Handle output buffer limits for Module blocked clients (#8141)

Module blocked clients cache the response in a temporary client,
the reply list in this client would be affected by the recent fix
in #7202, but when the reply is later copied into the real client,
it would have bypassed all the checks for output buffer limit, which
would have resulted in both: responding with a partial response to
the client, and also not disconnecting it at all.

src/networking.c

@@ -270,6 +270,9 @@ int prepareClientToWrite(client *c) {
  * Low level functions to add more data to output buffers.
  * -------------------------------------------------------------------------- */
 
+/* Attempts to add the reply to the static buffer in the client struct.
+ * Returns C_ERR if the buffer is full, or the reply list is not empty,
+ * in which case the reply must be added to the reply list. */
 int _addReplyToBuffer(client *c, const char *s, size_t len) {
     size_t available = sizeof(c->buf)-c->bufpos;
 
@@ -287,6 +290,8 @@ int _addReplyToBuffer(client *c, const char *s, size_t len) {
     return C_OK;
 }
 
+/* Adds the reply to the reply linked list.
+ * Note: some edits to this function need to be relayed to AddReplyFromClient. */
 void _addReplyProtoToList(client *c, const char *s, size_t len) {
     if (c->flags & CLIENT_CLOSE_AFTER_REPLY) return;
 
@@ -848,14 +853,40 @@ void addReplySubcommandSyntaxError(client *c) {
 /* Append 'src' client output buffers into 'dst' client output buffers. 
  * This function clears the output buffers of 'src' */
 void AddReplyFromClient(client *dst, client *src) {
+    /* If the source client contains a partial response due to client output
+     * buffer limits, propagate that to the dest rather than copy a partial
+     * reply. We don't wanna run the risk of copying partial response in case
+     * for some reason the output limits don't reach the same decision (maybe
+     * they changed) */
+    if (src->flags & CLIENT_CLOSE_ASAP) {
+        sds client = catClientInfoString(sdsempty(),dst);
+        freeClientAsync(dst);
+        serverLog(LL_WARNING,"Client %s scheduled to be closed ASAP for overcoming of output buffer limits.", client);
+        sdsfree(client);
+        return;
+    }
+
+    /* First add the static buffer (either into the static buffer or reply list) */
+    addReplyProto(dst,src->buf, src->bufpos);
+
+    /* We need to check with prepareClientToWrite again (after addReplyProto)
+     * since addReplyProto may have changed something (like CLIENT_CLOSE_ASAP) */
     if (prepareClientToWrite(dst) != C_OK)
         return;
-    addReplyProto(dst,src->buf, src->bufpos);
+
+    /* We're bypassing _addReplyProtoToList, so we need to add the pre/post
+     * checks in it. */
+    if (dst->flags & CLIENT_CLOSE_AFTER_REPLY) return;
+
+    /* Concatenate the reply list into the dest */
     if (listLength(src->reply))
         listJoin(dst->reply,src->reply);
     dst->reply_bytes += src->reply_bytes;
     src->reply_bytes = 0;
     src->bufpos = 0;
+
+    /* Check output buffer limits */
+    asyncCloseClientOnOutputBufferLimitReached(dst);
 }
 
 /* Copy 'src' client output buffers into 'dst' client output buffers.

tests/modules/blockedclient.c

@@ -85,6 +85,88 @@ int acquire_gil(RedisModuleCtx *ctx, RedisModuleString **argv, int argc)
     return REDISMODULE_OK;
 }
 
