[cfe-dev] Locking files on Windows with clangd

Ilya Biryukov via cfe-dev cfe-dev at lists.llvm.org
Tue Jul 24 23:56:43 PDT 2018

It's not something we do in clangd, it's something we do in our buildsystem
for usual compilations as well. And the idea is not to avoid warnings from
headers (we want those), but to avoid warnings from headers of third_party
dependencies (those we usually don't control, so we can't fix them).

On Wed, Jul 25, 2018 at 8:31 AM Ivan Donchevskii <ivan.donchevskii at qt.io>

> There are different ways to suppress warnings other than passing -isystem
> so we're trying to make fewer -isystem paths (for example
> https://reviews.llvm.org/D48116 which i really like and am planing to
> update in the nearest future)
> ------------------------------
> *From:* Ilya Biryukov <ibiryukov at google.com>
> *Sent:* Wednesday, July 25, 2018 8:20:40 AM
> *To:* Ivan Donchevskii
> *Cc:* Dmitry.Kozhevnikov at jetbrains.com; via cfe-dev
> *Subject:* Re: [cfe-dev] Locking files on Windows with clangd
> On Wed, Jul 25, 2018 at 8:09 AM Ivan Donchevskii <ivan.donchevskii at qt.io>
> wrote:
> There's no way I know to fix it because AFAIK windows always locks mmapped
> files.
> That's unfortunate, have you seen any pointers of this behavior in
> official docs?
> I think I was able to modify the mmapped file when I was playing around
> with Windows APIs while looking at D35200, but I'll need to rerun my
> experiments and make them reproducible.
> But i can't say that this behavior is broken because you are not supposed
> to mark file 'system' if you plan to modify it. What would I do if I had
> bugreport with '#pragma system_header' - I would simply remove that line
> from the unsaved file content I pass to clang.
> I'm not sure there's a single definition of what a "system" header is,
> e.g. we pass -isystem when adding include paths to third_party code to
> suppress warnings in that code.
> Regards,
> Ivan
> ------------------------------
> *From:* Ilya Biryukov <ibiryukov at google.com>
> *Sent:* Tuesday, July 24, 2018 9:13:47 AM
> *To:* Dmitry.Kozhevnikov at jetbrains.com; Ivan Donchevskii
> *Cc:* via cfe-dev
> *Subject:* Re: [cfe-dev] Locking files on Windows with clangd
> Hi Dmitry,
> We've had a pull request with a similar fix at some point:
> https://reviews.llvm.org/D35200, but it was abandoned and never made it
> in.
> @ivan.donchevskii at qt.io <ivan.donchevskii at qt.io>, did you manage to
> figure out a way to make this work on Windows? Any ideas on what's the best
> way to fix this?
> On Sun, Jul 22, 2018 at 7:22 PM Dmitry.Kozhevnikov at jetbrains.com via
> cfe-dev <cfe-dev at lists.llvm.org> wrote:
> Hi!
> A short background: we at JetBrains are experimenting with using clangd
> with
> CLion, and we're very happy with the results so far. Unfortunately, we've
> hit
> a problem when header files are locked on Windows, so they can't be saved,
> they break `git rebase` (leading to the loss of work), etc.
> It happens when the file is opened using a memory mapped file. It's fairly
> easy to reproduce with clangd:
> 1. Create a header big enough to pass the threshold in `shouldUseMmap()`
> (lib/Support/MemoryBuffer.cpp)
> 2. Include it in a source file
> 3. Keep this source file opened via clangd
> 4. Now an attempt to modify this header will fail
> The situation is especially unpleasant because the handle is locked not
> only
> during the parse, but for all the time clangd holds the preamble containing
> this header.
> This issue is mitigated to an extent in libclang: non-system files are
> considered "volatile" (see r160074, r334070), so memory mapped files are
> not
> used for them. However, I feel like it's not enough: you can easily have a
> `#pragma system_header` in your codebase (IIUC it affects the mentioned
> behavior), locking such file and i.e. losing user's work during `git
> rebase`
> is still unacceptable.
> Also, I think the fact that the proper compiler has this behavior is also
> unfortunate (forgotten build in the background might lead to the loss of
> data).
> IIUC the main reason for having memory mapped files is to reduce the memory
> footprint. However, for a regular translation unit, headers usually take
> several
> megabytes, which IMO is tolerable, and is usually topped by other per-TU
> data
> anyway.
> Hence, what do you think about not using memory mapped files on Windows at
> all
> for source files? Are there any implications that I'm not aware of?
> The naive patch (obviously not for commit, just for the illustration):
> --- a/lib/Basic/SourceManager.cpp
> +++ b/lib/Basic/SourceManager.cpp
> @@ -109,7 +109,12 @@ llvm::MemoryBuffer
> *ContentCache::getBuffer(DiagnosticsEngine &Diag,
> return Buffer.getPointer();
> }
> +#ifndef _WIN32
> bool isVolatile = SM.userFilesAreVolatile() && !IsSystemFile;
> +#else
> + bool isVolatile = true;
> +#endif
> +
> auto BufferOrError =
> SM.getFileManager().getBufferForFile(ContentsEntry, isVolatile);
> If I've not missed something, this would effectively disable memory mapped
> files for source files on Windows, but they would still be used for
> other potentially large files (including PCHs and preambles).
> If this is unacceptable, at the very least, clangd should treat user files
> similar to libclang (we can work on a patch in this case).
> Note that there is a similar bug report for QtCreator/libclang with some
> interesting comments: https://bugreports.qt.io/browse/QTCREATORBUG-15449
> Cheers,
> Dmitry.
> _______________________________________________
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> --
> Regards,
> Ilya Biryukov
> --
> Regards,
> Ilya Biryukov

Ilya Biryukov
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