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Previously to spawn a process with a closed standard handle, we would use `posix_spawn_file_action_addclose`. However, it turns out that POSIX specifies that `spawnp()` may fail if `addclose()` is used on an fd that is already closed. While glibc and musl appear to ignore this aspect of the specification, Darwin indeed follows it leading to haskell#251. This behavior is rather unfortunate as `posix_spawn_file_action_addclose` is a convenient way to close a handle in a subprocess in a race-free manner (e.g. unlike `O_CLOEXEC`, which is global). To avoid haskell#251 we must first use `posix_spawn_file_action_addopen` on the fd (e.g. opening `/dev/null`) to be closed to ensure that it is valid, which has the side-effect of closing the inherited fd. We can then safely use `posix_spawn_file_action_addclose` to close the fd. Fixes haskell#251.
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Unfortunately I found that the current Cabal-driven testsuite isn't particularly well-suited to test this, consequently I ended up adding a test in GHC's testsuite. Testing this locally as we speak. |
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I have confirmed that the test reproduces #251 on Darwin without this change and that the change fixes it. |
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Note that I ultimately opted not to make the |
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Is this released? I can't find it in https://hackage.haskell.org/package/process-1.6.15.0/changelog |
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Previously to spawn a process with a closed standard handle, we
would use
posix_spawn_file_action_addclose. However, it turns out thatPOSIX specifies that
spawnp()may fail ifaddclose()is used on anfd that is already closed. While glibc and musl appear to ignore this
aspect of the specification, Darwin indeed follows it leading to #251.
This behavior is rather unfortunate as
posix_spawn_file_action_addcloseis a convenient way to close a handlein a subprocess in a race-free manner (e.g. unlike
O_CLOEXEC, which isglobal). To avoid #251 we must first use
posix_spawn_file_action_addopenon the fd (e.g. opening/dev/null)to be closed to ensure that it is valid, which has the side-effect of
closing the inherited fd. We can then safely use
posix_spawn_file_action_addcloseto close the fd.Fixes #251.