[rfc] Proposal: fake symlinks support on windows

[Alexander Belchenko]

Aaron Bentley пишет:
> Alexander Belchenko wrote:
>> I propose reuse cygwin format and gain full compatibility with cygwin.
>>
>> I propose to make special methods in bzrlib/osutils.py module:
> 
> I don't think they should go there, because if we switch on platform, 
> *nix won't handle these symlinks properly (i.e. if the tree is created 
> by Windows, then accessed by *nix.)
> 
> I think the symlink-handling approach should be per-working tree, and 
> that *nix should use this style when working with a tree that was 
> created by Windows.

I dig deeper. I played with symlinks on linux with vfat fylesystem. I 
played with copying links between directories on cygwin.

Short summary: make a full cross-platform support and access to symlinks 
in working tree from non-native filesystem is painful terrible work. 
Windows and Cygwin share the same filesystem therefore they have good 
compatibility. But anyway it's better don't mix them.

So I restrict my proposal for fake symlinks support to deal with 
symlinks only inside bounds of native filesystem, without mixing access 
to working tree from outside.

I wrote plugin that run-time-patches some modules:
* os module (adds symlink(), readlink() methods that absent on native 
windows),
* os.path module (os.path.islink() function),
* bzrlib.osutils module (function file_kind()).

In addition I slightly change selftest and run tests with and without 
patch. Fortunately all symlinks-specific tests passed, and I don't find 
any regression in overall test suite.

If you insist to make all work with symlinks via WorkingTree methods 
then there is need to grep and change all existing code and change 
explicit os.symlink/os.readlink. That will be *very* big patch.

So at this moment I'd like to hear comments from developers that 
interested in this symlink simulation. What my simple os-patching plugin 
is missed? What additional tests should be added, improved?

In attachment you'll find patch for selftest subsystem of bzr. This 
changes is not depend on windows nor cygwin. I've add test_symlinks.py 
module with tests somewhat specific for my plugin. This tests looks 
trivial for unix where  I want to keep it separately and independetly in 
main bzrlib code with reason to split namespaces.

I think this patch is worth to include in main bzr.dev.


My plugin you can reach here:

bzr branch http://bzr.onembedding.com/bzr.win/plugins/win32symlinks/


Results of symlink-specific tests
=================================
Command: python bzr selftest symlink -v

Run without plugin on win32 native
**********************************
        bzr: D:/user/python/bzr/bzr.dev.symlinks/bzr
     bzrlib: D:\user\python\bzr\bzr.dev.symlinks\bzrlib

test_inv.TestInventoryEntry.test_symlink_detect_changes             OK 
    9ms
test_merge_core.MergeTest.test_symlink_conflicts                  SKIP 
    0ms
      Symlinks are not supported on this platform
test_merge_core.MergeTest.test_symlink_merge                      SKIP 
   10ms
      Symlinks are not supported on this platform
test_symlinks.TestSymlinks.test_has_symlinks_support              SKIP 
  540ms
      Symlinks are not supported on this platform
test_symlinks.TestSymlinks.test_symlink_readlink                  SKIP 
    0ms
      Symlinks are not supported on this platform
test_testament.TestamentTests.test_testament_symlinks             SKIP 
1532ms
      Symlinks are not supported on this platform
test_transform.TestTreeTransform.test_symlinks                    SKIP 
  610ms
      Symlinks are not supported on this platform

----------------------------------------------------------------------
Ran 7 tests in 2.714s

OK


Run with plugin on win32 native
*******************************
        bzr: D:/user/python/bzr/bzr.dev.symlinks/bzr
     bzrlib: D:\user\python\bzr\bzr.dev.symlinks\bzrlib

test_inv.TestInventoryEntry.test_symlink_detect_changes             OK 
    9ms
test_merge_core.MergeTest.test_symlink_conflicts                    OK 
4185ms
test_merge_core.MergeTest.test_symlink_merge                        OK 
4746ms
test_symlinks.TestSymlinks.test_has_symlinks_support                OK 
  631ms
test_symlinks.TestSymlinks.test_symlink_readlink                    OK 
    9ms
test_testament.TestamentTests.test_testament_symlinks               OK 
2062ms
test_transform.TestTreeTransform.test_symlinks                      OK 
  842ms

----------------------------------------------------------------------
Ran 7 tests in 12.488s

OK


Run on cygwin
*************
        bzr: /cygdrive/d/user/python/bzr/bzr.dev.symlinks/bzr
     bzrlib: /cygdrive/d/user/python/bzr/bzr.dev.symlinks/bzrlib

test_inv.TestInventoryEntry.test_symlink_detect_changes             OK 
   20ms
test_merge_core.MergeTest.test_symlink_conflicts                    OK 
4275ms
test_merge_core.MergeTest.test_symlink_merge                        OK 
4497ms
test_symlinks.TestSymlinks.test_has_symlinks_support                OK 
  559ms
test_symlinks.TestSymlinks.test_symlink_readlink                    OK 
    9ms
test_testament.TestamentTests.test_testament_symlinks               OK 
1822ms
test_transform.TestTreeTransform.test_symlinks                      OK 
  791ms

----------------------------------------------------------------------
Ran 7 tests in 11.987s

OK


--
Alexander
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