In forkrun(), fix SIGCHLD waiting and test it

Spotted by Thomas Gleixner <tglx@linutronix.de>, THANK YOU

forkrun() never properly handled the case that a child exited before
the timeout expired, because we had failed to block the signal and
hence never received a SIGCHLD. This was overlooked because this
functionality was never relevant (it only delayed test execution) and
because we did not explicitly test it.

Related to #31

src/fellow_log_test.c

@@ -50,6 +50,7 @@
 #include "fellow_hash.h"
 #include "fellow_testenv.h"
 
+typedef void child_f(void);
 typedef void t_f(struct fellow_fd *ffd, void *t_priv);
 
 typedef void t_resur_init_f(void **priv);
@@ -57,6 +58,7 @@ typedef void t_resur_fini_f(void **priv);
 
 struct tc {
 	const char * const	name;
+	child_f		*child_f;
 	t_f			*t_f;
 	fellow_resurrect_f	*resur_f;
 	t_resur_init_f		*resur_init_f;
@@ -78,6 +80,20 @@ randalloc(buddy_t *buddy, double max)
 	return (e);
 }
 
+/* ----------------------------------------------------------------------
+ * simply exit from child
+ */
+static void v_matchproto_(child_f)
+t_exit(void)
+{
+	exit(0);
+}
+
+const struct tc tc_SIGCHLD = {
+	.name		 = "tc_SIGCHLD - just exit, test for forkrun()",
+	.child_f	 = t_exit
+};
+
 /* ----------------------------------------------------------------------
  * test log append
  */
@@ -668,6 +684,9 @@ run(const struct tc *tc, void *t_priv)
 	unsigned l, h;
 	int r = 0;
 
+	if (tc->child_f)
+		tc->child_f();
+
 #ifdef HAVE_XXHASH_H
 	l = FH_LIM;
 #else
@@ -687,9 +706,15 @@ run(const struct tc *tc, void *t_priv)
 int
 forkrun(const struct tc *tc, double killafter, void *t_priv)
 {
+	struct timespec timeo;
+	sigset_t mask;
 	pid_t pid;
 	int i;
 
+	sigemptyset(&mask);
+	sigaddset(&mask, SIGCHLD);
+	sigprocmask(SIG_BLOCK, &mask, NULL);
+
 	pid = fork();
 	assert(pid != -1);
 	if (pid == 0)
@@ -700,29 +725,29 @@ forkrun(const struct tc *tc, double killafter, void *t_priv)
 		assert(pid != -1);
 		if (! WIFEXITED(i) || WEXITSTATUS(i) != 0)
 			exit(i);
+		sigprocmask(SIG_UNBLOCK, &mask, NULL);
 		return (WEXITSTATUS(i));
 	}
 
-	// idea from https://stackoverflow.com/a/20173592
-	sigset_t mask;
-	struct timespec timeo;
-
-	sigemptyset(&mask);
-	sigaddset(&mask, SIGCHLD);
-
 	// from vtim.c
 	timeo.tv_sec = (time_t)trunc(killafter);
 	timeo.tv_nsec = (int)(1e9 * (killafter - timeo.tv_sec));
 
 	DBG("killtimer %f", killafter);
 	i = sigtimedwait(&mask, NULL, &timeo);
-	if (i < 0) {
+
+	if (tc == &tc_SIGCHLD)
+		assert(i == SIGCHLD);	// self test
+	else if (i < 0) {
 		// XXX WHY EINTR? #31
 		assert(errno == EAGAIN || errno == EINTR);
 		i = kill(pid, SIGABRT);
 		assert(i == 0 || i == ESRCH);
-	}
+	} else
+		assert(i == SIGCHLD);
+
 	(void) waitpid(pid, &i, 0);
+	sigprocmask(SIG_UNBLOCK, &mask, NULL);
 	return (0);
 }
 
@@ -763,6 +788,9 @@ main(int argc, char **argv)
 	if (argc == 2 && *argv[1] != '-')
 		filename = argv[1];
 
+	// test forkrun SIGCHLD
+	AZ(forkrun(&tc_SIGCHLD, 1.0, 0));
+
 	AZ(run(&tc_adddel_netzero, 0));
 
 	AZ(run(&tc_log_append, 0));