Make signal handling work on OCaml 5 and with Eio #993
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Before installing a signal handler, Lwt now checks first with the engine, which may now prevent Lwt from installing its handler and instead forward signals from some external handler. This allows e.g. sharing the SIGCHLD handler with Eio.
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Eio uses a plain OCaml signal handler for SIGCHLD, so this might make #994 more likely. |
If an OCaml signal handler runs in a Lwt worker thread then the process will dereference a NULL pointer and crash. The worker tried to avoid this by masking out all signals after starting, but by then it may be too late. Now, we mask out signals before creating a worker thread and then restore the mask in the main thread afterwards.
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I've added a fix for #994 to this PR too (and updated the PR description). |
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Looks good to me. |
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The first commit fixes #981 by using
SA_ONSTACKwith OCaml 5 so that signals do not cause memory corruption.The second commit is a solution to #991. Before installing a signal handler, Lwt now checks first with the engine, which may now prevent Lwt from installing its handler and instead forward signals from some external handler.
This allows e.g. sharing the SIGCHLD handler with Eio.
It would also be possible to do it the other way around (so that Eio uses Lwt's signal handler). However, Lwt only notifies a single domain and if that domain is busy then the signal will be delayed, whereas Eio's signal handler notifies all interested domains immediately.
The third commit fixes #994, which causes the process to segfault if an OCaml signal handler runs in a Lwt worker thread before that thread has set the signal mask. This bug is more likely to be seen when using Lwt with Eio.
These changes should be low-risk:
SA_ONSTACKchange only affects OCaml 5, where Lwt already segfaults.false, causing the existing code to behave exactly as before./cc @raphael-proust