Check OOM at script start to get stable lua OOM state.

Checking OOM by `getMaxMemoryState` inside script might get different result
with `freeMemoryIfNeededAndSafe` at script start, because lua stack and
arguments also consume memory.

This leads to memory `borderline` when memory grows near server.maxmemory:

- `freeMemoryIfNeededAndSafe` at script start detects no OOM, no memory freed
- `getMaxMemoryState` inside script detects OOM, script aborted

We solve this 'borderline' issue by saving OOM state at script start to get
stable lua OOM state.

related to issue #6565 and #5250.

3 files changed, 11 insertions, 4 deletions

diff --git a/src/scripting.c b/src/scripting.c
index 9282b7fd9..5ed24ffad 100644
--- a/src/scripting.c
+++ b/src/scripting.c
@@ -655,12 +655,11 @@ int luaRedisGenericCommand(lua_State *lua, int raise_error) {
         !server.loading &&              /* Don't care about mem if loading. */
         !server.masterhost &&           /* Slave must execute the script. */
         server.lua_write_dirty == 0 &&  /* Script had no side effects so far. */
+        server.lua_oom &&               /* Detected OOM when script start. */
         (cmd->flags & CMD_DENYOOM))
     {
-        if (getMaxmemoryState(NULL,NULL,NULL,NULL) != C_OK) {
-            luaPushError(lua, shared.oomerr->ptr);
-            goto cleanup;
-        }
+        luaPushError(lua, shared.oomerr->ptr);
+        goto cleanup;
     }
 
     if (cmd->flags & CMD_RANDOM) server.lua_random_dirty = 1;
diff --git a/src/server.c b/src/server.c
index 2b226f568..21d6aab27 100644
--- a/src/server.c
+++ b/src/server.c
@@ -3442,6 +3442,13 @@ int processCommand(client *c) {
             addReply(c, shared.oomerr);
             return C_OK;
         }
+
+        /* Save out_of_memory result at script start, otherwise if we check OOM
+         * untill first write within script, memory used by lua stack and
+         * arguments might interfere. */
+        if (c->cmd->proc == evalCommand || c->cmd->proc == evalShaCommand) {
+            server.lua_oom = out_of_memory;
+        }
     }
 
     /* Make sure to use a reasonable amount of memory for client side
diff --git a/src/server.h b/src/server.h
index 8e354c03d..b440beba3 100644
--- a/src/server.h
+++ b/src/server.h
@@ -1376,6 +1376,7 @@ struct redisServer {
                              execution. */
     int lua_kill;         /* Kill the script if true. */
     int lua_always_replicate_commands; /* Default replication type. */
+    int lua_oom;          /* OOM detected when script start? */
     /* Lazy free */
     int lazyfree_lazy_eviction;
     int lazyfree_lazy_expire;