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.

1 files changed, 94 insertions, 0 deletions

diff --git a/gdb/unittests/gdb_tilde_expand-selftests.c b/gdb/unittests/gdb_tilde_expand-selftests.c
new file mode 100644
index 0000000..b57f7be
--- /dev/null
+++ b/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);
+}