sdscatvprintf(): Try to use a static buffer.

For small content the function now tries to use a static buffer to avoid
a malloc/free cycle that is too costly when the function is used in the
context of performance critical code path such as INFO output generation.

This change was verified to have positive effects in the execution speed
of the INFO command.

src/sds.c

@@ -292,25 +292,37 @@ sds sdscpy(sds s, const char *t) {
 /* Like sdscatpritf() but gets va_list instead of being variadic. */
 sds sdscatvprintf(sds s, const char *fmt, va_list ap) {
     va_list cpy;
-    char *buf, *t;
-    size_t buflen = strlen(fmt)*4;
+    char staticbuf[1024], *buf = staticbuf, *t;
+    size_t buflen = strlen(fmt)*2;
 
-    if (buflen < 32) buflen = 32;
-    while(1) {
+    /* We try to start using a static buffer for speed.
+     * If not possible we revert to heap allocation. */
+    if (buflen > sizeof(staticbuf)) {
         buf = zmalloc(buflen);
         if (buf == NULL) return NULL;
+    } else {
+        buflen = sizeof(staticbuf);
+    }
+
+    /* Try with buffers two times bigger every time we fail to
+     * fit the string in the current buffer size. */
+    while(1) {
         buf[buflen-2] = '\0';
         va_copy(cpy,ap);
         vsnprintf(buf, buflen, fmt, cpy);
         if (buf[buflen-2] != '\0') {
-            zfree(buf);
+            if (buf != staticbuf) zfree(buf);
             buflen *= 2;
+            buf = zmalloc(buflen);
+            if (buf == NULL) return NULL;
             continue;
         }
         break;
     }
+
+    /* Finally concat the obtained string to the SDS string and return it. */
     t = sdscat(s, buf);
-    zfree(buf);
+    if (buf != staticbuf) zfree(buf);
     return t;
 }