Mailing List Archive

commit e13a9a2146952859c21c0a0c7b8b07757c2aba9d
Author: Edwin Török <edvin.torok@citrix.com>
AuthorDate: Mon Nov 7 17:41:36 2022 +0000
Commit: Andrew Cooper <andrew.cooper3@citrix.com>
CommitDate: Tue Dec 20 13:13:40 2022 +0000

tools/oxenstored: Set uncaught exception handler

Unhandled exceptions go to stderr by default, but this doesn't typically work
for oxenstored because:
* daemonize reopens stderr as /dev/null
* systemd redirects stderr to /dev/null too

Debugging an unhandled exception requires reproducing the issue locally when
using --no-fork, and is not conducive to figuring out what went wrong on a
remote system.

Install a custom handler which also tries to render the backtrace to the
configured syslog facility, and DAEMON|ERR otherwise.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
(cherry picked from commit ee7815f49faf743e960dac9e72809eb66393bc6d)
---
tools/ocaml/xenstored/logging.ml | 29 +++++++++++++++++++++++++++++
tools/ocaml/xenstored/xenstored.ml | 3 ++-
2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/tools/ocaml/xenstored/logging.ml b/tools/ocaml/xenstored/logging.ml
index 39c3036155..255051437d 100644
--- a/tools/ocaml/xenstored/logging.ml
+++ b/tools/ocaml/xenstored/logging.ml
@@ -342,3 +342,32 @@ let xb_answer ~tid ~con ~ty data =
let watch_not_fired ~con perms path =
let data = Printf.sprintf "EPERM perms=[%s] path=%s" perms path in
access_logging ~tid:0 ~con ~data Watch_not_fired ~level:Info
+
+let msg_of exn bt =
+ Printf.sprintf "Fatal exception: %s\n%s\n" (Printexc.to_string exn)
+ (Printexc.raw_backtrace_to_string bt)
+
+let fallback_exception_handler exn bt =
+ (* stderr goes to /dev/null, so use the logger where possible,
+ but always print to stderr too, in case everything else fails,
+ e.g. this can be used to debug with --no-fork
+
+ this function should try not to raise exceptions, but if it does
+ the ocaml runtime should still print the exception, both the original,
+ and the one from this function, but to stderr this time
+ *)
+ let msg = msg_of exn bt in
+ prerr_endline msg;
+ (* See Printexc.set_uncaught_exception_handler, need to flush,
+ so has to call stop and flush *)
+ match !xenstored_logger with
+ | Some l -> error "xenstored-fallback" "%s" msg; l.stop ()
+ | None ->
+ (* Too early, no logger set yet.
+ We normally try to use the configured logger so we don't flood syslog
+ during development for example, or if the user has a file set
+ *)
+ try Syslog.log Syslog.Daemon Syslog.Err msg
+ with e ->
+ let bt = Printexc.get_raw_backtrace () in
+ prerr_endline @@ msg_of e bt
diff --git a/tools/ocaml/xenstored/xenstored.ml b/tools/ocaml/xenstored/xenstored.ml
index 35b8cbd43f..4d5851c5cb 100644
--- a/tools/ocaml/xenstored/xenstored.ml
+++ b/tools/ocaml/xenstored/xenstored.ml
@@ -355,7 +355,8 @@ let tweak_gc () =
Gc.set { (Gc.get ()) with Gc.max_overhead = !Define.gc_max_overhead }


-let _ =
+let () =
+ Printexc.set_uncaught_exception_handler Logging.fallback_exception_handler;
let cf = do_argv in
let pidfile =
if Sys.file_exists (config_filename cf) then
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.17