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Block ValkeyModule_Fork while a save is in progress #4478

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Block ValkeyModule_Fork while a save is in progress #4478

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7 changes: 7 additions & 0 deletions src/module.c
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Original file line number Diff line number Diff line change
Expand Up @@ -12256,6 +12256,13 @@ int VM_ScanKey(ValkeyModuleKey *key, ValkeyModuleScanCursor *cursor, ValkeyModul
int VM_Fork(ValkeyModuleForkDoneHandler cb, void *user_data) {
pid_t childpid;

/* A module fork must not run concurrently with a background save. */
if (isSaveInProgress()) {
serverLog(LL_WARNING, "Can't fork for module: a background save is in progress");
errno = EALREADY;
return -1;
}

Comment on lines +12259 to +12265

@JimB123 JimB123 Aug 20, 2026

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I think the better place to do this is within serverFork.

The code in serverFork already checks for certain child types and calls hasActiveChildProcess.

I'd suggest that the best thing would be something like this:

int serverFork(int purpose) {
    if (isMutuallyExclusiveChildType(purpose)) {
        if (hasActiveChildProcess() || bgIteration_iterationActive()) {  // better?
            errno = EALREADY;
            return -1;
        }

        ...

if ((childpid = serverFork(CHILD_TYPE_MODULE)) == 0) {
/* Child */
if (strstr(server.exec_argv[0], "redis-server") != NULL) {
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