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.

Author: lsix

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2021-01-23  Lancelot SIX  <[email protected]>
+
+	* Makefile.in (SELFTESTS_SRCS): Add
+	unittests/gdb_tilde_expand-selftests.c.
+	* unittests/gdb_tilde_expand-selftests.c: New file.
+
 2021-01-22  Andrew Burgess  <[email protected]>
 
 	PR cli/25956

gdb/Makefile.in

@@ -446,6 +446,7 @@ SELFTESTS_SRCS = \
 	unittests/filtered_iterator-selftests.c \
 	unittests/format_pieces-selftests.c \
 	unittests/function-view-selftests.c \
+	unittests/gdb_tilde_expand-selftests.c \
 	unittests/gmp-utils-selftests.c \
 	unittests/lookup_name_info-selftests.c \
 	unittests/memory-map-selftests.c \

gdb/unittests/gdb_tilde_expand-selftests.c

@@ -0,0 +1,94 @@
+/* Self tests for gdb_tilde_expand
+
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   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 "gdbsupport/common-defs.h"
+#include "gdbsupport/selftest.h"
+#include <cstdlib>
+
+#include "gdbsupport/gdb_tilde_expand.h"
+
+namespace selftests {
+namespace gdb_tilde_expand_tests {
+
+static void
+do_test ()
+{
+  /* Home directory of the user running the test.  */
+  const char *c_home = std::getenv ("HOME");
+
+  /* Skip the test if $HOME is not available in the environment.  */
+  if (c_home == nullptr)
+    return;
+
+  const std::string home (c_home);
+
+  /* Basic situation.  */
+  SELF_CHECK (home == gdb_tilde_expand ("~"));
+
+  /* When given a path that begins by a tilde and refers to a file that
+     does not exist, gdb_tilde expand must still be able to do the tilde
+     expansion.  */
+  SELF_CHECK (gdb_tilde_expand ("~/non/existent/directory")
+              == home + "/non/existent/directory");
+
+  /* gdb_tilde_expand only expands tilde and does not try to do any other
+     substitution.  */
+  SELF_CHECK (gdb_tilde_expand ("~/*/a.out") == home + "/*/a.out");
+
+  /* gdb_tilde_expand does no modification to a non tilde leading path.  */
+  SELF_CHECK (gdb_tilde_expand ("/some/abs/path") == "/some/abs/path");
+  SELF_CHECK (gdb_tilde_expand ("relative/path") == "relative/path");
+
+  /* If $USER is available in the env variables, check the '~user'
+     expansion.  */
+  const char *c_user = std::getenv ("USER");
+  if (c_user != nullptr)
+    {
+      const std::string user (c_user);
+      SELF_CHECK (gdb_tilde_expand (("~" + user).c_str ()) == home);
+      SELF_CHECK (gdb_tilde_expand (("~" + user + "/a/b").c_str ())
+                  == home + "/a/b");
+    }
+
+  /* Check that an error is thrown when trying to expand home of a unknown
+     user.  */
+  try
+    {
+      gdb_tilde_expand ("~no_one_should_have_that_login/a");
+      SELF_CHECK (false);
+    }
+  catch (const gdb_exception_error &e)
+    {
+      SELF_CHECK (e.error == GENERIC_ERROR);
+      SELF_CHECK
+        (*e.message
+         == "Could not find a match for '~no_one_should_have_that_login'.");
+    }
+}
+
+} /* namespace gdb_tilde_expand_tests */
+} /* namespace selftests */
+
+void _initialize_gdb_tilde_expand_selftests ();
+void
+_initialize_gdb_tilde_expand_selftests ()
+{
+  selftests::register_test
+    ("gdb_tilde_expand", selftests::gdb_tilde_expand_tests::do_test);
+}

gdbsupport/ChangeLog

@@ -1,3 +1,10 @@
+2021-01-23  Lancelot SIX  <[email protected]>
+
+	* 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  <[email protected]>
 
 	* common-debug.h (debug_prefixed_printf_cond_nofunc): New.

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>
 
@@ -71,25 +73,41 @@ class gdb_glob
 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.  */
 
 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 ());
 }

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