[PATCH] spawn: proper signal handling for vfork

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

We cannot afford to fire Perl-level signal handlers in the
vforked child process since they're not designed to run in
the child like that.

Thus we need to block all signals before calling vfork, reset
signal dispositions in the child, and restore the signal mask in
the parent.

ref: https://ewontfix.com/7
---
 lib/PublicInbox/Spawn.pm | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/lib/PublicInbox/Spawn.pm b/lib/PublicInbox/Spawn.pm
index 23f303f..c5a5c01 100644
--- a/lib/PublicInbox/Spawn.pm
+++ b/lib/PublicInbox/Spawn.pm
@@ -24,6 +24,8 @@ my $vfork_spawn = <<'VFORK_SPAWN';
 #include <sys/uio.h>
 #include <unistd.h>
 #include <alloca.h>
+#include <signal.h>
+#include <assert.h>
 
 #define AV_ALLOCA(av, max) alloca((max = (av_len((av)) + 1)) * sizeof(char *))
 
@@ -81,6 +83,8 @@ int public_inbox_fork_exec(int in, int out, int err,
 	pid_t pid;
 	char **argv, **envp;
 	I32 max;
+	sigset_t set, old;
+	int ret;
 
 	argv = AV_ALLOCA(cmd, max);
 	av2c_copy(argv, cmd, max);
@@ -88,14 +92,26 @@ int public_inbox_fork_exec(int in, int out, int err,
 	envp = AV_ALLOCA(env, max);
 	av2c_copy(envp, env, max);
 
+	ret = sigfillset(&set);
+	assert(ret == 0 && "BUG calling sigfillset");
+	ret = sigprocmask(SIG_SETMASK, &set, &old);
+	assert(ret == 0 && "BUG calling sigprocmask to block");
 	pid = vfork();
 	if (pid == 0) {
+		int sig;
+
 		REDIR(in, 0);
 		REDIR(out, 1);
 		REDIR(err, 2);
+		for (sig = 1; sig < NSIG; sig++)
+			signal(sig, SIG_DFL); /* ignore errorrs on signals */
+		ret = sigprocmask(SIG_SETMASK, &old, NULL);
+		if (ret != 0) xerr("sigprocmask failed in vfork child");
 		execve(filename, argv, envp);
 		xerr("execve failed");
 	}
+	ret = sigprocmask(SIG_SETMASK, &old, NULL);
+	assert(ret == 0 && "BUG calling sigprocmask to restore");
 
 	return (int)pid;
 }
@@ -111,7 +127,7 @@ if (defined $vfork_spawn) {
 		my $f = "$inline_dir/.public-inbox.lock";
 		open my $fh, '>', $f or die "failed to open $f: $!\n";
 		flock($fh, LOCK_EX) or die "LOCK_EX failed on $f: $!\n";
-		eval 'use Inline C => $vfork_spawn';
+		eval 'use Inline C => $vfork_spawn'; #, BUILD_NOISY => 1';
 		my $err = $@;
 		flock($fh, LOCK_UN) or die "LOCK_UN failed on $f: $!\n";
 		die $err if $err;
-- 
EW