From 6c001bfc0ddf8e7a93e86dad250e416166253d85 Mon Sep 17 00:00:00 2001
From: antirez <antirez@gmail.com>
Date: Mon, 3 Sep 2018 18:39:18 +0200
Subject: [PATCH] Unblocked clients API refactoring. See #4418.

---
 src/blocked.c    | 32 ++++++++++++++++++++++++++------
 src/networking.c |  9 ++++-----
 src/scripting.c  |  6 ++----
 src/server.h     |  1 +
 4 files changed, 33 insertions(+), 15 deletions(-)

diff --git a/src/blocked.c b/src/blocked.c
index aeca87a6..00212ed6 100644
--- a/src/blocked.c
+++ b/src/blocked.c
@@ -132,6 +132,31 @@ void processUnblockedClients(void) {
     }
 }
 
+/* This function will schedule the client for reprocessing at a safe time.
+ *
+ * This is useful when a client was blocked for some reason (blocking opeation,
+ * CLIENT PAUSE, or whatever), because it may end with some accumulated query
+ * buffer that needs to be processed ASAP:
+ *
+ * 1. When a client is blocked, its readable handler is still active.
+ * 2. However in this case it only gets data into the query buffer, but the
+ *    query is not parsed or executed once there is enough to proceed as
+ *    usually (because the client is blocked... so we can't execute commands).
+ * 3. When the client is unblocked, without this function, the client would
+ *    have to write some query in order for the readable handler to finally
+ *    call processQueryBuffer*() on it.
+ * 4. With this function instead we can put the client in a queue that will
+ *    process it for queries ready to be executed at a safe time.
+ */
+void queueClientForReprocessing(client *c) {
+    /* The client may already be into the unblocked list because of a previous
+     * blocking operation, don't add back it into the list multiple times. */
+    if (!(c->flags & CLIENT_UNBLOCKED)) {
+        c->flags |= CLIENT_UNBLOCKED;
+        listAddNodeTail(server.unblocked_clients,c);
+    }
+}
+
 /* Unblock a client calling the right function depending on the kind
  * of operation the client is blocking for. */
 void unblockClient(client *c) {
@@ -152,12 +177,7 @@ void unblockClient(client *c) {
     server.blocked_clients_by_type[c->btype]--;
     c->flags &= ~CLIENT_BLOCKED;
     c->btype = BLOCKED_NONE;
-    /* The client may already be into the unblocked list because of a previous
-     * blocking operation, don't add back it into the list multiple times. */
-    if (!(c->flags & CLIENT_UNBLOCKED)) {
-        c->flags |= CLIENT_UNBLOCKED;
-        listAddNodeTail(server.unblocked_clients,c);
-    }
+    queueClientForReprocessing(c);
 }
 
 /* This function gets called when a blocked client timed out in order to
diff --git a/src/networking.c b/src/networking.c
index 824d8824..0c1b3016 100644
--- a/src/networking.c
+++ b/src/networking.c
@@ -2134,11 +2134,10 @@ int clientsArePaused(void) {
         while ((ln = listNext(&li)) != NULL) {
             c = listNodeValue(ln);
 
-            /* Don't touch slaves and blocked or unblocked clients.
-             * The latter pending requests be processed when unblocked. */
-            if (c->flags & (CLIENT_SLAVE|CLIENT_BLOCKED|CLIENT_UNBLOCKED)) continue;
-            c->flags |= CLIENT_UNBLOCKED;
-            listAddNodeTail(server.unblocked_clients,c);
+            /* Don't touch slaves and blocked clients.
+             * The latter pending requests will be processed when unblocked. */
+            if (c->flags & (CLIENT_SLAVE|CLIENT_BLOCKED)) continue;
+            queueClientForReprocessing(c);
         }
     }
     return server.clients_paused;
diff --git a/src/scripting.c b/src/scripting.c
index 6c311dbe..8aa62071 100644
--- a/src/scripting.c
+++ b/src/scripting.c
@@ -1367,10 +1367,8 @@ void evalGenericCommand(client *c, int evalsha) {
          * script timeout was detected. */
         aeCreateFileEvent(server.el,c->fd,AE_READABLE,
                           readQueryFromClient,c);
-        if (server.masterhost && server.master && !(server.master->flags & CLIENT_UNBLOCKED)) {
-            server.master->flags |= CLIENT_UNBLOCKED;
-            listAddNodeTail(server.unblocked_clients,server.master);
-        }
+        if (server.masterhost && server.master)
+            queueClientForReprocessing(server.master);
     }
     server.lua_caller = NULL;
 
diff --git a/src/server.h b/src/server.h
index 4a5967f1..09348585 100644
--- a/src/server.h
+++ b/src/server.h
@@ -1884,6 +1884,7 @@ sds luaCreateFunction(client *c, lua_State *lua, robj *body);
 void processUnblockedClients(void);
 void blockClient(client *c, int btype);
 void unblockClient(client *c);
+void queueClientForReprocessing(client *c);
 void replyToBlockedClientTimedOut(client *c);
 int getTimeoutFromObjectOrReply(client *c, robj *object, mstime_t *timeout, int unit);
 void disconnectAllBlockedClients(void);