Fix crashes with io-threads-do-reads enabled. (#8230)

Normally IO threads should simply read data from the socket into the
buffer and attempt to parse it.

If a protocol error is detected, a reply is generated which may result
with installing a write handler which is not thread safe. This fix
delays that until the client is processed back in the main thread.

Fixes #8220

(cherry picked from commit e7047ec2fcc20e150c0c8cf29addf582638d7e80)

1 files changed, 14 insertions, 2 deletions

diff --git a/src/networking.c b/src/networking.c
index 2c3a1d067..65a67021f 100644
--- a/src/networking.c
+++ b/src/networking.c
@@ -249,8 +249,14 @@ int prepareClientToWrite(client *c) {
     if (!c->conn) return C_ERR; /* Fake client for AOF loading. */
 
     /* Schedule the client to write the output buffers to the socket, unless
-     * it should already be setup to do so (it has already pending data). */
-    if (!clientHasPendingReplies(c)) clientInstallWriteHandler(c);
+     * it should already be setup to do so (it has already pending data).
+     *
+     * If CLIENT_PENDING_READ is set, we're in an IO thread and should
+     * not install a write handler. Instead, it will be done by
+     * handleClientsWithPendingReadsUsingThreads() upon return.
+     */
+    if (!clientHasPendingReplies(c) && !(c->flags & CLIENT_PENDING_READ))
+            clientInstallWriteHandler(c);
 
     /* Authorize the caller to queue in the output buffer of this client. */
     return C_OK;
@@ -3340,6 +3346,12 @@ int handleClientsWithPendingReadsUsingThreads(void) {
             }
         }
         processInputBuffer(c);
+
+        /* We may have pending replies if a thread readQueryFromClient() produced
+         * replies and did not install a write handler (it can't).
+         */
+        if (!(c->flags & CLIENT_PENDING_WRITE) && clientHasPendingReplies(c))
+            clientInstallWriteHandler(c);
     }
 
     /* Update processed count on server */