Fix RANDOMKEY infinite loop during CLIENT PAUSE (#1850)

When the `client pause write` is set and all the keys in the server
are expired keys, executing the `randomkey` command will lead to an
infinite loop.

The reason is that expired keys are not deleted in this case. Limit
the number of tries and return an expired key after the max number
tries in this case.

Closes #1848.

---------

Signed-off-by: li-benson <[email protected]>
Signed-off-by: youngmore <[email protected]>
Signed-off-by: Viktor Söderqvist <[email protected]>
Co-authored-by: youngmore <[email protected]>
Co-authored-by: Viktor Söderqvist <[email protected]>

Author: 3 people

src/db.c

@@ -445,7 +445,7 @@ robj *dbRandomKey(serverDb *db) {
         sds key = objectGetKey(valkey);
         robj *keyobj = createStringObject(key, sdslen(key));
         if (objectIsExpired(valkey)) {
-            if (allvolatile && (server.primary_host || server.import_mode) && --maxtries == 0) {
+            if (allvolatile && (server.primary_host || server.import_mode || isPausedActions(PAUSE_ACTION_EXPIRE)) && --maxtries == 0) {
                 /* If the DB is composed only of keys with an expire set,
                  * it could happen that all the keys are already logically
                  * expired in the replica, so the function cannot stop because

tests/unit/pause.tcl

@@ -412,6 +412,28 @@ start_server {tags {"pause network"}} {
         } {bar2}
     }
 
+    test "Test the randomkey command will not cause the server to get into an infinite loop during the client pause write" {
+        # first, clear the database to avoid interference from existing keys on the test results 
+        r flushall
+
+        # then set a key with expire time
+        r set key value px 3
+
+        # set pause-write model and wait key expired
+        r client pause 10000 write
+        after 5
+
+        wait_for_condition 50 100 {
+            [r randomkey] == "key"
+        } else {
+            fail "execute randomkey failed, caused by the infinite loop"
+        }
+
+        r client unpause
+        assert_equal [r randomkey] {}
+
+    }
+
     # Make sure we unpause at the end
     r client unpause
 }