Hi! We’ve just had a bad experience with the nginx service on berlin, where ‘herd restart nginx’ would cause shepherd to get stuck forever in ‘waitpid’ on the process that was supposed to start nginx. The details are unclear, but one thing is clear is that using ‘waitpid’ (either directly or indirectly with ‘system*’, which is what ‘nginx-service-type’ does) is not great: 1. In the best case, shepherd (as of 0.9.1) is stuck while ‘system*’ is in ‘waitpid’ waiting for child process completion (“stuck” as in: doesn’t do anything, not even answering ‘herd’ requests or inetd connections.) 2. I don’t think that can happen with ‘system*’ (because it’s in C), but generally speaking, there’s a possibility that shepherd’s event loop will handle child process termination before some other user-made ‘waitpid’ call does. Anyway, that’s a bad situation. So I can think of several ways to address it: 1. Change the nginx service ‘stop’ method to just (make-kill-destructor), which should work just as well as invoking “nginx -s stop”. 2. Have Shepherd provide a replacement for ‘system*’. Thoughts? Ludo’.
[-- Attachment #1.1.1: Type: text/plain, Size: 2307 bytes --] On 20-07-2022 23:39, Ludovic Courtès wrote: > Hi! > > We’ve just had a bad experience with the nginx service on berlin, where > ‘herd restart nginx’ would cause shepherd to get stuck forever in > ‘waitpid’ on the process that was supposed to start nginx. > > The details are unclear, but one thing is clear is that using ‘waitpid’ > (either directly or indirectly with ‘system*’, which is what > ‘nginx-service-type’ does) is not great: > > 1. In the best case, shepherd (as of 0.9.1) is stuck while ‘system*’ > is in ‘waitpid’ waiting for child process completion (“stuck” as > in: doesn’t do anything, not even answering ‘herd’ requests or > inetd connections.) > > 2. I don’t think that can happen with ‘system*’ (because it’s in C), > but generally speaking, there’s a possibility that shepherd’s event > loop will handle child process termination before some other > user-made ‘waitpid’ call does. > > Anyway, that’s a bad situation. > > So I can think of several ways to address it: > > 1. Change the nginx service ‘stop’ method to just > (make-kill-destructor), which should work just as well as invoking > “nginx -s stop”. > > 2. Have Shepherd provide a replacement for ‘system*’. Why Shepherd and not guile fibers? Is this a Shepherd-specific problem? > > Thoughts? 3. Make waitpid (or a variant that does what we need) interact well with guile-fibers, like how 'accept' is doesn't inhibit switching to another fiber. There some Linux API with signal handlers or pid fds or such that might be useful here, though I don't recall the name. Presumably something similar can be done for the Hurd, though some C glue may be needed to access the right Hurd APIs if the signal handler API isn't portable. Alternatively: 4. Do the waitpid in a separate thread (needs work-around for the multi-threaded fork problem, probably C things? Or modifying Guile and maybe glibc to avoid async-unsafe things or make more things async-safe or whatever the appropriate ...-safe is here.) If not a Guile Fibers interaction problem, then the asynchronous signal handler API might still be useful. Greetings, Maxime [-- Attachment #1.1.2: OpenPGP public key --] [-- Type: application/pgp-keys, Size: 929 bytes --] [-- Attachment #2: OpenPGP digital signature --] [-- Type: application/pgp-signature, Size: 236 bytes --]
Maxime Devos <maximedevos@telenet.be> skribis: > Why Shepherd and not guile fibers? Is this a Shepherd-specific problem? Blocking calls are a problem for Fibers in general, and ‘waitpid’ is no exception. The problem here is Shepherd-specific in the sense that we’re more likely to use ‘system*’ and ‘waitpid’ in this context. It’s also Shepherd-specific because shepherd already runs an event loop that tracks signal FDs and will thus “see” SIGCHLD events. > 3. Make waitpid (or a variant that does what we need) interact well > with guile-fibers, like how 'accept' is doesn't inhibit switching to > another fiber. There some Linux API with signal handlers or pid fds or > such that might be useful here, though I don't recall the > name. Presumably something similar can be done for the Hurd, though > some C glue may be needed to access the right Hurd APIs if the signal > handler API isn't portable. Yes, that’s roughly what I had in mind when I mentioned providing a replacement for ‘system*’ (but you’re right, it’s a replacement for ‘waitpid’ at its core). > Alternatively: > > 4. Do the waitpid in a separate thread (needs work-around for the > multi-threaded fork problem, probably C things? Or modifying Guile and > maybe glibc to avoid async-unsafe things or make more things > async-safe or whatever the appropriate ...-safe is here.) For shepherd, multithreading is not an option due to the semantics of fork in the presence of threads. Ludo’.
