Modules API: blocked client disconnection callback.

src/module.c

@@ -158,7 +158,9 @@ typedef struct RedisModuleKey RedisModuleKey;
 
 /* Function pointer type of a function representing a command inside
  * a Redis module. */
+struct RedisModuleBlockedClient;
 typedef int (*RedisModuleCmdFunc) (RedisModuleCtx *ctx, void **argv, int argc);
+typedef void (*RedisModuleDisconnectFunc) (RedisModuleCtx *ctx, struct RedisModuleBlockedClient *bc);
 
 /* This struct holds the information about a command registered by a module.*/
 struct RedisModuleCommandProxy {
@@ -201,6 +203,7 @@ typedef struct RedisModuleBlockedClient {
     RedisModule *module;    /* Module blocking the client. */
     RedisModuleCmdFunc reply_callback; /* Reply callback on normal completion.*/
     RedisModuleCmdFunc timeout_callback; /* Reply callback on timeout. */
+    RedisModuleDisconnectFunc disconnect_callback; /* Called on disconnection.*/
     void (*free_privdata)(RedisModuleCtx*,void*);/* privdata cleanup callback.*/
     void *privdata;     /* Module private data that may be used by the reply
                            or timeout callback. It is set via the
@@ -3438,6 +3441,17 @@ void moduleBlockedClientPipeReadable(aeEventLoop *el, int fd, void *privdata, in
  * running the list of clients blocked by a module that need to be unblocked. */
 void unblockClientFromModule(client *c) {
     RedisModuleBlockedClient *bc = c->bpop.module_blocked_handle;
+
+    /* Call the disconnection callback if any. */
+    if (bc->disconnect_callback) {
+        RedisModuleCtx ctx = REDISMODULE_CTX_INIT;
+        ctx.blocked_privdata = bc->privdata;
+        ctx.module = bc->module;
+        ctx.client = bc->client;
+        bc->disconnect_callback(&ctx,bc);
+        moduleFreeContext(&ctx);
+    }
+
     bc->client = NULL;
     /* Reset the client for a new query since, for blocking commands implemented
      * into modules, we do not it immediately after the command returns (and
@@ -3478,6 +3492,7 @@ RedisModuleBlockedClient *RM_BlockClient(RedisModuleCtx *ctx, RedisModuleCmdFunc
     bc->module = ctx->module;
     bc->reply_callback = reply_callback;
     bc->timeout_callback = timeout_callback;
+    bc->disconnect_callback = NULL; /* Set by RM_SetDisconnectCallback() */
     bc->free_privdata = free_privdata;
     bc->privdata = NULL;
     bc->reply_client = createClient(-1);
@@ -3525,6 +3540,26 @@ int RM_AbortBlock(RedisModuleBlockedClient *bc) {
     return RM_UnblockClient(bc,NULL);
 }
 
+/* Set a callback that will be called if a blocked client disconnects
+ * before the module has a chance to call RedisModule_UnblockClient()
+ *
+ * Usually what you want to do there, is to cleanup your module state
+ * so that you can call RedisModule_UnblockClient() safely, otherwise
+ * the client will remain blocked forever if the timeout is large.
+ *
+ * Notes:
+ *
+ * 1. It is not safe to call Reply* family functions here, it is also
+ *    useless since the client is gone.
+ *
+ * 2. This callback is not called if the client disconnects because of
+ *    a timeout. In such a case, the client is unblocked automatically
+ *    and the timeout callback is called.
+ */
+void RM_SetDisconnectCallback(RedisModuleBlockedClient *bc, RedisModuleDisconnectFunc callback) {
+    bc->disconnect_callback = callback;
+}
+
 /* This function will check the moduleUnblockedClients queue in order to
  * call the reply callback and really unblock the client.
  *
@@ -3592,6 +3627,10 @@ void moduleHandleBlockedClients(void) {
         freeClient(bc->reply_client);
 
         if (c != NULL) {
+            /* Before unblocking the client, set the disconnect callback
+             * to NULL, because if we reached this point, the client was
+             * properly unblocked by the module. */
+            bc->disconnect_callback = NULL;
             unblockClient(c);
             /* Put the client in the list of clients that need to write
              * if there are pending replies here. This is needed since
@@ -3627,6 +3666,10 @@ void moduleBlockedClientTimedOut(client *c) {
     ctx.client = bc->client;
     bc->timeout_callback(&ctx,(void**)c->argv,c->argc);
     moduleFreeContext(&ctx);
+    /* For timeout events, we do not want to call the disconnect callback,
+     * because the blocekd client will be automatically disconnected in
+     * this case, and the user can still hook using the timeout callback. */
+    bc->disconnect_callback = NULL;
 }
 
