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winsymlinks: add a mode preferring native symlinks with a deepcopy fa…
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…llback

When native symlinks are available, it is a shame to create deep copies
by default.

However, since there are many scenarios where symlinks are not available
(e.g. when running on FAT, or on older Windows versions, or when
Developer Mode is not enabled), we've got to have a fallback.

In the regular Cygwin world, it is legitimate to fall back to WSL
symlinks and/or to the system file emulation (where a file is created
that is marked with the "system" attribute and with content that adheres
to a specific, magic form that is recognized specifically by the Cygwin
runtime).

However, in the world of MSYS2, the assumption is that the result of the
operation should be as interoperable with regular Win32 programs as
possible. Hence the default to "deepcopy".

As a "best of both worlds" mode, let's implement one that tries to
create native symlinks by default, and if that fails, uses the
"deepcopy" method as a fallback.

This addresses #113.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
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dscho committed Nov 7, 2022
1 parent 6f2e6f0 commit e69c9f4
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Showing 4 changed files with 23 additions and 2 deletions.
2 changes: 2 additions & 0 deletions winsup/cygwin/environ.cc
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
Original file line number Diff line number Diff line change
Expand Up @@ -59,7 +59,8 @@ 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
Original file line number Diff line number Diff line change
Expand Up @@ -2109,6 +2109,9 @@ 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 @@ -2143,6 +2146,7 @@ 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 @@ -2153,6 +2157,12 @@ 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)
{
wsym_type = WSYM_deepcopy;
break;
}
/* Otherwise, fall back to default symlink type. */
wsym_type = WSYM_default;
fallthrough;
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10 changes: 9 additions & 1 deletion winsup/doc/cygwinenv.xml
Original file line number Diff line number Diff line change
Expand Up @@ -76,7 +76,7 @@ in addition to the normal UNIX argv list. Defaults to set.</para>
</listitem>

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

<itemizedlist mark="square">
<listitem>
Expand Down Expand Up @@ -113,6 +113,14 @@ the source directory will be copied recursively). This mode makes a trade-off
between compatibility and interoperability with Win32 programs, favoring the
latter.
</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.
</listitem>
</itemizedlist>

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