Shared reusable client for RM_Call() (#8516)

A single client pointer is added in the server struct. This is
initialized by the first RM_Call() and reused for every subsequent
RM_Call() except if it's already in use, which means that it's not
used for (recursive) module calls to modules. For these, a new
"fake" client is created each time.

Other changes:
* Avoid allocating a dict iterator in pubsubUnsubscribeAllChannels
  when not needed

src/module.c

@@ -3969,16 +3969,26 @@ RedisModuleCallReply *RM_Call(RedisModuleCtx *ctx, const char *cmdname, const ch
     RedisModuleCallReply *reply = NULL;
     int replicate = 0; /* Replicate this command? */
 
-    /* Create the client and dispatch the command. */
+    /* Handle arguments. */
     va_start(ap, fmt);
-    c = createClient(NULL);
-    c->user = NULL; /* Root user. */
     argv = moduleCreateArgvFromUserFormat(cmdname,fmt,&argc,&flags,ap);
     replicate = flags & REDISMODULE_ARGV_REPLICATE;
     va_end(ap);
 
     /* Setup our fake client for command execution. */
-    c->flags |= CLIENT_MODULE;
+    if (server.module_client == NULL) {
+        /* This is the first RM_Call() ever. Create reusable client. */
+        c = server.module_client = createClient(NULL);
+    } else if (server.module_client->argv == NULL) {
+        /* The reusable module client is not busy with a command. Use it. */
+        c = server.module_client;
+    } else {
+        /* The reusable module client is busy. (It is probably used in a
+         * recursive call to this module.) */
+        c = createClient(NULL);
+    }
+    c->user = NULL; /* Root user. */
+    c->flags = CLIENT_MODULE;
 
     /* We do not want to allow block, the module do not expect it */
     c->flags |= CLIENT_DENY_BLOCKING;
@@ -4066,7 +4076,18 @@ RedisModuleCallReply *RM_Call(RedisModuleCtx *ctx, const char *cmdname, const ch
 
 cleanup:
     if (ctx->module) ctx->module->in_call--;
-    freeClient(c);
+    if (c == server.module_client) {
+        /* reset shared client so it can be reused */
+        discardTransaction(c);
+        pubsubUnsubscribeAllChannels(c,0);
+        pubsubUnsubscribeAllPatterns(c,0);
+        resetClient(c); /* frees the contents of argv */
+        zfree(c->argv);
+        c->argv = NULL;
+        c->resp = 2;
+    } else {
+        freeClient(c); /* temporary client */
+    }
     return reply;
 }
 
@@ -8269,6 +8290,9 @@ void moduleInitModulesSystem(void) {
     /* Set up filter list */
     moduleCommandFilters = listCreate();
 
+    /* Reusable client for RM_Call() is created on first use */
+    server.module_client = NULL;
+
     moduleRegisterCoreAPI();
     if (pipe(server.module_blocked_pipe) == -1) {
         serverLog(LL_WARNING,

src/pubsub.c

@@ -250,18 +250,20 @@ int pubsubUnsubscribePattern(client *c, robj *pattern, int notify) {
 /* Unsubscribe from all the channels. Return the number of channels the
  * client was subscribed to. */
 int pubsubUnsubscribeAllChannels(client *c, int notify) {
-    dictIterator *di = dictGetSafeIterator(c->pubsub_channels);
-    dictEntry *de;
     int count = 0;
+    if (dictSize(c->pubsub_channels) > 0) {
+        dictIterator *di = dictGetSafeIterator(c->pubsub_channels);
+        dictEntry *de;
 
-    while((de = dictNext(di)) != NULL) {
-        robj *channel = dictGetKey(de);
+        while((de = dictNext(di)) != NULL) {
+            robj *channel = dictGetKey(de);
 
-        count += pubsubUnsubscribeChannel(c,channel,notify);
+            count += pubsubUnsubscribeChannel(c,channel,notify);
+        }
+        dictReleaseIterator(di);
     }
     /* We were subscribed to nothing? Still reply to the client. */
     if (notify && count == 0) addReplyPubsubUnsubscribed(c,NULL);
-    dictReleaseIterator(di);
     return count;
 }
 

src/server.h

@@ -1195,6 +1195,7 @@ struct redisServer {
                                    to be processed. */
     pid_t child_pid;            /* PID of current child */
     int child_type;             /* Type of current child */
+    client *module_client;      /* "Fake" client to call Redis from modules */
     /* Networking */
     int port;                   /* TCP listening port */
     int tls_port;               /* TLS listening port */

tests/unit/moduleapi/misc.tcl

@@ -16,6 +16,11 @@ start_server {tags {"modules"}} {
         assert { [string match "*cmdstat_module*" $info] }
     }
 
+    test {test RM_Call recursive} {
+        set info [r test.call_generic test.call_generic info commandstats]
+        assert { [string match "*cmdstat_module*" $info] }
+    }
+
     test {test redis version} {
         set version [s redis_version]
         assert_equal $version [r test.redisversion]