 /* Return non-zero if a module command was called in order to fill the
@@ -4625,4 +4668,5 @@ void moduleRegisterCoreAPI(void) {
     REGISTER_API(GetRandomBytes);
     REGISTER_API(GetRandomHexChars);
     REGISTER_API(BlockedClientDisconnected);
+    REGISTER_API(SetDisconnectCallback);
 }

src/modules/helloblock.c

@@ -74,6 +74,23 @@ void *HelloBlock_ThreadMain(void *arg) {
     return NULL;
 }
 
+/* An example blocked client disconnection callback.
+ *
+ * Note that in the case of the HELLO.BLOCK command, the blocked client is now
+ * owned by the thread calling sleep(). In this speciifc case, there is not
+ * much we can do, however normally we could instead implement a way to
+ * signal the thread that the client disconnected, and sleep the specified
+ * amount of seconds with a while loop calling sleep(1), so that once we
+ * detect the client disconnection, we can terminate the thread ASAP. */
+void HelloBlock_Disconnected(RedisModuleCtx *ctx, RedisModuleBlockedClient *bc) {
+    RedisModule_Log(ctx,"warning","Blocked client %p disconnected!",
+        (void*)bc);
+
+    /* Here you should cleanup your state / threads, and if possible
+     * call RedisModule_UnblockClient(), or notify the thread that will
+     * call the function ASAP. */
+}
+
 /* HELLO.BLOCK <delay> <timeout> -- Block for <count> seconds, then reply with
  * a random number. Timeout is the command timeout, so that you can test
  * what happens when the delay is greater than the timeout. */
@@ -93,6 +110,11 @@ int HelloBlock_RedisCommand(RedisModuleCtx *ctx, RedisModuleString **argv, int a
     pthread_t tid;
     RedisModuleBlockedClient *bc = RedisModule_BlockClient(ctx,HelloBlock_Reply,HelloBlock_Timeout,HelloBlock_FreeData,timeout);
 
+    /* Here we set a disconnection handler, however since this module will
+     * block in sleep() in a thread, there is not much we can do in the
+     * callback, so this is just to show you the API. */
+    RedisModule_SetDisconnectCallback(bc,HelloBlock_Disconnected);
+
     /* Now that we setup a blocking client, we need to pass the control
      * to the thread. However we need to pass arguments to the thread:
      * the delay and a reference to the blocked client handle. */

src/redismodule.h

@@ -143,6 +143,7 @@ typedef struct RedisModuleBlockedClient RedisModuleBlockedClient;
 typedef struct RedisModuleClusterInfo RedisModuleClusterInfo;
 
 typedef int (*RedisModuleCmdFunc)(RedisModuleCtx *ctx, RedisModuleString **argv, int argc);
+typedef void (*RedisModuleDisconnectFunc)(RedisModuleCtx *ctx, RedisModuleBlockedClient *bc);
 typedef int (*RedisModuleNotificationFunc)(RedisModuleCtx *ctx, int type, const char *event, RedisModuleString *key);
 typedef void *(*RedisModuleTypeLoadFunc)(RedisModuleIO *rdb, int encver);
 typedef void (*RedisModuleTypeSaveFunc)(RedisModuleIO *rdb, void *value);
@@ -275,7 +276,7 @@ void REDISMODULE_API_FUNC(RedisModule_DigestEndSequence)(RedisModuleDigest *md);
 
 /* Experimental APIs */
 #ifdef REDISMODULE_EXPERIMENTAL_API
-#define REDISMODULE_EXPERIMENTAL_API_VERSION 2
+#define REDISMODULE_EXPERIMENTAL_API_VERSION 3
 RedisModuleBlockedClient *REDISMODULE_API_FUNC(RedisModule_BlockClient)(RedisModuleCtx *ctx, RedisModuleCmdFunc reply_callback, RedisModuleCmdFunc timeout_callback, void (*free_privdata)(RedisModuleCtx*,void*), long long timeout_ms);
 int REDISMODULE_API_FUNC(RedisModule_UnblockClient)(RedisModuleBlockedClient *bc, void *privdata);
 int REDISMODULE_API_FUNC(RedisModule_IsBlockedReplyRequest)(RedisModuleCtx *ctx);
@@ -300,6 +301,7 @@ const char *REDISMODULE_API_FUNC(RedisModule_GetMyClusterID)(void);
 size_t REDISMODULE_API_FUNC(RedisModule_GetClusterSize)(void);
 void REDISMODULE_API_FUNC(RedisModule_GetRandomBytes)(unsigned char *dst, size_t len);
 void REDISMODULE_API_FUNC(RedisModule_GetRandomHexChars)(char *dst, size_t len);
+void REDISMODULE_API_FUNC(RedisModule_SetDisconnectCallback)(RedisModuleBlockedClient *bc, RedisModuleDisconnectFunc callback);
 #endif
 
 /* This is included inline inside each Redis module. */
@@ -421,6 +423,7 @@ static int RedisModule_Init(RedisModuleCtx *ctx, const char *name, int ver, int
     REDISMODULE_GET_API(IsBlockedTimeoutRequest);
     REDISMODULE_GET_API(GetBlockedClientPrivateData);
     REDISMODULE_GET_API(AbortBlock);
+    REDISMODULE_GET_API(SetDisconnectCallback);
     REDISMODULE_GET_API(SubscribeToKeyspaceEvents);
     REDISMODULE_GET_API(BlockedClientDisconnected);
     REDISMODULE_GET_API(RegisterClusterMessageReceiver);