xap_helper: implement alarm(2)-based timeout

alarm(2) delivering SIGALRM seems sufficient for Xapian since
Xapian doesn't block signals (which would necessitate the use of
SIGKILL via RLIMIT_CPU hard limit).  When Xapian gets stuck in
`D' state on slow storage, SIGKILL would not make a difference,
either (at least not on Linux).

Relying on RLIMIT_CPU is also trickier since we must account for
CPU time already consumed by a process for unrelated requests.
Thus we just rely on a simple alarm-based timeout.  This also
avoids requiring the optional BSD::Resource module in the (mostly)
Perl implementation (and avoids potential bugs given my meager
arithmetic skills).

2 files changed, 19 insertions, 1 deletions

diff --git a/lib/PublicInbox/XapHelper.pm b/lib/PublicInbox/XapHelper.pm
index 746b4d62..2e20660e 100644
--- a/lib/PublicInbox/XapHelper.pm
+++ b/lib/PublicInbox/XapHelper.pm
@@ -27,6 +27,8 @@ sub cmd_test_inspect {
                 ($req->{srch}->has_threadid ? 1 : 0)
 }
 
+sub cmd_test_sleep { select(undef, undef, undef, 0.01) while 1 }
+
 sub iter_retry_check ($) {
         if (ref($@) =~ /\bDatabaseModifiedError\b/) {
                 $_[0]->{srch}->reopen;
@@ -193,7 +195,10 @@ sub dispatch {
                 $new->{qp} = $new->qparse_new;
                 $new;
         };
+        my $timeo = $req->{K};
+        alarm($timeo) if $timeo;
         $fn->($req, @argv);
+        alarm(0) if $timeo;
 }
 
 sub recv_loop {
@@ -212,7 +217,7 @@ sub recv_loop {
                 }
                 scalar(@fds) or exit(66); # EX_NOINPUT
                 die "recvmsg: $!" if !defined($fds[0]);
-                PublicInbox::DS::block_signals();
+                PublicInbox::DS::block_signals(POSIX::SIGALRM);
                 my $req = bless {}, __PACKAGE__;
                 my $i = 0;
                 open($req->{$i++}, '+<&=', $_) for @fds;
diff --git a/lib/PublicInbox/xap_helper.h b/lib/PublicInbox/xap_helper.h
index 7ecea264..3df3ce91 100644
--- a/lib/PublicInbox/xap_helper.h
+++ b/lib/PublicInbox/xap_helper.h
@@ -27,6 +27,7 @@
 #include <sys/types.h>
 #include <sys/uio.h>
 #include <sys/wait.h>
+#include <poll.h>
 
 #include <assert.h>
 #include <err.h> // BSD, glibc, and musl all have this
@@ -413,6 +414,11 @@ static bool cmd_test_inspect(struct req *req)
         return false;
 }
 
+static bool cmd_test_sleep(struct req *req)
+{
+        for (;;) poll(NULL, 0, 10);
+        return false;
+}
 #include "xh_mset.h" // read-only (WWW, IMAP, lei) stuff
 #include "xh_cidx.h" // CodeSearchIdx.pm stuff
 
@@ -427,6 +433,7 @@ static const struct cmd_entry {
         CMD(dump_ibx), // many inboxes
         CMD(dump_roots), // per-cidx shard
         CMD(test_inspect), // least common commands last
+        CMD(test_sleep), // least common commands last
 };
 
 #define MY_ARRAY_SIZE(x)        (sizeof(x)/sizeof((x)[0]))
@@ -680,6 +687,9 @@ static void dispatch(struct req *req)
                 free_srch(kbuf.srch);
                 goto cmd_err; // srch_init already warned
         }
+        if (req->timeout_sec)
+                alarm(req->timeout_sec > UINT_MAX ?
+                        UINT_MAX : (unsigned)req->timeout_sec);
         try {
                 if (!req->fn(req))
                         warnx("`%s' failed", req->argv[0]);
@@ -688,6 +698,8 @@ static void dispatch(struct req *req)
         } catch (...) {
                 warn("unhandled exception");
         }
+        if (req->timeout_sec)
+                alarm(0);
 cmd_err:
         return; // just be silent on errors, for now
 }
@@ -1025,6 +1037,7 @@ int main(int argc, char *argv[])
         DELSET(SIGXFSZ);
 #undef DELSET
         CHECK(int, 0, sigdelset(&workerset, SIGUSR1));
+        CHECK(int, 0, sigdelset(&fullset, SIGALRM));
 
         if (nworker == 0) { // no SIGTERM handling w/o workers
                 recv_loop();