[cfe-dev] libclang locking files for write/delete on windows

[Reid Kleckner]

Can the memory mapping outlive the file handle? I bet it can.

On Tue, Dec 9, 2014 at 3:17 PM, Ivan Koster <ivankoster at gmail.com> wrote:

> Hi Ivan,
>>
>
> Hello!
>
> This is good info; MemoryBuffer::getOpenFile conditionally uses file
>> mappings or normal file reads depending on file size (see
>> MemoryBuffer.cpp/shouldUseMmap).
>>
>> Since the mapping uses a stricter sharing mode than the normal file
>> read, it's more likely to fail. Still not sure who's writing/deleting
>> the file while Clang attempts to map it, can you make that out from
>> your PM logs?
>>
>
> I've exported the log to csv and you can find it here:
> https://drive.google.com/file/d/0B8xFlIzMhHcpdzR6OW4wM3BZdGM/view?usp=sharing
>
> This is a log of sublime text opening dummy.c, commanding YCMD
> (python.exe) to analyze the dummy.c, which so and so on then finally makes
> libclang.dll open dummy.h, and finally me killing the python.exe process.
> As you can see, killing the proces does not log anything, even though it
> "frees" the file.
>
> The trace shows two threads of libclang.dll (TID 6888 and TID 4040), but
> they do not operate concurrently. 4040 only starts after 6888 is done with
> dummy.h.
> When I examine the stack traces of the CreateFileMapping operation that
> appears in pairs, they both happen in the same kernel32.dll
> CreateFileMappingA function.
> So the kernel seems to work around the FILE LOCKED WITH ONLY HEADERS thing
> here?
>
> There are no dangling file handles, the trace shows only pairs of open and
> close events. This is confirmed by the windows handle.exe tool, which can
> not find any filehandles to dummy.h even though the file is locked.
> The weird thing is that the windows Process Explorer has a "find file
> handle or DLL" function.
> When I make it search for dummy.h, it finds not a file handle, but a DLL
> inside python.exe called dummy.h.
> Somehow Process Explorer reports that dummy.h is loaded as a DLL? (When I
> kill python.exe this disappears).
>
> I haven't dived so deep into windows stuff before so I'm hoping this helps!
> But to be honest, this is starting to not make sense to me. The file is
> locked even though there is no open file handle. Process Explorer reporting
> that it is loaded as a DLL...
>
> Regards,
>
> Ivan
>
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