My response here is in addition to Dscho's remarks on this topic. He makes excellent points that I'll just #include here. I do want to add my own $0.02 here. On 7/1/21 6:18 PM, Ævar Arnfjörð Bjarmason wrote:
On Thu, Jul 01 2021, Jeff Hostetler via GitGitGadget wrote:+#ifdef GIT_WINDOWS_NATIVE +/* + * Create a background process to run the daemon. It should be completely + * disassociated from the terminal. + * + * Conceptually like `daemonize()` but different because Windows does not + * have `fork(2)`. Spawn a normal Windows child process but without the + * limitations of `start_command()` and `finish_command()`. + * + * The child process execs the "git fsmonitor--daemon run" command. + * + * The current process returns so that the caller can wait for the child + * to startup before exiting. + */ +static int spawn_background_fsmonitor_daemon(pid_t *pid) +{ + char git_exe[MAX_PATH]; + struct strvec args = STRVEC_INIT; + int in, out; + + GetModuleFileNameA(NULL, git_exe, MAX_PATH); + + in = open("/dev/null", O_RDONLY); + out = open("/dev/null", O_WRONLY); + + strvec_push(&args, git_exe); + strvec_push(&args, "fsmonitor--daemon"); + strvec_push(&args, "run"); + strvec_pushf(&args, "--ipc-threads=%d", fsmonitor__ipc_threads); + + *pid = mingw_spawnvpe(args.v[0], args.v, NULL, NULL, in, out, out); + close(in); + close(out); + + strvec_clear(&args); + + if (*pid < 0) + return error(_("could not spawn fsmonitor--daemon in the background")); + + return 0; +} +#else +/* + * Create a background process to run the daemon. It should be completely + * disassociated from the terminal. + * + * This is adapted from `daemonize()`. Use `fork()` to directly + * create and run the daemon in the child process. + * + * The fork-child can just call the run code; it does not need to exec + * it. + * + * The fork-parent returns the child PID so that we can wait for the + * child to startup before exiting. + */ +static int spawn_background_fsmonitor_daemon(pid_t *pid) +{ + *pid = fork(); + + switch (*pid) { + case 0: + if (setsid() == -1) + error_errno(_("setsid failed")); + close(0); + close(1); + close(2); + sanitize_stdfds(); + + return !!fsmonitor_run_daemon(); + + case -1: + return error_errno(_("could not spawn fsmonitor--daemon in the background")); + + default: + return 0; + } +} +#endifThe spawn_background_fsmonitor_daemon() function here is almost the same as daemonize(). I wonder if this & the Windows-specific one you have here can't be refactored into an API from what's now in setup.c. Then we could make builtin/gc.c and daemon.c use that, so Windows could have background GC, and we'd have a more battle-tested central codepath for this tricky bit.
I'd rather not refactor all of this and add unnecessary generality and complexity just to save duplicating some of the code in daemonize(). And I'd rather not destabilize existing commands like gc and daemon by changing the daemonize() layer on them. If those commands need help, let's have a separate conversation _later_ about what help they need and if it makes sense to combine them.
It seems to me like the only limitations on it are to have this return slightly more general things (e.g. not set its own errors, return structured data), and maybe some callback for what to do in the child/parent.
There are several issues here when trying to start a background process and we're already on the edge of the behavioral differences between Windows and Unix -- let's not make things more confusing with multiple callbacks, returning structures, custom errors, and etc. Also, since Windows doesn't do fork(), we don't have child/parent branches in the call, so this whole "just pretend it's all Unix" model doesn't work. Even if we did pretend I'd still need ifdef'd callback routines to either call `fsmonitor_run_daemon()` or build a command line (or have blocks of functions that "just happen to never be called on one platform or the other"). What I have here is an API that the primary (read: parent) calls and gets back a 0 or -1 (with error message). And that's it. The primary can then wait for the child (whether from fork or CreateProcess) to become responsive or fail to start. And then the primary can exit (with or without error). So I think we're good. Yes, there is an ifdef here, but I think it is worth it.
+/* + * This is adapted from `wait_or_whine()`. Watch the child process and + * let it get started and begin listening for requests on the socket + * before reporting our success. + */ +static int wait_for_background_startup(pid_t pid_child) +{ + int status; + pid_t pid_seen; + enum ipc_active_state s; + time_t time_limit, now; + + time(&time_limit); + time_limit += fsmonitor__start_timeout_sec; + + for (;;) { + pid_seen = waitpid(pid_child, &status, WNOHANG); + + if (pid_seen == -1) + return error_errno(_("waitpid failed")); + else if (pid_seen == 0) { + /* + * The child is still running (this should be + * the normal case). Try to connect to it on + * the socket and see if it is ready for + * business. + * + * If there is another daemon already running, + * our child will fail to start (possibly + * after a timeout on the lock), but we don't + * care (who responds) if the socket is live. + */ + s = fsmonitor_ipc__get_state(); + if (s == IPC_STATE__LISTENING) + return 0; + + time(&now); + if (now > time_limit) + return error(_("fsmonitor--daemon not online yet")); + } else if (pid_seen == pid_child) { + /* + * The new child daemon process shutdown while + * it was starting up, so it is not listening + * on the socket. + * + * Try to ping the socket in the odd chance + * that another daemon started (or was already + * running) while our child was starting. + * + * Again, we don't care who services the socket. + */ + s = fsmonitor_ipc__get_state(); + if (s == IPC_STATE__LISTENING) + return 0; + + /* + * We don't care about the WEXITSTATUS() nor + * any of the WIF*(status) values because + * `cmd_fsmonitor__daemon()` does the `!!result` + * trick on all function return values. + * + * So it is sufficient to just report the + * early shutdown as an error. + */ + return error(_("fsmonitor--daemon failed to start")); + } else + return error(_("waitpid is confused")); + } +}Ditto this. could we extend the wait_or_whine() function (or some extended version thereof) to do what you need with callbacks? It seems the main difference is just being able to pass down a flag for waitpid(), and the loop needing to check EINTR or not depending on whether WNOHANG is passed. For e.g. the "We don't care about the WEXITSTATUS()" you'd get that behavior with an adjusted wait_or_whine(). Wouldn't it be better to report what exit status it exits with e.g. if the top-level process is signalled? We do so in trace2 for other things we spawn...
Again, I don't want to mix my usage here with the existing code and destabilize all existing callers. Here we are spinning to give the child a chance to *start* and confirm that it is in a listening state and ready for connections. We do not wait for the child to exit (unless it dies quickly without becoming ready). We want to end our wait as soon as we confirm that the child is ready and return. All I really need from the system is `waitpid()`. Also, since we started the child in my `spawn_background...()`, it is not in the `children_to_clean` list, so there is no need to mess with that. So I'd like to leave this as is. Jeff
