Fix race in module fork kill test (#10717)

The purpose of the test is to kill the child while it is running.
From the last two lines we can see the child exits before being killed.
```
- Module fork started pid: 56998
* <fork> fork child started
- Killing running module fork child: 56998
* <fork> fork child exiting
signal-handler (1652267501) Received SIGUSR1 in child, exiting now.
```

In this commit, we pass an argument to `fork.create` indicating how
long it should sleep. For the fork kill test, we use a longer time to
avoid the child exiting before being killed.

Other changes:
use wait_for_condition instead of hardcoded `after 250`.
Unify the test for failing fork with the one for killing it (save time) 

1 files changed, 8 insertions, 5 deletions

diff --git a/tests/modules/fork.c b/tests/modules/fork.c
index 44e3971bd..d7a0d154f 100644
--- a/tests/modules/fork.c
+++ b/tests/modules/fork.c
@@ -23,7 +23,8 @@ void done_handler(int exitcode, int bysignal, void *user_data) {
 int fork_create(RedisModuleCtx *ctx, RedisModuleString **argv, int argc)
 {
     long long code_to_exit_with;
-    if (argc != 2) {
+    long long usleep_us;
+    if (argc != 3) {
         RedisModule_WrongArity(ctx);
         return REDISMODULE_OK;
     }
@@ -34,20 +35,22 @@ int fork_create(RedisModuleCtx *ctx, RedisModuleString **argv, int argc)
     }
 
     RedisModule_StringToLongLong(argv[1], &code_to_exit_with);
+    RedisModule_StringToLongLong(argv[2], &usleep_us);
     exitted_with_code = -1;
-    child_pid = RedisModule_Fork(done_handler, (void*)0xdeadbeef);
-    if (child_pid < 0) {
+    int fork_child_pid = RedisModule_Fork(done_handler, (void*)0xdeadbeef);
+    if (fork_child_pid < 0) {
         RedisModule_ReplyWithError(ctx, "Fork failed");
         return REDISMODULE_OK;
-    } else if (child_pid > 0) {
+    } else if (fork_child_pid > 0) {
         /* parent */
+        child_pid = fork_child_pid;
         RedisModule_ReplyWithLongLong(ctx, child_pid);
         return REDISMODULE_OK;
     }
 
     /* child */
     RedisModule_Log(ctx, "notice", "fork child started");
-    usleep(500000);
+    usleep(usleep_us);
     RedisModule_Log(ctx, "notice", "fork child exiting");
     RedisModule_ExitFromChild(code_to_exit_with);
     /* unreachable */