Add a user-defined callback to set custom socket options.

Fixes #698

hiredis.c

@@ -829,6 +829,9 @@ redisContext *redisConnectWithOptions(const redisOptions *options) {
         c->flags |= REDIS_PREFER_IPV6;
     }
 
+    /* Set any configured socket callback function */
+    c->socket_cb = options->socket_cb;
+
     /* Set any user supplied RESP3 PUSH handler or use freeReplyObject
      * as a default unless specifically flagged that we don't want one. */
     if (options->push_cb != NULL)

hiredis.h

@@ -216,6 +216,10 @@ typedef struct {
         redisFD fd;
     } endpoint;
 
+    /* Socket callback function executed after socket creation but before
+     * we connect. This can be used to set OS specific socket options. */
+    int (*socket_cb)(redisFD fd, void *privdata, char **errstr);
+
     /* Optional user defined data/destructor */
     void *privdata;
     void (*free_privdata)(void *);
@@ -239,6 +243,10 @@ typedef struct {
         (opts)->endpoint.unix_socket = path;    \
     } while(0)
 
+#define REDIS_OPTIONS_SET_SOCKET_CB(opts, cb) do { \
+        (opts)->socket_cb = cb;                    \
+    } while(0)
+
 #define REDIS_OPTIONS_SET_PRIVDATA(opts, data, dtor) do {  \
         (opts)->privdata = data;                           \
         (opts)->free_privdata = dtor;                      \
@@ -288,6 +296,11 @@ typedef struct redisContext {
     struct sockaddr *saddr;
     size_t addrlen;
 
+    /* A callback to be invoked after successful socket creation but before we
+     * connect.  This can be usefull for setting OS specific socket flags.
+     * The callback may optionally set errstr in the event of an error. */
+    int (*socket_cb)(redisFD fd, void *privdata, char **errstr);
+
     /* Optional data and corresponding destructor users can use to provide
      * context to a given redisContext.  Not used by hiredis. */
     void *privdata;

net.c

@@ -402,6 +402,7 @@ static int _redisContextConnectTcp(redisContext *c, const char *addr, int port,
     int rv, n;
     char _port[6];  /* strlen("65535"); */
     struct addrinfo hints, *servinfo, *bservinfo, *p, *b;
+    char *errstr;
     int blocking = (c->flags & REDIS_BLOCK);
     int reuseaddr = (c->flags & REDIS_REUSEADDR);
     int reuses = 0;
@@ -477,6 +478,13 @@ addrretry:
         if ((s = socket(p->ai_family,p->ai_socktype,p->ai_protocol)) == REDIS_INVALID_FD)
             continue;
 
+        /* Invoke any user-provided socket callback */
+        errstr = NULL;
+        if (c->socket_cb && c->socket_cb(s, c->privdata, &errstr) != REDIS_OK) {
+            __redisSetError(c, REDIS_ERR_OTHER, errstr ? errstr : "Failure in user-defined socket callback");
+            goto error;
+        }
+
         c->fd = s;
         if (redisSetBlocking(c,0) != REDIS_OK)
             goto error;