[PATCH] Improve gdb_tilde_expand logic

[Lancelot SIX via Gdb-patches <gdb-patches@sourceware.org>]

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 leave 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:

	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. I am unsure where to place unittests for this
function.

My copyright assignment is still pending, but should be signed soon.

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.
---
 gdbsupport/gdb_tilde_expand.cc | 45 +++++++++++++++++++++++++---------
 gdbsupport/gdb_tilde_expand.h  |  3 +--
 2 files changed, 34 insertions(+), 14 deletions(-)

diff --git a/gdbsupport/gdb_tilde_expand.cc b/gdbsupport/gdb_tilde_expand.cc
index b31fc48446..d060e26f50 100644
--- a/gdbsupport/gdb_tilde_expand.cc
+++ b/gdbsupport/gdb_tilde_expand.cc
@@ -17,7 +17,9 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+#include <algorithm>
 #include "common-defs.h"
+#include "filenames.h"
 #include "gdb_tilde_expand.h"
 #include <glob.h>
 
@@ -71,14 +73,37 @@ private:
 std::string
 gdb_tilde_expand (const char *dir)
 {
-  gdb_glob glob (dir, GLOB_TILDE_CHECK, NULL);
+  if (dir[0] == '~')
+    {
+      /* 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.
 
-  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];
+	 In order to avoid such limitation, we only use
+	 glob to expand "~" and keep "/does/not/exist" unchanged.
 
-  return expanded_dir;
+	 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)
+      const auto first_sep =
+	std::find_if (d.cbegin (), d.cend(),
+		      [](const auto c) { return IS_DIR_SEPARATOR (c); });
+
+      // split d according to the first separator
+      const std::string to_expand (d.cbegin (), first_sep);
+      const std::string remainder (first_sep, d.cend ());
+
+      // Expand the first element
+      const gdb_glob glob (to_expand.c_str (), GLOB_TILDE_CHECK, nullptr);
+      gdb_assert (glob.pathc () == 1);
+
+      return std::string (glob.pathv ()[0]) + remainder;
+    }
+  else
+    return std::string (dir);
 }
 
 /* See gdbsupport/gdb_tilde_expand.h.  */
@@ -86,10 +111,6 @@ 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]);
+  auto const 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 e2d85cadad..a61f246329 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
-- 
2.29.2