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fscache: teach fscache to use NtQueryDirectoryFile #1937

Merged
merged 2 commits into from Nov 27, 2018
Merged

fscache: teach fscache to use NtQueryDirectoryFile #1937

merged 2 commits into from Nov 27, 2018

Commits on Nov 27, 2018

  1. fixup! mingw: Windows Docker volumes are *not* symbolic links 

    The code in question is unclear, and not everbody has the time to dig up
the commit message for the commit that added it, so let's play nice and
add an explanation as a code comment.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
    @dscho
    dscho committed Nov 27, 2018
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  2. fscache: teach fscache to use NtQueryDirectoryFile 

    Using FindFirstFileExW() requires the OS to allocate a 64K buffer for each
directory and then free it when we call FindClose().  Update fscache to call
the underlying kernel API NtQueryDirectoryFile so that we can do the buffer
management ourselves.  That allows us to allocate a single buffer for the
lifetime of the cache and reuse it for each directory.

This change improves performance of 'git status' by 18% in a repo with ~200K
files and 30k folders.

Documentation for NtQueryDirectoryFile can be found at:

https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/content/ntifs/nf-ntifs-ntquerydirectoryfile
https://docs.microsoft.com/en-us/windows/desktop/FileIO/file-attribute-constants
https://docs.microsoft.com/en-us/windows/desktop/fileio/reparse-point-tags

To determine if the specified directory is a symbolic link, inspect the
FileAttributes member to see if the FILE_ATTRIBUTE_REPARSE_POINT flag is
set. If so, EaSize will contain the reparse tag (this is a so far
undocumented feature, but confirmed by the NTFS developers). To
determine if the reparse point is a symbolic link (and not some other
form of reparse point), test whether the tag value equals the value
IO_REPARSE_TAG_SYMLINK.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
    @benpeart @dscho
    benpeart authored and dscho committed Nov 27, 2018
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