Fix protocol error (#1106)

Fix ProtocolError

This commit attempts to fix hiredis such that a recoverable write error
will be retried rather than throwing a hard error.

Since our read/write functions are now behind function pointers, we
specify semantically that a return value of < 0 is a hard error, 0 a
recoverable error, and > 0 a success.

Our default `redisNetRead` function was already doing something similar
so this also improves code consistency.

Resolves #961

Co-authored-by: Maksim Tuleika <maksim.tuleika@appcast.io>

hiredis.c

@@ -986,8 +986,8 @@ int redisBufferRead(redisContext *c) {
  * successfully written to the socket. When the buffer is empty after the
  * write operation, "done" is set to 1 (if given).
  *
- * Returns REDIS_ERR if an error occurred trying to write and sets
- * c->errstr to hold the appropriate error string.
+ * Returns REDIS_ERR if an unrecoverable error occurred in the underlying
+ * c->funcs->write function.
  */
 int redisBufferWrite(redisContext *c, int *done) {
 

hiredis.h

@@ -249,10 +249,16 @@ typedef struct redisContextFuncs {
     void (*free_privctx)(void *);
     void (*async_read)(struct redisAsyncContext *);
     void (*async_write)(struct redisAsyncContext *);
+
+    /* Read/Write data to the underlying communication stream, returning the
+     * number of bytes read/written.  In the event of an unrecoverable error
+     * these functions shall return a value < 0.  In the event of a
+     * recoverable error, they should return 0. */
     ssize_t (*read)(struct redisContext *, char *, size_t);
     ssize_t (*write)(struct redisContext *);
 } redisContextFuncs;
 
+
 /* Context for a connection to Redis */
 typedef struct redisContext {
     const redisContextFuncs *funcs;   /* Function table */

net.c

@@ -70,7 +70,7 @@ ssize_t redisNetRead(redisContext *c, char *buf, size_t bufcap) {
             __redisSetError(c, REDIS_ERR_TIMEOUT, "recv timeout");
             return -1;
         } else {
-            __redisSetError(c, REDIS_ERR_IO, NULL);
+            __redisSetError(c, REDIS_ERR_IO, strerror(errno));
             return -1;
         }
     } else if (nread == 0) {
@@ -82,15 +82,19 @@ ssize_t redisNetRead(redisContext *c, char *buf, size_t bufcap) {
 }
 
 ssize_t redisNetWrite(redisContext *c) {
-    ssize_t nwritten = send(c->fd, c->obuf, sdslen(c->obuf), 0);
+    ssize_t nwritten;
+
+    nwritten = send(c->fd, c->obuf, sdslen(c->obuf), 0);
     if (nwritten < 0) {
         if ((errno == EWOULDBLOCK && !(c->flags & REDIS_BLOCK)) || (errno == EINTR)) {
-            /* Try again later */
+            /* Try again */
+            return 0;
         } else {
-            __redisSetError(c, REDIS_ERR_IO, NULL);
+            __redisSetError(c, REDIS_ERR_IO, strerror(errno));
             return -1;
         }
     }
+
     return nwritten;
 }