Fix MIGRATE closing of cached socket on error.

After investigating issue #3796, it was discovered that MIGRATE
could call migrateCloseSocket() after the original MIGRATE c->argv
was already rewritten as a DEL operation. As a result the host/port
passed to migrateCloseSocket() could be anything, often a NULL pointer
that gets deferenced crashing the server.

Now the socket is closed at an earlier time when there is a socket
error in a later stage where no retry will be performed, before we
rewrite the argument vector. Moreover a check was added so that later,
in the socket_err label, there is no further attempt at closing the
socket if the argument was rewritten.

This fix should resolve the bug reported in #3796.

src/cluster.c

@@ -4756,6 +4756,7 @@ void migrateCommand(client *c) {
     rio cmd, payload;
     int may_retry = 1;
     int write_error = 0;
+    int argv_rewritten = 0;
 
     /* To support the KEYS option we need the following additional state. */
     int first_key = 3; /* Argument index of the first key. */
@@ -4939,12 +4940,20 @@ try_again:
         goto socket_err; /* A retry is guaranteed because of tested conditions.*/
     }
 
+    /* On socket errors, close the migration socket now that we still have
+     * the original host/port in the ARGV. Later the original command may be
+     * rewritten to DEL and will be too later. */
+    if (socket_error) migrateCloseSocket(c->argv[1],c->argv[2]);
+
     if (!copy) {
-        /* Translate MIGRATE as DEL for replication/AOF. */
+        /* Translate MIGRATE as DEL for replication/AOF. Note that we do
+         * this only for the keys for which we received an acknowledgement
+         * from the receiving Redis server, by using the del_idx index. */
         if (del_idx > 1) {
             newargv[0] = createStringObject("DEL",3);
             /* Note that the following call takes ownership of newargv. */
             replaceClientCommandVector(c,del_idx,newargv);
+            argv_rewritten = 1;
         } else {
             /* No key transfer acknowledged, no need to rewrite as DEL. */
             zfree(newargv);
@@ -4953,8 +4962,8 @@ try_again:
     }
 
     /* If we are here and a socket error happened, we don't want to retry.
-     * Just signal the problem to the client, but only do it if we don't
-     * already queued a different error reported by the destination server. */
+     * Just signal the problem to the client, but only do it if we did not
+     * already queue a different error reported by the destination server. */
     if (!error_from_target && socket_error) {
         may_retry = 0;
         goto socket_err;
@@ -4962,7 +4971,11 @@ try_again:
 
     if (!error_from_target) {
         /* Success! Update the last_dbid in migrateCachedSocket, so that we can
-         * avoid SELECT the next time if the target DB is the same. Reply +OK. */
+         * avoid SELECT the next time if the target DB is the same. Reply +OK.
+         *
+         * Note: If we reached this point, even if socket_error is true
+         * still the SELECT command succeeded (otherwise the code jumps to
+         * socket_err label. */
         cs->last_dbid = dbid;
         addReply(c,shared.ok);
     } else {
@@ -4972,7 +4985,6 @@ try_again:
 
     sdsfree(cmd.io.buffer.ptr);
     zfree(ov); zfree(kv); zfree(newargv);
-    if (socket_error) migrateCloseSocket(c->argv[1],c->argv[2]);
     return;
 
 /* On socket errors we try to close the cached socket and try again.
@@ -4982,7 +4994,12 @@ socket_err:
     /* Cleanup we want to perform in both the retry and no retry case.
      * Note: Closing the migrate socket will also force SELECT next time. */
     sdsfree(cmd.io.buffer.ptr);
-    migrateCloseSocket(c->argv[1],c->argv[2]);
+
+    /* If the command was rewritten as DEL and there was a socket error,
+     * we already closed the socket earlier. While migrateCloseSocket()
+     * is idempotent, the host/port arguments are now gone, so don't do it
+     * again. */
+    if (!argv_rewritten) migrateCloseSocket(c->argv[1],c->argv[2]);
     zfree(newargv);
     newargv = NULL; /* This will get reallocated on retry. */