Signal handler API has no way to set SA_RESTART

[ghost]

@nomis commented on Oct 29, 2018, 8:14 PM UTC:

I'm handling SIGCHLD using asio while also doing writes to standard output and reads/writes to files.

If I receive SIGCHLD during a read() or write() then it gets interrupted. The simplest way to deal with this is to set SA_RESTART when setting up the signal handler, but there is no support for doing this and it doesn't happen by default.

This issue was moved by chriskohlhoff from boostorg/asio#157.

[ghost]

@nomis commented on Nov 3, 2018, 10:33 AM UTC:

I'm going to workaround this by using ld --wrap=sigaction to force the use of SA_RESTART and cause a compile error if sigaction or SA_RESTART does not exist.

I'm not convinced that the read() and write() calls used to pass signals over a pipe to the I/O service are implemented safely if another signal interrupts them, because the return value is never checked for EINTR, but this is not easy to test.

[ghost]

@nomis commented on Nov 4, 2018, 8:10 PM UTC:

It's a bit fragile to wrap sigaction because on some platforms (NetBSD) there's more than one version of it and while the linker is instructed to wrap sigaction, the header files have defined it as __sigaction14. I need to wrap both versions of the function and linking will fail if __sigaction14 does not exist.

It would be a lot easier if asio could be configured to use SA_RESTART.

[vinipsmaker]

SA_RESTART should be the default. Any other default is a bad default. Code that doesn't use UNIX signals will never be written with signals in mind and it won't be prepared for EINTR. And most (all?) code that will handle EINTR will just use a macro such as TEMP_FAILURE_RETRY() that only contributes to code noise.

For signal() (the old way of setting the signal handling disposition), 4.2BSD changed the bad default from non-SA_RESTART to SA_RESTART. Unfortunately you have to explicitly request for SA_RESTART on the new sigaction() (and not the other way around).

[vinipsmaker]

Commit 50ad5e4 closes this issue. It's now possible to set SA_RESTART when establishing a signal handler.

I'm handling SIGCHLD using asio

You might be interested in PR #1258 which will enhance commit 11c6caf enhances support to handle SIGCHLD signals.

[nomis]

This is now fixed in Asio 1.27.0 / Boost 1.82.0 but I can't close it because GitHub has set the reporter to @ghost instead of me.