Diskless replication: rio fdset target new supports buffering.

To perform a socket write() for each RDB rio API write call was
extremely unefficient, so now rio has minimal buffering capabilities.
Writes are accumulated into a buffer and only when a given limit is
reacehd are actually wrote to the N slaves FDs.

Trivia: rio lacked support for buffering since our targets were:

1) Memory buffers.
2) C standard I/O.

Both were buffered already.

src/rio.c

@@ -78,10 +78,18 @@ static off_t rioBufferTell(rio *r) {
     return r->io.buffer.pos;
 }
 
+/* Flushes any buffer to target device if applicable. Returns 1 on success
+ * and 0 on failures. */
+static int rioBufferFlush(rio *r) {
+    REDIS_NOTUSED(r);
+    return 1; /* Nothing to do, our write just appends to the buffer. */
+}
+
 static const rio rioBufferIO = {
     rioBufferRead,
     rioBufferWrite,
     rioBufferTell,
+    rioBufferFlush,
     NULL,           /* update_checksum */
     0,              /* current checksum */
     0,              /* bytes read or written */
@@ -124,10 +132,17 @@ static off_t rioFileTell(rio *r) {
     return ftello(r->io.file.fp);
 }
 
+/* Flushes any buffer to target device if applicable. Returns 1 on success
+ * and 0 on failures. */
+static int rioFileFlush(rio *r) {
+    return (fflush(r->io.file.fp) == 0) ? 1 : 0;
+}
+
 static const rio rioFileIO = {
     rioFileRead,
     rioFileWrite,
     rioFileTell,
+    rioFileFlush,
     NULL,           /* update_checksum */
     0,              /* current checksum */
     0,              /* bytes read or written */
@@ -146,11 +161,29 @@ void rioInitWithFile(rio *r, FILE *fp) {
 
 /* Returns 1 or 0 for success/failure.
  * The function returns success as long as we are able to correctly write
- * to at least one file descriptor. */
+ * to at least one file descriptor.
+ *
+ * When buf is NULL adn len is 0, the function performs a flush operation
+ * if there is some pending buffer, so this function is also used in order
+ * to implement rioFdsetFlush(). */
 static size_t rioFdsetWrite(rio *r, const void *buf, size_t len) {
     size_t retval;
     int j;
     unsigned char *p = (unsigned char*) buf;
+    int doflush = (buf == NULL && len == 0);
+
+    /* To start we always append to our buffer. If it gets larger than
+     * a given size, we actually write to the sockets. */
+    if (len) {
+        r->io.fdset.buf = sdscatlen(r->io.fdset.buf,buf,len);
+        len = 0; /* Prevent entering the while belove if we don't flush. */
+        if (sdslen(r->io.fdset.buf) > REDIS_IOBUF_LEN) doflush = 1;
+    }
+
+    if (doflush) {
+        p = (unsigned char*) r->io.fdset.buf;
+        len = sdslen(r->io.fdset.buf);
+    }
 
     /* Write in little chunchs so that when there are big writes we
      * parallelize while the kernel is sending data in background to
@@ -176,6 +209,8 @@ static size_t rioFdsetWrite(rio *r, const void *buf, size_t len) {
         len -= count;
         r->io.fdset.pos += count;
     }
+
+    if (doflush) sdsclear(r->io.fdset.buf);
     return 1;
 }
 
@@ -192,10 +227,19 @@ static off_t rioFdsetTell(rio *r) {
     return r->io.fdset.pos;
 }
 
+/* Flushes any buffer to target device if applicable. Returns 1 on success
+ * and 0 on failures. */
+static int rioFdsetFlush(rio *r) {
+    /* Our flush is implemented by the write method, that recognizes a
+     * buffer set to NULL with a count of zero as a flush request. */
+    return rioFdsetWrite(r,NULL,0);
+}
+
 static const rio rioFdsetIO = {
     rioFdsetRead,
     rioFdsetWrite,
     rioFdsetTell,
+    rioFdsetFlush,
     NULL,           /* update_checksum */
     0,              /* current checksum */
     0,              /* bytes read or written */
@@ -213,11 +257,13 @@ void rioInitWithFdset(rio *r, int *fds, int numfds) {
     for (j = 0; j < numfds; j++) r->io.fdset.state[j] = 0;
     r->io.fdset.numfds = numfds;
     r->io.fdset.pos = 0;
+    r->io.fdset.buf = sdsempty();
 }
 
 void rioFreeFdset(rio *r) {
     zfree(r->io.fdset.fds);
     zfree(r->io.fdset.state);
+    sdsfree(r->io.fdset.buf);
 }
 
 /* ---------------------------- Generic functions ---------------------------- */