[-- Attachment #1.1.1.1: Type: text/plain, Size: 782 bytes --] On 21-07-2022 17:39, Ludovic Courtès wrote: >> Alternatively: >> >> 4. Do the waitpid in a separate thread (needs work-around for the >> multi-threaded fork problem, probably C things? Or modifying Guile and >> maybe glibc to avoid async-unsafe things or make more things >> async-safe or whatever the appropriate ...-safe is here.) > For shepherd, multithreading is not an option due to the semantics of > fork in the presence of threads. From what I've read, multi-threaded fork is safe as long as you do an exec 'immediately' afterwards, without doing things like taking locks or allocating memory with malloc in-between the fork and exec. I don't think it's possible to do that in Guile code, but that's what the C things are for. Greetings, Maxime. [-- Attachment #1.1.1.2: Type: text/html, Size: 1272 bytes --] [-- Attachment #1.1.2: OpenPGP public key --] [-- Type: application/pgp-keys, Size: 929 bytes --] [-- Attachment #2: OpenPGP digital signature --] [-- Type: application/pgp-signature, Size: 236 bytes --]
Hello, When running the following command: --8<---------------cut here---------------start------------->8--- sudo herd restart service-that-hangs-upon-restart --8<---------------cut here---------------end--------------->8--- then hitting C-c, Shepherd becomes totally unresponsive: --8<---------------cut here---------------start------------->8--- sudo herd status --8<---------------cut here---------------end--------------->8--- and all further Shpeherd commands hang forever. I was able to reproduce it in two different configurations: 1. On my laptop with a Wireguard service trying to reach a non-existing DNS server. --8<---------------cut here---------------start------------->8--- (service wireguard-service-type (wireguard-configuration (addresses (list "10.0.0.2/24")) (dns '("10.0.0.50")) #does not exit --8<---------------cut here---------------end--------------->8--- 2. On Berlin, while trying to restart nginx. In both situations, the "reboot" command was also hanging. Thanks, Mathieu
Hi, Mathieu Othacehe <othacehe@gnu.org> skribis: > sudo herd restart service-that-hangs-upon-restart > > > then hitting C-c, Shepherd becomes totally unresponsive: > > sudo herd status > > > and all further Shpeherd commands hang forever. I was able to reproduce > it in two different configurations: > > 1. On my laptop with a Wireguard service trying to reach a non-existing > DNS server. > > (service wireguard-service-type > (wireguard-configuration > (addresses (list "10.0.0.2/24")) > (dns '("10.0.0.50")) #does not exit > > 2. On Berlin, while trying to restart nginx. I experienced case #2: in that case ‘strace -p1’ showed that shepherd was stuck on waitpid of the nginx process, which was not terminating. Killing that process would unlock shepherd. This might be <https://issues.guix.gnu.org/56674>. Would be good to see what’s up with WireGuard. Ludo’.
Mathieu Othacehe <othacehe@gnu.org> skribis: > 1. On my laptop with a Wireguard service trying to reach a non-existing > DNS server. > > (service wireguard-service-type > (wireguard-configuration > (addresses (list "10.0.0.2/24")) > (dns '("10.0.0.50")) #does not exit This one is similar to: https://issues.guix.gnu.org/53225 https://issues.guix.gnu.org/53381 It has to do with the fact that “wg-quick up” blocks until it succeeds and that ‘invoke’ gets stuck on ‘waitpid’ until the “wg-quick” process terminates. The solution will be to use something non-blocking instead of ‘invoke’; I’m looking into it. Ludo’.
Mathieu Othacehe <othacehe@gnu.org> skribis: > then hitting C-c, Shepherd becomes totally unresponsive: > > sudo herd status > > > and all further Shpeherd commands hang forever. I was able to reproduce > it in two different configurations: [...] > 2. On Berlin, while trying to restart nginx. I can’t reproduce it in a VM. Before I try it on a production system :-), does anyone have a tip on how to reproduce it? Or perhaps strace output from a system that exhibits this bug? TIA! Ludo’.
