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Make poll return errno::inval if there are no subscriptions. #193

Merged
merged 1 commit into from
Jan 16, 2020
Merged

Make poll return errno::inval if there are no subscriptions. #193

merged 1 commit into from
Jan 16, 2020

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sunfishcode
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poll with no subscriptions is effectively an infinite hang. Since wasm
has no signals, use cases which involve waking up a libc pause call, which
is implemented in terms of a poll with no subscriptions, aren't applicable.
So all pause can do is suspend the program until the host environment
terminates it.

One situation where that's useful is in debugging, as it's sometimes useful
to be able to suspend a process in place so that it can be inspected by a
debugger. However, that doesn't require a fully infinite suspend; a suspend
with a long timeout is adequate.

Making the no-subscriptions case an error can help catch cases where
applications unintentionally enter infinite loops.

@sunfishcode
Make poll return errno::inval if there are no subscriptions.
d9f7d7c 
`poll` with no subscriptions is effectively an infinite hang. Since wasm
has no signals, use cases which involve waking up a libc `pause` call, which
is implemented in terms of a poll with no subscriptions, aren't applicable.
So all `pause` can do is suspend the program until the host environment
terminates it.

One situation where that's useful is in debugging, as it's sometimes useful
to be able to suspend a process in place so that it can be inspected by a
debugger. However, that doesn't require a fully infinite suspend; a suspend
with a long timeout is adequate.

Making the no-subscriptions case an error can help catch cases where
applications unintentionally enter infinite loops.
@cjihrig cjihrig mentioned this pull request Jan 8, 2020
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@sunfishcode sunfishcode mentioned this pull request Jan 9, 2020
Merged
@marmistrz
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I think that ENOTSUP would be more fortunate. On Linux this is a valid value for poll(2):

EINVAL (ppoll()) The timeout value expressed in *ip is invalid (negative).

But it's not supported by WASI.

@sunfishcode
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That's a good point.

From a pure WASI perspective, this issue feels more like an EINVAL situation; then we'd just say that calling the poll syscall with no subscriptions is a bug, which is what EINVAL is for. But from a POSIX and Linux perspective the poll libc function does support poll with 0 subscriptions, so maybe we could have WASI libc return ENOTSUP in that case. Does that make sense?

@sunfishcode
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We discussed this in the video call today, and there were no objections, so I'm calling it a consensus.

@sunfishcode sunfishcode merged commit 77629f3 into WebAssembly:master Jan 16, 2020
@sunfishcode sunfishcode deleted the pause branch January 16, 2020 22:42
sunfishcode added a commit to WebAssembly/wasi-libc that referenced this pull request Jan 16, 2020
@sunfishcode
Avoid calling poll_oneoff with zero subscriptions.
237abf7 
With WebAssembly/WASI#193 merged, WASI is moving
to make `poll_oneoff` with no arguments an error. Even though that's in
ephemeral and not yet in a snapshot, we can start to anticipate it in
libc:
 - Remove the `pause` function, since WASI has no signals and thus no
   way to ever wake it up short of having the host terminate it.
 - Make `poll` and `pselect` return `ENOTSUP` in the case of having no
   events to wait for.
@sunfishcode sunfishcode mentioned this pull request Jan 16, 2020
Merged
sunfishcode added a commit to WebAssembly/wasi-libc that referenced this pull request Jan 17, 2020
@sunfishcode
Avoid calling poll_oneoff with zero subscriptions.
468e73a 
With WebAssembly/WASI#193 merged, WASI is moving
to make `poll_oneoff` with no arguments an error. Even though that's in
ephemeral and not yet in a snapshot, we can start to anticipate it in
libc:
 - Remove the `pause` function, since WASI has no signals and thus no
   way to ever wake it up short of having the host terminate it.
 - Make `poll` and `pselect` return `ENOTSUP` in the case of having no
   events to wait for.
marmistrz added a commit to marmistrz/wasmtime that referenced this pull request Jan 17, 2020
@marmistrz
Adhere to WebAssembly/WASI#193
3a8c7e9
marmistrz added a commit to marmistrz/wasmtime that referenced this pull request Jan 17, 2020
@marmistrz
Return EINVAL in poll_oneoff with no events.
ea779fd 
We adhere to WebAssembly/WASI#193.
@marmistrz marmistrz mentioned this pull request Jan 17, 2020
Merged
marmistrz added a commit to marmistrz/wasmtime that referenced this pull request Jan 17, 2020
@marmistrz
Return EINVAL in poll_oneoff with no events.
b572a32 
We adhere to WebAssembly/WASI#193.
sunfishcode pushed a commit to bytecodealliance/wasmtime that referenced this pull request Jan 17, 2020
@marmistrz @sunfishcode
Return EINVAL in poll_oneoff with no events. (#838)
815576e 
* Return EINVAL in poll_oneoff with no events.