+typedef struct {
+    RedisModuleString **argv;
+    int argc;
+    RedisModuleBlockedClient *bc;
+} bg_call_data;
+
+void *bg_call_worker(void *arg) {
+    bg_call_data *bg = arg;
+
+    // Get Redis module context
+    RedisModuleCtx *ctx = RedisModule_GetThreadSafeContext(bg->bc);
+
+    // Acquire GIL
+    RedisModule_ThreadSafeContextLock(ctx);
+
+    // Call the command
+    const char* cmd = RedisModule_StringPtrLen(bg->argv[1], NULL);
+    RedisModuleCallReply* rep = RedisModule_Call(ctx, cmd, "v", bg->argv + 2, bg->argc - 2);
+
+    // Release GIL
+    RedisModule_ThreadSafeContextUnlock(ctx);
+
+    // Reply to client
+    if (!rep) {
+        RedisModule_ReplyWithError(ctx, "NULL reply returned");
+    } else {
+        RedisModule_ReplyWithCallReply(ctx, rep);
+        RedisModule_FreeCallReply(rep);
+    }
+
+    // Unblock client
+    RedisModule_UnblockClient(bg->bc, NULL);
+
+    /* Free the arguments */
+    for (int i=0; i<bg->argc; i++)
+        RedisModule_FreeString(ctx, bg->argv[i]);
+    RedisModule_Free(bg->argv);
+    RedisModule_Free(bg);
+
+    // Free the Redis module context
+    RedisModule_FreeThreadSafeContext(ctx);
+
+    return NULL;
+}
+
+int do_bg_rm_call(RedisModuleCtx *ctx, RedisModuleString **argv, int argc)
+{
+    UNUSED(argv);
+    UNUSED(argc);
+
+    /* Make sure we're not trying to block a client when we shouldn't */
+    int flags = RedisModule_GetContextFlags(ctx);
+    int allFlags = RedisModule_GetContextFlagsAll();
+    if ((allFlags & REDISMODULE_CTX_FLAGS_MULTI) &&
+        (flags & REDISMODULE_CTX_FLAGS_MULTI)) {
+        RedisModule_ReplyWithSimpleString(ctx, "Blocked client is not supported inside multi");
+        return REDISMODULE_OK;
+    }
+    if ((allFlags & REDISMODULE_CTX_FLAGS_DENY_BLOCKING) &&
+        (flags & REDISMODULE_CTX_FLAGS_DENY_BLOCKING)) {
+        RedisModule_ReplyWithSimpleString(ctx, "Blocked client is not allowed");
+        return REDISMODULE_OK;
+    }
+
+    /* Make a copy of the arguments and pass them to the thread. */
+    bg_call_data *bg = RedisModule_Alloc(sizeof(bg_call_data));
+    bg->argv = RedisModule_Alloc(sizeof(RedisModuleString*)*argc);
+    bg->argc = argc;
+    for (int i=0; i<argc; i++)
+        bg->argv[i] = RedisModule_HoldString(ctx, argv[i]);
+
+    /* Block the client */
+    bg->bc = RedisModule_BlockClient(ctx, NULL, NULL, NULL, 0);
+
+    /* Start a thread to handle the request */
+    pthread_t tid;
+    int res = pthread_create(&tid, NULL, bg_call_worker, bg);
+    assert(res == 0);
+
+    return REDISMODULE_OK;
+}
+
 int do_rm_call(RedisModuleCtx *ctx, RedisModuleString **argv, int argc){
     UNUSED(argv);
     UNUSED(argc);
@@ -120,5 +202,8 @@ int RedisModule_OnLoad(RedisModuleCtx *ctx, RedisModuleString **argv, int argc)
     if (RedisModule_CreateCommand(ctx, "do_rm_call", do_rm_call, "", 0, 0, 0) == REDISMODULE_ERR)
         return REDISMODULE_ERR;
 
+    if (RedisModule_CreateCommand(ctx, "do_bg_rm_call", do_bg_rm_call, "", 0, 0, 0) == REDISMODULE_ERR)
+        return REDISMODULE_ERR;
+
     return REDISMODULE_OK;
 }

tests/unit/moduleapi/blockedclient.tcl

@@ -72,4 +72,22 @@ start_server {tags {"modules"}} {
         } e
         set e
     } {*ERR*DENY BLOCKING*}
+
+    test {RM_Call from blocked client} {
+        r hset hash foo bar
+        r do_bg_rm_call hgetall hash
+    } {foo bar}
+
+    test {blocked client reaches client output buffer limit} {
+        r hset hash big [string repeat x 50000]
+        r hset hash bada [string repeat x 50000]
+        r hset hash boom [string repeat x 50000]
+        r config set client-output-buffer-limit {normal 100000 0 0}
+        r client setname myclient
+        catch {r do_bg_rm_call hgetall hash} e
+        assert_match "*I/O error*" $e
+        reconnect
+        set clients [r client list]
+        assert_no_match "*name=myclient*" $clients
+    }
 }