EXEC with only read commands should not be rejected when OOM (#7696)

If the server gets MULTI command followed by only read
commands, and right before it gets the EXEC it reaches OOM,
the client will get OOM response.

So, from now on, it will get OOM response only if there was
at least one command that was tagged with `use-memory` flag

src/server.c

@@ -3625,14 +3625,19 @@ int processCommand(client *c) {
          * into a slave, that may be the active client, to be freed. */
         if (server.current_client == NULL) return C_ERR;
 
-        /* It was impossible to free enough memory, and the command the client
-         * is trying to execute is denied during OOM conditions or the client
-         * is in MULTI/EXEC context? Error. */
-        if (out_of_memory &&
-            (is_denyoom_command ||
-             (c->flags & CLIENT_MULTI &&
-              c->cmd->proc != discardCommand)))
-        {
+        int reject_cmd_on_oom = is_denyoom_command;
+        /* If client is in MULTI/EXEC context, queuing may consume an unlimited
+         * amount of memory, so we want to stop that.
+         * However, we never want to reject DISCARD, or even EXEC (unless it
+         * contains denied commands, in which case is_denyoom_command is already
+         * set. */
+        if (c->flags & CLIENT_MULTI &&
+            c->cmd->proc != execCommand &&
+            c->cmd->proc != discardCommand) {
+            reject_cmd_on_oom = 1;
+        }
+
+        if (out_of_memory && reject_cmd_on_oom) {
             rejectCommand(c, shared.oomerr);
             return C_OK;
         }

tests/unit/multi.tcl

@@ -466,4 +466,42 @@ start_server {tags {"multi"}} {
         assert { $xx == 1 }
         $r1 close;
     }
+
+    test {EXEC with only read commands should not be rejected when OOM} {
+        set r2 [redis_client]
+
+        r set x value
+        r multi
+        r get x
+        r ping
+
+        # enforcing OOM
+        $r2 config set maxmemory 1
+
+        # finish the multi transaction with exec
+        assert { [r exec] == {value PONG} }
+
+        # releasing OOM
+        $r2 config set maxmemory 0
+        $r2 close
+    }
+
+    test {EXEC with at least one use-memory command should fail} {
+        set r2 [redis_client]
+
+        r multi
+        r set x 1
+        r get x
+
+        # enforcing OOM
+        $r2 config set maxmemory 1
+
+        # finish the multi transaction with exec
+        catch {r exec} e
+        assert_match {EXECABORT*OOM*} $e
+
+        # releasing OOM
+        $r2 config set maxmemory 0
+        $r2 close
+    }
 }