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Implement a winsymlinks mode that prefers native symlinks, falling back to the deep copy mode #114

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Implement a winsymlinks mode that prefers native symlinks, falling back to the deep copy mode #114

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f0d57f8
fixup! Instead of creating Cygwin symlinks, use deep copy by default
dscho Nov 7, 2022
b8e842e
winsymlinks: add a mode preferring native symlinks with a deepcopy fa…
dscho Nov 7, 2022
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2 changes: 2 additions & 0 deletions winsup/cygwin/environ.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -92,6 +92,8 @@ set_winsymlinks (const char *buf)
? WSYM_nativestrict : WSYM_native;
else if (ascii_strncasematch (buf, "deepcopy", 8))
allow_winsymlinks = WSYM_deepcopy;
else if (ascii_strncasematch (buf, "nativeordeepcopy", 8))
allow_winsymlinks = WSYM_native_or_deepcopy;
else
allow_winsymlinks = WSYM_sysfile;
}
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3 changes: 2 additions & 1 deletion winsup/cygwin/globals.cc
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Expand Up @@ -57,7 +57,8 @@ enum winsym_t
WSYM_nativestrict,
WSYM_nfs,
WSYM_sysfile,
WSYM_deepcopy
WSYM_deepcopy,
WSYM_native_or_deepcopy
};

exit_states NO_COPY exit_state;
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10 changes: 10 additions & 0 deletions winsup/cygwin/path.cc
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Expand Up @@ -2110,6 +2110,9 @@ symlink_worker (const char *oldpath, path_conv &win32_newpath, bool isdevice)
else if ((wsym_type == WSYM_native || wsym_type == WSYM_nativestrict)
&& !(win32_newpath.fs_flags () & FILE_SUPPORTS_REPARSE_POINTS))
wsym_type = WSYM_default;
else if (wsym_type == WSYM_native_or_deepcopy
&& !(win32_newpath.fs_flags () & FILE_SUPPORTS_REPARSE_POINTS))
wsym_type = WSYM_deepcopy;

/* Attach .lnk suffix when shortcut is requested. */
if (wsym_type == WSYM_lnk && !win32_newpath.exists ()
Expand Down Expand Up @@ -2144,6 +2147,7 @@ symlink_worker (const char *oldpath, path_conv &win32_newpath, bool isdevice)
__leave;
case WSYM_native:
case WSYM_nativestrict:
case WSYM_native_or_deepcopy:
res = symlink_native (oldpath, win32_newpath);
if (!res)
__leave;
Expand All @@ -2154,6 +2158,12 @@ symlink_worker (const char *oldpath, path_conv &win32_newpath, bool isdevice)
__seterrno ();
__leave;
}
/* With deepcopy fall back? Let's do that, then */
if (res == -1 && wsym_type == WSYM_native_or_deepcopy)
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Note that if target isspecial res will be -2, so will still fall back to default. This is probably reasonable, since deepcopy would just fall back to sysfile in that case. default will let it try wsl before falling back to sysfile. I'm thinking it might be best to make deepcopy fall back to default instead of sysfile anyway.

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making deepcopy fall back to wsl instead of sysfile breaks one git test which tries to commit a symlink to /dev/null. The result of doing so is almost certainly not helpful (I bet it commits the sysfile, not a symlink), but when it's a wsl symlink it fails with Function not implemented

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when it's a wsl symlink it fails with Function not implemented

That's my fault, as I have not managed to spend time on upstreaming Git for Windows' symlink support: git/git@master...dscho:git:support-symlinks-on-windows

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I had a quick look at those changes, and I didn't see any handling of wsl symlinks in there, so I don't think that would help.

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Huh. I misremembered. A git grep IO_REPARSE_TAG_LX_SYMLINK indeed comes up empty...

{
wsym_type = WSYM_deepcopy;
break;
}
/* Otherwise, fall back to default symlink type. */
wsym_type = WSYM_default;
fallthrough;
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18 changes: 17 additions & 1 deletion winsup/doc/cygwinenv.xml
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Expand Up @@ -104,7 +104,7 @@ back to the parent process.</para>
</listitem>

<listitem>
<para><envar>winsymlinks:{lnk,native,nativestrict,sys}</envar></para>
<para><envar>winsymlinks:{lnk,native,nativestrict,sys,deepcopy,nativeordeepcopy}</envar></para>

<itemizedlist mark="square">
<listitem>
Expand Down Expand Up @@ -133,6 +133,22 @@ system call will immediately fail.</para>
plain files with the <literal>system</literal> attribute, containing a magic
cookie followed by the path to which the link points.</para>
</listitem>

<listitem>
<para>If set to <literal>winsymlinks:deepcopy</literal>, a copy of the source
will be created at the target (a "deep" copy in the case of directories, i.e.
the source directory will be copied recursively). This mode makes a trade-off
between compatibility and interoperability with Win32 programs, favoring the
latter.</para>
</listitem>

<listitem>
<para>If set to <literal>winsymlinks:nativeordeepcopy</literal> Cygwin creates
symlinks as native Windows symlinks if supported (i.e. on file systems
supporting symbolic links, and when the current user is permitted to create
symbolic links, e.g. in Windows 10's "Developer Mode"), and fall back to
creating a deep copy in case symlinks are not supported.</para>
</listitem>
</itemizedlist>

<para>Note that this setting has no effect where Cygwin knows that the
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