Improve gdb_tilde_expand logic.

Before this patch, gdb_tilde_expand would use glob(3) in order to expand
tilde at the begining of a path. This implementation has limitation when
expanding a tilde leading path to a non existing file since glob fails to
expand.

This patch proposes to use glob only to expand the tilde component of the
path and leaves the rest of the path unchanged.

This patch is a followup to the following discution:
https://sourceware.org/pipermail/gdb-patches/2021-January/174776.html

Before the patch:

	gdb_tilde_expand("~") -> "/home/lsix"
	gdb_tilde_expand("~/a/c/b") -> error() is called

After the patch:

	gdb_tilde_expand("~") -> "/home/lsix"
	gdb_tilde_expand("~/a/c/b") -> "/home/lsix/a/c/b"

Tested on x84_64 linux.

gdb/ChangeLog:

	* Makefile.in (SELFTESTS_SRCS): Add
	unittests/gdb_tilde_expand-selftests.c.
	* unittests/gdb_tilde_expand-selftests.c: New file.

gdbsupport/ChangeLog:

	* gdb_tilde_expand.cc (gdb_tilde_expand): Improve
	implementation.
	(gdb_tilde_expand_up): Delegate logic to gdb_tilde_expand.
	* gdb_tilde_expand.h (gdb_tilde_expand): Update description.

3 files changed, 40 insertions, 16 deletions

diff --git a/gdbsupport/ChangeLog b/gdbsupport/ChangeLog
index 487107e..fe6138c 100644
--- a/gdbsupport/ChangeLog
+++ b/gdbsupport/ChangeLog
@@ -1,3 +1,10 @@
+2021-01-23  Lancelot SIX  <lsix@lancelotsix.com>
+
+	* gdb_tilde_expand.cc (gdb_tilde_expand): Improve
+	implementation.
+	(gdb_tilde_expand_up): Delegate logic to gdb_tilde_expand.
+	* gdb_tilde_expand.h (gdb_tilde_expand): Update description.
+
 2021-01-22  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* common-debug.h (debug_prefixed_printf_cond_nofunc): New.
diff --git a/gdbsupport/gdb_tilde_expand.cc b/gdbsupport/gdb_tilde_expand.cc
index b31fc48..d9fb115 100644
--- a/gdbsupport/gdb_tilde_expand.cc
+++ b/gdbsupport/gdb_tilde_expand.cc
@@ -18,6 +18,8 @@
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include "common-defs.h"
+#include <algorithm>
+#include "filenames.h"
 #include "gdb_tilde_expand.h"
 #include <glob.h>
 
@@ -71,14 +73,34 @@ private:
 std::string
 gdb_tilde_expand (const char *dir)
 {
-  gdb_glob glob (dir, GLOB_TILDE_CHECK, NULL);
-
-  gdb_assert (glob.pathc () > 0);
-  /* "glob" may return more than one match to the path provided by the
-     user, but we are only interested in the first match.  */
-  std::string expanded_dir = glob.pathv ()[0];
-
-  return expanded_dir;
+  if (dir[0] != '~')
+    return std::string (dir);
+
+  /* This function uses glob in order to expand the ~.  However, this function
+     will fail to expand if the actual dir we are looking for does not exist.
+     Given "~/does/not/exist", glob will fail.
+
+     In order to avoid such limitation, we only use glob to expand "~" and keep
+     "/does/not/exist" unchanged.
+
+     Similarly, to expand ~gdb/might/not/exist, we only expand "~gdb" using
+     glob and leave "/might/not/exist" unchanged.  */
+  const std::string d (dir);
+
+  /* Look for the first dir separator (if any) and split d around it.  */
+  const auto first_sep
+    = std::find_if (d.cbegin (), d.cend(),
+                    [] (const char c) -> bool
+                    {
+                      return IS_DIR_SEPARATOR (c);
+                    });
+  const std::string to_expand (d.cbegin (), first_sep);
+  const std::string remainder (first_sep, d.cend ());
+
+  const gdb_glob glob (to_expand.c_str (), GLOB_TILDE_CHECK, nullptr);
+
+  gdb_assert (glob.pathc () == 1);
+  return std::string (glob.pathv ()[0]) + remainder;
 }
 
 /* See gdbsupport/gdb_tilde_expand.h.  */
@@ -86,10 +108,6 @@ gdb_tilde_expand (const char *dir)
 gdb::unique_xmalloc_ptr<char>
 gdb_tilde_expand_up (const char *dir)
 {
-  gdb_glob glob (dir, GLOB_TILDE_CHECK, NULL);
-
-  gdb_assert (glob.pathc () > 0);
-  /* "glob" may return more than one match to the path provided by the
-     user, but we are only interested in the first match.  */
-  return make_unique_xstrdup (glob.pathv ()[0]);
+  const std::string expanded = gdb_tilde_expand (dir);
+  return make_unique_xstrdup (expanded.c_str ());
 }
diff --git a/gdbsupport/gdb_tilde_expand.h b/gdbsupport/gdb_tilde_expand.h
index e2d85ca..a61f246 100644
--- a/gdbsupport/gdb_tilde_expand.h
+++ b/gdbsupport/gdb_tilde_expand.h
@@ -20,8 +20,7 @@
 #ifndef COMMON_GDB_TILDE_EXPAND_H
 #define COMMON_GDB_TILDE_EXPAND_H
 
-/* Perform path expansion (i.e., tilde expansion) on DIR, and return
-   the full path.  */
+/* Perform tilde expansion on DIR, and return the full path.  */
 extern std::string gdb_tilde_expand (const char *dir);
 
 /* Same as GDB_TILDE_EXPAND, but return the full path as a