Modules: Add event for fork child birth and termination (#8289)

Useful to avoid doing background jobs that can cause excessive COW

src/module.c

@@ -7899,6 +7899,14 @@ void ModuleForkDoneHandler(int exitcode, int bysignal) {
  *     * `REDISMODULE_SUBEVENT_REPL_BACKUP_RESTORE`
  *     * `REDISMODULE_SUBEVENT_REPL_BACKUP_DISCARD`
  *
+ * * RedisModuleEvent_ForkChild
+ *
+ *     Called when a fork child (AOFRW, RDBSAVE, module fork...) is born/dies
+ *     The following sub events are available:
+ *
+ *     * `REDISMODULE_SUBEVENT_FORK_CHILD_BORN`
+ *     * `REDISMODULE_SUBEVENT_FORK_CHILD_DIED`
+ *
  * The function returns REDISMODULE_OK if the module was successfully subscribed
  * for the specified event. If the API is called from a wrong context or unsupported event
  * is given then REDISMODULE_ERR is returned. */
@@ -7971,6 +7979,8 @@ int RM_IsSubEventSupported(RedisModuleEvent event, int64_t subevent) {
         return subevent < _REDISMODULE_SUBEVENT_SWAPDB_NEXT;
     case REDISMODULE_EVENT_REPL_BACKUP:
         return subevent < _REDISMODULE_SUBEVENT_REPL_BACKUP_NEXT;
+    case REDISMODULE_EVENT_FORK_CHILD:
+        return subevent < _REDISMODULE_SUBEVENT_FORK_CHILD_NEXT;
     default:
         break;
     }

src/redismodule.h

@@ -230,9 +230,8 @@ typedef uint64_t RedisModuleTimerID;
 #define REDISMODULE_EVENT_LOADING_PROGRESS 10
 #define REDISMODULE_EVENT_SWAPDB 11
 #define REDISMODULE_EVENT_REPL_BACKUP 12
-
-/* Next event flag, should be updated if a new event added. */
-#define _REDISMODULE_EVENT_NEXT 13
+#define REDISMODULE_EVENT_FORK_CHILD 13
+#define _REDISMODULE_EVENT_NEXT 14 /* Next event flag, should be updated if a new event added. */
 
 typedef struct RedisModuleEvent {
     uint64_t id;        /* REDISMODULE_EVENT_... defines. */
@@ -295,6 +294,10 @@ static const RedisModuleEvent
     RedisModuleEvent_ReplBackup = {
         REDISMODULE_EVENT_REPL_BACKUP,
         1
+    },
+    RedisModuleEvent_ForkChild = {
+        REDISMODULE_EVENT_FORK_CHILD,
+        1
     };
 
 /* Those are values that are used for the 'subevent' callback argument. */
@@ -345,6 +348,10 @@ static const RedisModuleEvent
 #define REDISMODULE_SUBEVENT_REPL_BACKUP_DISCARD 2
 #define _REDISMODULE_SUBEVENT_REPL_BACKUP_NEXT 3
 
+#define REDISMODULE_SUBEVENT_FORK_CHILD_BORN 0
+#define REDISMODULE_SUBEVENT_FORK_CHILD_DIED 1
+#define _REDISMODULE_SUBEVENT_FORK_CHILD_NEXT 2
+
 #define _REDISMODULE_SUBEVENT_SHUTDOWN_NEXT 0
 #define _REDISMODULE_SUBEVENT_CRON_LOOP_NEXT 0
 #define _REDISMODULE_SUBEVENT_SWAPDB_NEXT 0

src/server.c

@@ -1600,6 +1600,9 @@ void resetChildState() {
     server.stat_current_cow_bytes = 0;
     updateDictResizePolicy();
     closeChildInfoPipe();
+    moduleFireServerEvent(REDISMODULE_EVENT_FORK_CHILD,
+                          REDISMODULE_SUBEVENT_FORK_CHILD_DIED,
+                          NULL);
 }
 
 /* Return if child type is mutual exclusive with other fork children */
@@ -5567,6 +5570,9 @@ int redisFork(int purpose) {
         }
 
         updateDictResizePolicy();
+        moduleFireServerEvent(REDISMODULE_EVENT_FORK_CHILD,
+                              REDISMODULE_SUBEVENT_FORK_CHILD_BORN,
+                              NULL);
     }
     return childpid;
 }