[PATCH 2/2] spawn: better error handling

[Eric Wong <e@80x24.org>]

Since vfork always shares memory between the child and parent,
we can propagate errors to the parent errno using shared memory
instead of just dumping to stderr and hoping somebody sees it.
---
 lib/PublicInbox/Spawn.pm | 42 +++++++++++++++-------------------------
 1 file changed, 16 insertions(+), 26 deletions(-)

diff --git a/lib/PublicInbox/Spawn.pm b/lib/PublicInbox/Spawn.pm
index d624c521..6eea2b9c 100644
--- a/lib/PublicInbox/Spawn.pm
+++ b/lib/PublicInbox/Spawn.pm
@@ -56,26 +56,10 @@ my $vfork_spawn = <<'VFORK_SPAWN';
 	dst[real_len] = 0; \
 } while (0)
 
-static void *deconst(const char *s)
-{
-	union { const char *in; void *out; } u;
-	u.in = s;
-	return u.out;
-}
-
 /* needs to be safe inside a vfork'ed process */
-static void xerr(const char *msg)
+static void exit_err(int *cerrnum)
 {
-	struct iovec iov[3];
-	const char *err = strerror(errno); /* should be safe in practice */
-
-	iov[0].iov_base = deconst(msg);
-	iov[0].iov_len = strlen(msg);
-	iov[1].iov_base = deconst(err);
-	iov[1].iov_len = strlen(err);
-	iov[2].iov_base = deconst("\n");
-	iov[2].iov_len = 1;
-	writev(2, iov, 3);
+	*cerrnum = errno;
 	_exit(1);
 }
 
@@ -95,7 +79,7 @@ int pi_fork_exec(SV *redirref, SV *file, SV *cmdref, SV *envref, SV *rlimref,
 	pid_t pid;
 	char **argv, **envp;
 	sigset_t set, old;
-	int ret, errnum;
+	int ret, perrnum, cerrnum = 0;
 
 	AV2C_COPY(argv, cmd);
 	AV2C_COPY(envp, env);
@@ -115,12 +99,12 @@ int pi_fork_exec(SV *redirref, SV *file, SV *cmdref, SV *envref, SV *rlimref,
 			if (parent_fd == child_fd)
 				continue;
 			if (dup2(parent_fd, child_fd) < 0)
-				xerr("dup2");
+				exit_err(&cerrnum);
 		}
 		for (sig = 1; sig < NSIG; sig++)
 			signal(sig, SIG_DFL); /* ignore errors on signals */
 		if (*cd && chdir(cd) < 0)
-			xerr("chdir");
+			exit_err(&cerrnum);
 
 		max = av_len(rlim);
 		for (i = 0; i < max; i += 3) {
@@ -132,7 +116,7 @@ int pi_fork_exec(SV *redirref, SV *file, SV *cmdref, SV *envref, SV *rlimref,
 			rl.rlim_cur = SvIV(*soft);
 			rl.rlim_max = SvIV(*hard);
 			if (setrlimit(SvIV(*res), &rl) < 0)
-				xerr("sertlimit");
+				exit_err(&cerrnum);
 		}
 
 		/*
@@ -140,13 +124,19 @@ int pi_fork_exec(SV *redirref, SV *file, SV *cmdref, SV *envref, SV *rlimref,
 		 * to the group taking out a subprocess
 		 */
 		execve(filename, argv, envp);
-		xerr("execve failed");
+		exit_err(&cerrnum);
 	}
-	errnum = errno;
+	perrnum = errno;
 	ret = sigprocmask(SIG_SETMASK, &old, NULL);
 	assert(ret == 0 && "BUG calling sigprocmask to restore");
-	errno = errnum;
-
+	if (cerrnum) {
+		if (pid > 0)
+			waitpid(pid, NULL, 0);
+		pid = -1;
+		errno = cerrnum;
+	} else if (perrnum) {
+		errno = perrnum;
+	}
 	return (int)pid;
 }
 VFORK_SPAWN