add test case and comments for active expiry in the writeable replica (#11789)

This test case is to cover a edge scenario: when a writable replica enabled AOF
at the same time, active expiry keys which was created in writable replicas should
propagate to the AOF file, and some versions might crash (fixed by #11615).
For details, please refer to #11778

src/expire.c

@@ -385,7 +385,8 @@ void expireSlaveKeys(void) {
                     activeExpireCycleTryExpire(server.db+dbid,expire,start))
                 {
                     expired = 1;
-                    /* DELs aren't propagated, but modules may want their hooks. */
+                    /* Propagate the DEL (writable replicas do not propagate anything to other replicas,
+                     * but they might propagate to AOF) and trigger module hooks. */
                     postExecutionUnitOperations();
                 }
 

tests/unit/expire.tcl

@@ -571,6 +571,23 @@ start_server {tags {"expire"}} {
                 assert_equal [$primary pexpiretime $key] [$replica pexpiretime $key]
             }
         }
+
+        test {expired key which is created in writeable replicas should be deleted by active expiry} {
+            $primary flushall
+            $replica config set replica-read-only no
+            foreach {yes_or_no} {yes no} {
+                $replica config set appendonly $yes_or_no
+                waitForBgrewriteaof $replica
+                set prev_expired [s expired_keys]
+                $replica set foo bar PX 1
+                wait_for_condition 100 10 {
+                    [s expired_keys] eq $prev_expired + 1
+                } else {
+                    fail "key not expired"
+                }
+                assert_equal {} [$replica get foo]
+            }
+        }
     }
 
     test {SET command will remove expire} {