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

…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>

winsup/cygwin/environ.cc

@@ -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;
 }

winsup/cygwin/globals.cc

@@ -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;

winsup/cygwin/path.cc

@@ -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 ()
@@ -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;
@@ -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)
+	    {
+	      wsym_type = WSYM_deepcopy;
+	      break;
+	    }
 	  /* Otherwise, fall back to default symlink type. */
 	  wsym_type = WSYM_default;
 	  fallthrough;

winsup/doc/cygwinenv.xml

@@ -104,7 +104,7 @@ back to the parent process.</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>
@@ -141,6 +141,14 @@ 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