Diskless-load emptyDb-related fixes

1. Call emptyDb even in case of diskless-load: We want modules
   to get the same FLUSHDB event as disk-based replication.
2. Do not fire any module events when flushing the backups array.
3. Delete redundant call to signalFlushedDb (Called from emptyDb).

src/db.c

@@ -347,7 +347,10 @@ robj *dbUnshareStringValue(redisDb *db, robj *key, robj *o) {
  * DB number if we want to flush only a single Redis database number.
  *
  * Flags are be EMPTYDB_NO_FLAGS if no special flags are specified or
- * EMPTYDB_ASYNC if we want the memory to be freed in a different thread
+ * 1. EMPTYDB_ASYNC if we want the memory to be freed in a different thread.
+ * 2. EMPTYDB_BACKUP if we want to empty the backup dictionaries created by
+ *    disklessLoadMakeBackups. In that case we only free memory and avoid
+ *    firing module events.
  * and the function to return ASAP.
  *
  * On success the fuction returns the number of keys removed from the
@@ -355,6 +358,8 @@ robj *dbUnshareStringValue(redisDb *db, robj *key, robj *o) {
  * DB number is out of range, and errno is set to EINVAL. */
 long long emptyDbGeneric(redisDb *dbarray, int dbnum, int flags, void(callback)(void*)) {
     int async = (flags & EMPTYDB_ASYNC);
+    int backup = (flags & EMPTYDB_BACKUP); /* Just free the memory, nothing else */
+    RedisModuleFlushInfoV1 fi = {REDISMODULE_FLUSHINFO_VERSION,!async,dbnum};
     long long removed = 0;
 
     if (dbnum < -1 || dbnum >= server.dbnum) {
@@ -362,16 +367,18 @@ long long emptyDbGeneric(redisDb *dbarray, int dbnum, int flags, void(callback)(
         return -1;
     }
 
-    /* Fire the flushdb modules event. */
-    RedisModuleFlushInfoV1 fi = {REDISMODULE_FLUSHINFO_VERSION,!async,dbnum};
-    moduleFireServerEvent(REDISMODULE_EVENT_FLUSHDB,
-                          REDISMODULE_SUBEVENT_FLUSHDB_START,
-                          &fi);
-
-    /* Make sure the WATCHed keys are affected by the FLUSH* commands.
-     * Note that we need to call the function while the keys are still
-     * there. */
-    signalFlushedDb(dbnum);
+    /* Pre-flush actions */
+    if (!backup) {
+        /* Fire the flushdb modules event. */
+        moduleFireServerEvent(REDISMODULE_EVENT_FLUSHDB,
+                              REDISMODULE_SUBEVENT_FLUSHDB_START,
+                              &fi);
+
+        /* Make sure the WATCHed keys are affected by the FLUSH* commands.
+         * Note that we need to call the function while the keys are still
+         * there. */
+        signalFlushedDb(dbnum);
+    }
 
     int startdb, enddb;
     if (dbnum == -1) {
@@ -390,20 +397,24 @@ long long emptyDbGeneric(redisDb *dbarray, int dbnum, int flags, void(callback)(
             dictEmpty(dbarray[j].expires,callback);
         }
     }
-    if (server.cluster_enabled) {
-        if (async) {
-            slotToKeyFlushAsync();
-        } else {
-            slotToKeyFlush();
+
+    /* Post-flush actions */
+    if (!backup) {
+        if (server.cluster_enabled) {
+            if (async) {
+                slotToKeyFlushAsync();
+            } else {
+                slotToKeyFlush();
+            }
         }
-    }
-    if (dbnum == -1) flushSlaveKeysWithExpireList();
+        if (dbnum == -1) flushSlaveKeysWithExpireList();
 
-    /* Also fire the end event. Note that this event will fire almost
-     * immediately after the start event if the flush is asynchronous. */
-    moduleFireServerEvent(REDISMODULE_EVENT_FLUSHDB,
-                          REDISMODULE_SUBEVENT_FLUSHDB_END,
-                          &fi);
+        /* Also fire the end event. Note that this event will fire almost
+         * immediately after the start event if the flush is asynchronous. */
+        moduleFireServerEvent(REDISMODULE_EVENT_FLUSHDB,
+                              REDISMODULE_SUBEVENT_FLUSHDB_END,
+                              &fi);
+    }
 
     return removed;
 }

src/replication.c

@@ -1339,8 +1339,8 @@ void disklessLoadRestoreBackups(redisDb *backup, int restore, int empty_db_flags
             server.db[i] = backup[i];
         }
     } else {
-        /* Delete. */
-        emptyDbGeneric(backup,-1,empty_db_flags,replicationEmptyDbCallback);
+        /* Delete (Pass EMPTYDB_BACKUP in order to avoid firing module events) . */
+        emptyDbGeneric(backup,-1,empty_db_flags|EMPTYDB_BACKUP,replicationEmptyDbCallback);
         for (int i=0; i<server.dbnum; i++) {
             dictRelease(backup[i].dict);
             dictRelease(backup[i].expires);
@@ -1524,7 +1524,6 @@ void readSyncBulkPayload(connection *conn) {
     /* We need to stop any AOF rewriting child before flusing and parsing
      * the RDB, otherwise we'll create a copy-on-write disaster. */
     if (server.aof_state != AOF_OFF) stopAppendOnly();
-    signalFlushedDb(-1);
 
     /* When diskless RDB loading is used by replicas, it may be configured
      * in order to save the current DB instead of throwing it away,
@@ -1532,10 +1531,15 @@ void readSyncBulkPayload(connection *conn) {
     if (use_diskless_load &&
         server.repl_diskless_load == REPL_DISKLESS_LOAD_SWAPDB)
     {
+        /* Create a backup of server.db[] and initialize to empty
+         * dictionaries */
         diskless_load_backup = disklessLoadMakeBackups();
-    } else {
-        emptyDb(-1,empty_db_flags,replicationEmptyDbCallback);
     }
+    /* We call to emptyDb even in case of REPL_DISKLESS_LOAD_SWAPDB
+     * (Where disklessLoadMakeBackups left server.db empty) because we
+     * want to execute all the auxiliary logic of emptyDb (Namely,
+     * fire module events) */
+    emptyDb(-1,empty_db_flags,replicationEmptyDbCallback);
 
     /* Before loading the DB into memory we need to delete the readable
      * handler, otherwise it will get called recursively since

src/server.h

@@ -2045,6 +2045,7 @@ robj *dbUnshareStringValue(redisDb *db, robj *key, robj *o);
 
 #define EMPTYDB_NO_FLAGS 0      /* No flags. */
 #define EMPTYDB_ASYNC (1<<0)    /* Reclaim memory in another thread. */
+#define EMPTYDB_BACKUP (1<<2)   /* DB array is a backup for REPL_DISKLESS_LOAD_SWAPDB. */
 long long emptyDb(int dbnum, int flags, void(callback)(void*));
 long long emptyDbGeneric(redisDb *dbarray, int dbnum, int flags, void(callback)(void*));
 void flushAllDataAndResetRDB(int flags);