[master] fd3d71b17 Try to handle reentrant panics more sensibly.

[Poul-Henning Kamp]

commit fd3d71b17eb9b3595b4d0fda297159b713d70f80
Author: Poul-Henning Kamp <phk at FreeBSD.org>
Date:   Tue Jan 4 07:34:25 2022 +0000

    Try to handle reentrant panics more sensibly.
    
    Relevant to: #3764

diff --git a/bin/varnishd/cache/cache_panic.c b/bin/varnishd/cache/cache_panic.c
index 3ac6af736..734b801e3 100644
--- a/bin/varnishd/cache/cache_panic.c
+++ b/bin/varnishd/cache/cache_panic.c
@@ -67,6 +67,7 @@
 
 static struct vsb pan_vsb_storage, *pan_vsb;
 static pthread_mutex_t panicstr_mtx;
+static pthread_t panicy;
 
 static void pan_sess(struct vsb *, const struct sess *);
 static void pan_req(struct vsb *, const struct req *);
@@ -720,12 +721,17 @@ pan_ic(const char *func, const char *file, int line, const char *cond,
 	struct sigaction sa;
 	int err = errno;
 
-	AZ(pthread_mutex_lock(&panicstr_mtx));
-
-	/* If we already panic'ed, do nothing */
-	while (heritage.panic_str[0])
+	/* If we already panicing in another thread, do nothing */
+	while (heritage.panic_str[0] && panicy != pthread_self())
 		sleep(1);
 
+	if (pthread_mutex_lock(&panicstr_mtx)) {
+		/* Reentrant panic */
+		VSB_printf(pan_vsb,"\n\nPANIC REENTRANCY\n\n");
+		abort();
+	}
+	panicy = pthread_self();
+
 	/*
 	 * should we trigger a SIGSEGV while handling a panic, our sigsegv
 	 * handler would hide the panic, so we need to reset the handler to