObjectIdleTime(robj *o);
+void trimStringObjectIfNeeded(robj *o);
 #define sdsEncodedObject(objptr) (objptr->encoding == OBJ_ENCODING_RAW || objptr->encoding == OBJ_ENCODING_EMBSTR)
 
 /* Synchronous I/O with timeout */