Fix lookup of separate debug file on MS-Windows.

If you put the separate debug file in a global debug directory, GDB on
MS-Windows would fail to find it.  This happens because we obtain the
directory to look up the debug file by concatenating the debug
directory name with the leading directories of the executable, and the
latter includes the drive letter on MS-Windows.  So we get an invalid
file name like

   d:/usr/lib/debug/d:/usr/bin/foo.debug

This commit fixes that by removing the colon of the drive letter,
thus producing

   d:/usr/lib/debug/d/usr/bin/foo.debug

gdb/ChangeLog:
2019-05-03  Eli Zaretskii  <eliz@gnu.org>

	* symfile.c (find_separate_debug_file): Remove colon from the
	drive spec of DOS/Windows file names of the target, so that the
	file name produced from DEBUGDIR and the target's directory will
	be valid on DOS/Windows systems.

gdb/doc/ChangeLog:
2019-05-03  Eli Zaretskii  <eliz@gnu.org>

	* gdb.texinfo (Separate Debug Files): Document how the
	subdirectory of the global debug directory is computed on
	MS-Windows/MS-DOS.

1 files changed, 22 insertions, 0 deletions

diff --git a/gdb/symfile.c b/gdb/symfile.c
index 5736666..af99da1 100644
--- a/gdb/symfile.c
+++ b/gdb/symfile.c
@@ -1443,11 +1443,33 @@ find_separate_debug_file (const char *dir,
     = dirnames_to_char_ptr_vec (debug_file_directory);
   gdb::unique_xmalloc_ptr<char> canon_sysroot = gdb_realpath (gdb_sysroot);
 
+ /* MS-Windows/MS-DOS don't allow colons in file names; we must
+    convert the drive letter into a one-letter directory, so that the
+    file name resulting from splicing below will be valid.
+
+    FIXME: The below only works when GDB runs on MS-Windows/MS-DOS.
+    There are various remote-debugging scenarios where such a
+    transformation of the drive letter might be required when GDB runs
+    on a Posix host, see
+
+    https://sourceware.org/ml/gdb-patches/2019-04/msg00605.html
+
+    If some of those scenarions need to be supported, we will need to
+    use a different condition for HAS_DRIVE_SPEC and a different macro
+    instead of STRIP_DRIVE_SPEC, which work on Posix systems as well.  */
+  std::string drive;
+  if (HAS_DRIVE_SPEC (dir_notarget))
+    {
+      drive = dir_notarget[0];
+      dir_notarget = STRIP_DRIVE_SPEC (dir_notarget);
+    }
+
   for (const gdb::unique_xmalloc_ptr<char> &debugdir : debugdir_vec)
     {
       debugfile = target_prefix ? "target:" : "";
       debugfile += debugdir.get ();
       debugfile += "/";
+      debugfile += drive;
       debugfile += dir_notarget;
       debugfile += debuglink;