We adhere to WebAssembly/WASI#193.

* Add a test for empty poll.
@pchickey pchickey mentioned this pull request Jan 22, 2020
Merged
sunfishcode added a commit to WebAssembly/wasi-libc that referenced this pull request Feb 24, 2020
@sunfishcode
Avoid calling poll_oneoff with zero subscriptions.
f213cc7 
With WebAssembly/WASI#193 merged, WASI is moving
to make `poll_oneoff` with no arguments an error. Even though that's in
ephemeral and not yet in a snapshot, we can start to anticipate it in
libc:
 - Remove the `pause` function, since WASI has no signals and thus no
   way to ever wake it up short of having the host terminate it.
 - Make `poll` and `pselect` return `ENOTSUP` in the case of having no
   events to wait for.
sunfishcode added a commit to WebAssembly/wasi-libc that referenced this pull request Mar 3, 2020
@sunfishcode
Avoid calling poll_oneoff with zero subscriptions.
11ec052 
With WebAssembly/WASI#193 merged, WASI is moving
to make `poll_oneoff` with no arguments an error. Even though that's in
ephemeral and not yet in a snapshot, we can start to anticipate it in
libc:
 - Remove the `pause` function, since WASI has no signals and thus no
   way to ever wake it up short of having the host terminate it.
 - Make `poll` and `pselect` return `ENOTSUP` in the case of having no
   events to wait for.
sunfishcode added a commit to WebAssembly/wasi-libc that referenced this pull request Mar 18, 2020
@sunfishcode
Avoid calling poll_oneoff with zero subscriptions.
0258ef5 
With WebAssembly/WASI#193 merged, WASI is moving
to make `poll_oneoff` with no arguments an error. Even though that's in
ephemeral and not yet in a snapshot, we can start to anticipate it in
libc:
 - Remove the `pause` function, since WASI has no signals and thus no
   way to ever wake it up short of having the host terminate it.
 - Make `poll` and `pselect` return `ENOTSUP` in the case of having no
   events to wait for.
sunfishcode added a commit to WebAssembly/wasi-libc that referenced this pull request Jun 2, 2020
@sunfishcode
Avoid calling poll_oneoff with zero subscriptions.
1c020cc 
With WebAssembly/WASI#193 merged, WASI is moving
to make `poll_oneoff` with no arguments an error. Even though that's in
ephemeral and not yet in a snapshot, we can start to anticipate it in
libc:
 - Remove the `pause` function, since WASI has no signals and thus no
   way to ever wake it up short of having the host terminate it.
 - Make `poll` and `pselect` return `ENOTSUP` in the case of having no
   events to wait for.
sunfishcode added a commit to WebAssembly/wasi-libc that referenced this pull request Jun 2, 2020
@sunfishcode
Avoid calling poll_oneoff with zero subscriptions. (#162)
5a7ba74 
* Avoid calling `poll_oneoff` with zero subscriptions.

With WebAssembly/WASI#193 merged, WASI is moving
to make `poll_oneoff` with no arguments an error. Even though that's in
ephemeral and not yet in a snapshot, we can start to anticipate it in
libc:
 - Remove the `pause` function, since WASI has no signals and thus no
   way to ever wake it up short of having the host terminate it.
 - Make `poll` and `pselect` return `ENOTSUP` in the case of having no
   events to wait for.

* Remove `pause` from the defined-symbols.txt list.

* Fix __wasilibc_unmodified_upstream markers.

* Check for zero subscriptions, rather than zero events.

Make `poll` and `pselect` return `ENOTSUP` when asked to poll on zero
subscriptions, rather than when the systerm returns zero events.

While here, drop the `__wasilibc_unmodified_upstream` markers, which
were already pretty noisy here, and would be significantly worse with
this change.

* Add comments about the subtle relationship between nfds and nsubscriptions.

* Rewrite the comment.

* Fix code quotes.
@sunfishcode sunfishcode mentioned this pull request Dec 16, 2022
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@peterhuene peterhuene peterhuene approved these changes

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@sunfishcode @marmistrz @peterhuene