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Document atomicity of write and pwrite, and technical fix about POSIX #257

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merged 7 commits into from
May 18, 2020
Merged

Document atomicity of write and pwrite, and technical fix about POSIX #257

merged 7 commits into from
May 18, 2020

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igrep
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@igrep igrep commented Apr 15, 2020

See the commit message of each commit for details.

@programmerjake
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Is it technically possible to provide atomic writes with respect to non-WASI code, such as other processes? (Seems unlikely when a file is mmaped.) If not, wording should be added to limit the scope of the atomicity to the WASI process (implementable with a mutex or similar) or some other suitable scope.

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igrep commented Apr 16, 2020

Thanks for pointing out. Reviewing the related chapter of POSIX, it refers only to threads.
I'll fix.

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The related chapter in POSIX only provides these guarantees when the functions are operating on regular files or symbolic links. Unless we intend WASI to differ, we should preserve that aspect of the guarantee.

write etc. don't have a guarantee to write all the data or nothing. POSIX only says these "functions shall be atomic with respect to each other", which I read as saying that these functions' I/O won't be interleaved with other I/O, however they can presumably still produce incomplete results if they encounter errors such as running out of space on an output device. And in practice, some OS's also impose their own limitations -- on 64-bit Linux for example, calling write with a 4 GiB buffer (which is below SSIZE_MAX) to a normal file writes only 0x7ffff000 bytes.

igrep added a commit to igrep/WASI that referenced this pull request Apr 22, 2020
@igrep
Fix after review
13ab34b 
WebAssembly#257 (comment)

The atomicity is guaranteed only for regular files or symbolic links (only symbolic links for a regular files, right?)

Delete expressions indicating 'all or nothing' to reflect the actual behavior of POSIX's`write` etc.
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igrep commented Apr 22, 2020 •
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The related chapter in POSIX only provides these guarantees when the functions are operating on regular files or symbolic links

Thanks! I missed the phrase "when they operate on regular files or symbolic links".

write etc. don't have a guarantee to write all the data or nothing. POSIX only says these "functions shall be atomic with respect to each other", which I read as saying that these functions' I/O won't be interleaved with other I/O, however they can presumably still produce incomplete results if they encounter errors such as running out of space on an output device

I agree. So I rewrote about what "atomicity" stands for. 13ab34b 3339842.

igrep added a commit to igrep/WASI that referenced this pull request Apr 22, 2020
@igrep
Fix after review
3339842 
WebAssembly#257 (comment)

The atomicity is guaranteed only for regular files or symbolic links (only symbolic links for a regular files, right?)

Delete expressions indicating 'all or nothing' to reflect the actual behavior of POSIX's`write` etc.
igrep added a commit to igrep/WASI that referenced this pull request May 8, 2020
@igrep
Fix after review
5a8b789 
WebAssembly#257 (comment)

Delete the sentences related to 'all data' completely

Don't refer to symlink
igrep added 6 commits May 8, 2020 11:57
@igrep
Techinically speaking, preadv and pwritev are not in POSIX.
ecac47f 
Ref: https://pubs.opengroup.org/onlinepubs/9699919799/idx/ip.html
@igrep
Document about atomicity of write and pwrite
fed67f9 
Related thread of ZULIP: https://bytecodealliance.zulipchat.com/#narrow/stream/219900-wasi/topic/Why.20is.20.60iovecs.60.20an.20array.20of.20strings.3F

The thread refers atomicity of `read`, but I couldn't make a good expression
for the atomicity of `read`.
So I add document only in `write` and `pwrite`.
@igrep
Limit the scope of atomicity
29ac897 
The related POSIX document refers only to threads. https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_09_07

So the atomicity is limited in the process
@igrep
Fix test
f2e3c22
@igrep
Fix after review
879a017 
WebAssembly#257 (comment)

The atomicity is guaranteed only for regular files or symbolic links (only symbolic links for a regular files, right?)

Delete expressions indicating 'all or nothing' to reflect the actual behavior of POSIX's`write` etc.
@igrep
Fix after review
793be7b 
WebAssembly#257 (comment)

Delete the sentences related to 'all data' completely

Don't refer to symlink
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igrep commented May 12, 2020

@MaxDesiatov Thank you for the review!

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This looks good to me, thanks for seeing this through!

@sunfishcode sunfishcode merged commit 7408db4 into WebAssembly:master May 18, 2020
@igrep igrep deleted the write-atomicity branch May 19, 2020 00:15
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