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.

4 files changed, 42 insertions, 3 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index bef807d..efe07af 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,10 @@
+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.
+
 2019-05-02  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* rust-lang.c (val_print_struct): Handle printing structures
diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index 4166f19..a26e20c 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,3 +1,9 @@
+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.
+
 2019-04-29  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdb.texinfo (Print Settings): Document 'print max-depth'.
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 2282c80..dd8ae91 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -20007,9 +20007,13 @@ uses two different methods of looking for the debug file:
 @item
 For the ``debug link'' method, @value{GDBN} looks up the named file in
 the directory of the executable file, then in a subdirectory of that
-directory named @file{.debug}, and finally under each one of the global debug
-directories, in a subdirectory whose name is identical to the leading
-directories of the executable's absolute file name.
+directory named @file{.debug}, and finally under each one of the
+global debug directories, in a subdirectory whose name is identical to
+the leading directories of the executable's absolute file name.  (On
+MS-Windows/MS-DOS, the drive letter of the executable's leading
+directories is converted to a one-letter subdirectory, i.e.@:
+@file{d:/usr/bin/} is converted to @file{/d/usr/bin/}, because Windows
+filesystems disallow colons in file names.)
 
 @item
 For the ``build ID'' method, @value{GDBN} looks in the
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;