XACK should be executed in a "all or nothing" fashion.

First, we must parse the IDs, so that we abort ASAP.
The return value of this command cannot be an error if
the client successfully acknowledged some messages,
so it should be executed in a "all or nothing" fashion.

src/t_stream.c

@@ -1921,11 +1921,21 @@ void xackCommand(client *c) {
         return;
     }
 
+    /* Start parsing the IDs, so that we abort ASAP if there is a syntax
+     * error: the return value of this command cannot be an error in case
+     * the client successfully acknowledged some messages, so it should be
+     * executed in a "all or nothing" fashion. */
+    for (int j = 3; j < c->argc; j++) {
+        streamID id;
+        if (streamParseStrictIDOrReply(c,c->argv[j],&id,0) != C_OK) return;
+    }
+
     int acknowledged = 0;
     for (int j = 3; j < c->argc; j++) {
         streamID id;
         unsigned char buf[sizeof(streamID)];
-        if (streamParseStrictIDOrReply(c,c->argv[j],&id,0) != C_OK) return;
+        if (streamParseStrictIDOrReply(c,c->argv[j],&id,0) != C_OK)
+            serverPanic("StreamID invalid after check. Should not be possible.");
         streamEncodeID(buf,&id);
 
         /* Lookup the ID in the group PEL: it will have a reference to the

tests/unit/type/stream-cgroups.tcl

@@ -93,6 +93,18 @@ start_server {
         assert {[r XACK mystream mygroup $id1 $id2] eq 1}
     }
 
+    test {XACK should fail if got at least one invalid ID} {
+        r del mystream
+        r xgroup create s g $ MKSTREAM
+        r xadd s * f1 v1
+        set c [llength [lindex [r xreadgroup group g c streams s >] 0 1]]
+        assert {$c == 1}
+        set pending [r xpending s g - + 10 c]
+        set id1 [lindex $pending 0 0]
+        assert_error "*Invalid stream ID specified*" {r xack s g $id1 invalid-id}
+        assert {[r xack s g $id1] eq 1}
+    }
+
     test {PEL NACK reassignment after XGROUP SETID event} {
         r del events
         r xadd events * f1 v1