mingw: kill unterminated child processes on signals

Git for Windows' MSYS2 runtime was just adjusted to kill processes
gently, by injecting a thread that calls ExitProcess(). In case of
signals (such as when handling Ctrl+C in a MinTTY window), the exit code
is 128 + sign_no, as expected by Git's source code.

However, as there is no POSIX signal handling on Windows, no signal
handlers are called. Instead, functions registered via atexit() are
called. We work around that by testing the exit code explicitly.

This fixes the Git for Windows side of the bug where  interrupting `git
clone https://...` would send the spawned-off `git remote-https` process
into the background instead of interrupting it, i.e. the clone would
continue and its progress would be reported mercilessly to the console
window without the user being able to do anything about it (short of
firing up the task manager and killing the appropriate task manually).

Signed-off-by: Johannes Schindelin <[email protected]>

compat/mingw.c

@@ -1499,10 +1499,44 @@ struct pinfo_t {
 static struct pinfo_t *pinfo = NULL;
 CRITICAL_SECTION pinfo_cs;
 
+#ifndef SIGRTMAX
+#define SIGRTMAX 63
+#endif
+
+static void kill_child_processes_on_signal(void)
+{
+	DWORD status;
+
+	/*
+	 * Only continue if the process was terminated by a signal, as
+	 * indicated by the exit status (128 + sig_no).
+	 *
+	 * As we are running in an atexit() handler, the exit code has been
+	 * set at this stage by the ExitProcess() function already.
+	 */
+	if (!GetExitCodeProcess(GetCurrentProcess(), &status) ||
+	    status <= 128 || status > 128 + SIGRTMAX)
+		return;
+
+	EnterCriticalSection(&pinfo_cs);
+
+	while (pinfo) {
+		struct pinfo_t *info = pinfo;
+		pinfo = pinfo->next;
+		if (exit_process(info->proc, status))
+			/* the handle is still valid in case of error */
+			CloseHandle(info->proc);
+		free(info);
+	}
+
+	LeaveCriticalSection(&pinfo_cs);
+}
+
 static pid_t mingw_spawnve_fd(const char *cmd, const char **argv, char **deltaenv,
 			      const char *dir,
 			      int prepend_cmd, int fhin, int fhout, int fherr)
 {
+	static int atexit_handler_initialized;
 	STARTUPINFOW si;
 	PROCESS_INFORMATION pi;
 	struct strbuf args;
@@ -1512,6 +1546,17 @@ static pid_t mingw_spawnve_fd(const char *cmd, const char **argv, char **deltaen
 	HANDLE cons;
 	const char *strace_env;
 
+	if (!atexit_handler_initialized) {
+		atexit_handler_initialized = 1;
+		/*
+		 * On Windows, there is no POSIX signaling. Instead, we inject
+		 * a thread calling ExitProcess(128 + sig_no); and that calls
+		 * the *atexit* handlers. Catch this condition and kill child
+		 * processes with the same signal.
+		 */
+		atexit(kill_child_processes_on_signal);
+	}
+
 	do_unset_environment_variables();
 
 	/* Determine whether or not we are associated to a console */