Support creating symlinks outside elevated sessions #1188

dscho · 2017-05-30T20:37:39Z

This Pull Request is intended to supersede #1184.

The main difference is that I fixed a rather serious bug (the return value was declared incorrectly, possibly fooling Git into believing that it could create a symbolic link when it really failed, and that really happened here during my tests), and that the file/directory flags are modified only once. I also use the GetVersion() API instead of the POSIX-y uname().

dscho · 2017-05-30T20:41:30Z

Oh, and I also decided to test for the build number of the first Insider build that supported the flag...

…only) This patch fixes signature of the CreateSymbolicLinkW() function's declaratoin. The previous declaration claimed that said function returns a BOOL, while it really returns a BOOLEAN. This is not an academic distinction: BOOL is defined as an int (i.e. 32-bit) and BOOLEAN as an unsigned char (i.e. 8-bit). Therefore, the return value 0 (meaning, the least-significant 8 bits are all zero) could be mistaken to indicate a successful creation of the symbolic link (because the remaining 24 bits are undefined, and quite likely non-zero). Signed-off-by: Johannes Schindelin <[email protected]>

With Windows 10 Build 14972 in Developer Mode, a new flag is supported by CreateSymbolicLink() to create symbolic links even when running outside of an elevated session (which was previously required). This new flag is called SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE and has the numeric value 0x02. Previous Windows 10 versions will not understand that flag and return an ERROR_INVALID_PARAMETER, therefore we have to be careful to try passing that flag only when the build number indicates that it is supported. For more information about the new flag, see this blog post: https://blogs.windows.com/buildingapps/2016/12/02/symlinks-windows-10/ This patch is loosely based on the patch submitted by Samuel D. Leslie as git-for-windows#1184. Signed-off-by: Johannes Schindelin <[email protected]>

ralish · 2017-05-31T09:40:41Z

Looks good to me and nice pick-up on the return type of CreateSymbolicLinkW!

Only comment is I think it'd be nice to preserve the #ifndefs corresponding to the CreateSymbolicLinkW flags for readability in the absence of importing the actual header. Otherwise we do end-up with a fair few magic numbers that are going to be potentially quite non-obvious to others. A minor point for sure, but in general I think the less magic numbers the better.

dscho · 2017-05-31T09:47:03Z

in general I think the less magic numbers the better.

I agree. However, the header files available in Git for Windows' SDK do not even have that flag yet. So even if we bumped _WINNT_VER to 0x600 (i.e. require Windows Vista or later, which leads to all kinds of unrelated compile errors e.g. because of the pollfd declarations), we would not be able to include the header and use the symbol.

As to #define'ing it, we also have to consider Git's pretty strict formatting rules: 80 columns per line, no more. And that symbol is really long (I thought only Germans use stupidly long names, but apparently I was wrong).

As the symbol's name has been mentioned in the code comment above, I am a bit less concerned about the magic-ness of the 2, too ;-)

Thanks for the look-over!

ralish · 2017-05-31T09:51:28Z

Excellent, thanks for the explanation as to the rationale. As for the missing flag, I addressed that in a separate patch a few days ago to the Mingw-w64 developers so it should be available in a future release (though doesn't solve the 80 columns per line issue, but indicating anyway just for reference).

Thanks for merging. Looking forward to finally having 100% usable symlinks on Windows in Git!

Git [now uses the flag introduced with Windows 10 Creators Update to create symbolic links without requiring elevated privileges](git-for-windows/git#1188) in Developer Mode. Signed-off-by: Johannes Schindelin <[email protected]>

dscho · 2017-05-31T15:37:43Z

Looking forward to finally having 100% usable symlinks on Windows in Git!

Yesss! And thanks for all your help!

Support creating symlinks outside elevated sessions

dscho added 2 commits May 30, 2017 22:44

dscho force-pushed the unprivileged-symlinks branch from 00813f0 to dbba5f1 Compare May 30, 2017 20:46

ralish mentioned this pull request May 31, 2017

Add support for creating symlinks as an unprivileged user #1184

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dscho merged commit a4d0d5f into git-for-windows:master May 31, 2017

dscho deleted the unprivileged-symlinks branch May 31, 2017 09:47

dscho added this to the v2.13.0(1) milestone May 31, 2017

This was referenced May 31, 2017

Tests fail locally on Windows python/mypy#3222

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Create symlinks when cloning repos with symlinks #117

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Should the installer default to enabling symlinks when running on the Windows 10 Creators Update? #1189

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corford mentioned this pull request Jun 2, 2017

Allow to save files that need user elevation microsoft/vscode#1614

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Implement pthread_cond_t with Win32 CONDITION_VARIABLE #1214

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Test real symlink handling on Windows gradle/gradle#9077

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Support creating symlinks outside elevated sessions #1188

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dscho commented May 30, 2017

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Support creating symlinks outside elevated sessions #1188

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