[-- Attachment #1: Type: text/plain, Size: 2121 bytes --] Hi, Ludovic Courtès <ludo@gnu.org> skribis: > 1. In the best case, shepherd (as of 0.9.1) is stuck while ‘system*’ > is in ‘waitpid’ waiting for child process completion (“stuck” as > in: doesn’t do anything, not even answering ‘herd’ requests or > inetd connections.) > > 2. I don’t think that can happen with ‘system*’ (because it’s in C), > but generally speaking, there’s a possibility that shepherd’s event > loop will handle child process termination before some other > user-made ‘waitpid’ call does. > > Anyway, that’s a bad situation. > > So I can think of several ways to address it: > > 1. Change the nginx service ‘stop’ method to just > (make-kill-destructor), which should work just as well as invoking > “nginx -s stop”. > > 2. Have Shepherd provide a replacement for ‘system*’. These fresh Shepherd commits install a non-blocking ‘system*’ replacement: 975b0aa service: Provide a non-blocking replacement of 'system*'. 039c7a8 service: Spawn a fiber responsible for process monitoring. We’ll have to do more testing and probably go for a 0.9.3 release soon. Protip: you can test the latest shepherd with: --8<---------------cut here---------------start------------->8--- (operating-system ;; … (essential-services (modify-services (operating-system-default-essential-services this-operating-system) (shepherd-root-service-type config => (shepherd-configuration (shepherd (package (inherit shepherd-0.9) (version "0.9.3pre") (source (git-checkout (url "https://git.savannah.gnu.org/git/shepherd.git"))) (native-inputs (modify-inputs (package-native-inputs shepherd-0.9) (append autoconf automake help2man texinfo gnu-gettext)))))))))) --8<---------------cut here---------------end--------------->8--- Full example attached. Ludo’. [-- Attachment #2: the example --] [-- Type: text/plain, Size: 3640 bytes --] ;; This is an operating system configuration template ;; for a "bare bones" setup, with no X11 display server. (use-modules (gnu) (guix) (guix git)) (use-service-modules networking ssh web vpn shepherd) (use-package-modules linux screen ssh admin autotools gettext man texinfo) (operating-system (host-name "komputilo") (timezone "Europe/Berlin") (locale "en_US.utf8") ;; Boot in "legacy" BIOS mode, assuming /dev/sdX is the ;; target hard disk, and "my-root" is the label of the target ;; root file system. (bootloader (bootloader-configuration (bootloader grub-bootloader) (targets '("/dev/sdX")))) ;; It's fitting to support the equally bare bones ‘-nographic’ ;; QEMU option, which also nicely sidesteps forcing QWERTY. (kernel-arguments (list "console=ttyS0,115200")) (file-systems (cons (file-system (device (file-system-label "my-root")) (mount-point "/") (type "ext4")) %base-file-systems)) ;; This is where user accounts are specified. The "root" ;; account is implicit, and is initially created with the ;; empty password. (users (cons (user-account (name "alice") (comment "Bob's sister") (group "users") ;; Adding the account to the "wheel" group ;; makes it a sudoer. Adding it to "audio" ;; and "video" allows the user to play sound ;; and access the webcam. (supplementary-groups '("wheel" "audio" "video"))) %base-user-accounts)) ;; Globally-installed packages. (packages (append (list screen strace) %base-packages)) (essential-services (modify-services (operating-system-default-essential-services this-operating-system) (shepherd-root-service-type config => (shepherd-configuration (shepherd (package (inherit shepherd-0.9) (version "0.9.3pre") (source (git-checkout (url "https://git.savannah.gnu.org/git/shepherd.git"))) (native-inputs (modify-inputs (package-native-inputs shepherd-0.9) (append autoconf automake help2man texinfo gnu-gettext))))))))) ;; Add services to the baseline: a DHCP client and ;; an SSH server. (services (append (list (service dhcp-client-service-type) (service nginx-service-type (nginx-configuration (server-blocks (list (nginx-server-configuration (listen '("80")) (server-name '("www.example.org")) (root "/srv/whatever")))))) (service wireguard-service-type (wireguard-configuration (addresses (list "10.0.0.2/24")) (dns '("10.0.0.50")))) ;does not exit (service openssh-service-type (openssh-configuration (openssh openssh-sans-x) (port-number 2222)))) %base-services)))
Hello!
Ludovic Courtès <ludo@gnu.org> skribis:
> These fresh Shepherd commits install a non-blocking ‘system*’ replacement:
>
> 975b0aa service: Provide a non-blocking replacement of 'system*'.
> 039c7a8 service: Spawn a fiber responsible for process monitoring.
>
> We’ll have to do more testing and probably go for a 0.9.3 release soon.
Shepherd commit ada88074f0ab7551fd0f3dce8bf06de971382e79 passes my
tests. It definitely solves the wireguard example and similar things
(uses of ‘system*’ in service constructors/destructors); I can’t tell
for sure about nginx because I haven’t been able to reproduce it in a
VM. I’m interested in ways to reproduce it.
It does look like we could go with 0.9.3 real soon now.
Ludo’.