Configurable reader max idle buffer size.

Hiredis used to free unused redisReader buffers bigger than 16k. Now
this limit is configurable (see the documentation updated by this commit)
in order to allow working with big payloads without incurring to speed
penalty.

README.md

@@ -320,6 +320,10 @@ The reply parsing API consists of the following functions:
     int redisReaderFeed(redisReader *reader, const char *buf, size_t len);
     int redisReaderGetReply(redisReader *reader, void **reply);
 
+The same set of functions are used internally by hiredis when creating a
+normal Redis context, the above API just exposes it to the user for a direct
+usage.
+
 ### Usage
 
 The function `redisReaderCreate` creates a `redisReader` structure that holds a
@@ -346,6 +350,29 @@ immediately after creating the `redisReader`.
 For example, [hiredis-rb](https://github.com/pietern/hiredis-rb/blob/master/ext/hiredis_ext/reader.c)
 uses customized reply object functions to create Ruby objects.
 
+### Reader max buffer
+
+Both when using the Reader API directly or when using it indirectly via a
+normal Redis context, the redisReader structure uses a buffer in order to
+accumulate data from the server.
+Usually this buffer is destroyed when it is empty and is larger than 16
+kb in order to avoid wasting memory in unused buffers
+
+However when working with very big payloads destroying the buffer may slow
+down performances considerably, so it is possible to modify the max size of
+an idle buffer changing the value of the `maxbuf` field of the reader structure
+to the desired value. The special value of 0 means that there is no maximum
+value for an idle buffer, so the buffer will never get freed.
+
+For instance if you have a normal Redis context you can set the maximum idle
+buffer to zero (unlimited) just with:
+
+    context->reader->maxbuf = 0;
+
+This should be done only in order to maximize performances when working with
+large payloads. The context should be set back to `REDIS_READER_MAX_BUF` again
+as soon as possible in order to prevent allocation of useless memory.
+
 ## AUTHORS
 
 Hiredis was written by Salvatore Sanfilippo (antirez at gmail) and

hiredis.c

@@ -564,6 +564,7 @@ redisReader *redisReaderCreate(void) {
     r->errstr[0] = '\0';
     r->fn = &defaultFunctions;
     r->buf = sdsempty();
+    r->maxbuf = REDIS_READER_MAX_BUF;
     if (r->buf == NULL) {
         free(r);
         return NULL;
@@ -591,7 +592,7 @@ int redisReaderFeed(redisReader *r, const char *buf, size_t len) {
     /* Copy the provided buffer. */
     if (buf != NULL && len >= 1) {
         /* Destroy internal buffer when it is empty and is quite large. */
-        if (r->len == 0 && sdsavail(r->buf) > 16*1024) {
+        if (r->len == 0 && r->maxbuf != 0 && sdsavail(r->buf) > r->maxbuf) {
             sdsfree(r->buf);
             r->buf = sdsempty();
             r->pos = 0;

hiredis.h

@@ -86,6 +86,8 @@
 #define REDIS_REPLY_STATUS 5
 #define REDIS_REPLY_ERROR 6
 
+#define REDIS_READER_MAX_BUF (1024*16)  /* Default max unused reader buffer. */
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -125,6 +127,7 @@ typedef struct redisReader {
     char *buf; /* Read buffer */
     size_t pos; /* Buffer cursor */
     size_t len; /* Buffer length */
+    size_t maxbuf; /* Max length of unused buffer */
 
     redisReadTask rstack[4];
     int ridx; /* Index of current read task */