gdbsupport: add path_join function

In this review [1], Eli pointed out that we should be careful when
concatenating file names to avoid duplicated slashes.  On Windows, a
double slash at the beginning of a file path has a special meaning.  So
naively concatenating "/"  and "foo/bar" would give "//foo/bar", which
would not give the desired results.  We already have a few spots doing:

  if (first_path ends with a slash)
    path = first_path + second_path
  else
    path = first_path + slash + second_path

In general, I think it's nice to avoid superfluous slashes in file
paths, since they might end up visible to the user and look a bit
unprofessional.

Introduce the path_join function that can be used to join multiple path
components together (along with unit tests).

I initially wanted to make it possible to join two absolute paths, to
support the use case of prepending a sysroot path to a target file path,
or the prepending the debug-file-directory to a target file path.  But
the code in solib_find_1 shows that it is more complex than this anyway
(for example, when the right hand side is a Windows path with a drive
letter).  So I don't think we need to support that case in path_join.
That also keeps the implementation simpler.

Change a few spots to use path_join to show how it can be used.  I
believe that all the spots I changed are guarded by some checks that
ensure the right hand side operand is not an absolute path.

Regression-tested on Ubuntu 18.04.  Built-tested on Windows, and I also
ran the new unit-test there.

[1] https://sourceware.org/pipermail/gdb-patches/2022-April/187559.html

Change-Id: I0df889f7e3f644e045f42ff429277b732eb6c752

1 files changed, 73 insertions, 0 deletions

diff --git a/gdb/unittests/path-join-selftests.c b/gdb/unittests/path-join-selftests.c
new file mode 100644
index 0000000..d074cda
--- /dev/null
+++ b/gdb/unittests/path-join-selftests.c
@@ -0,0 +1,73 @@
+/* Self tests for path_join for GDB, the GNU debugger.
+
+   Copyright (C) 2022 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 "defs.h"
+#include "gdbsupport/pathstuff.h"
+#include "gdbsupport/selftest.h"
+
+namespace selftests {
+namespace path_join {
+
+template <typename ...Args>
+static void
+test_one (const char *expected, Args... paths)
+{
+  std::string actual = ::path_join (paths...);
+
+  SELF_CHECK (actual == expected);
+}
+
+/* Test path_join.  */
+
+static void
+test ()
+{
+  test_one ("/foo/bar", "/foo", "bar");
+  test_one ("/bar", "/", "bar");
+  test_one ("foo/bar/", "foo", "bar", "");
+  test_one ("foo", "", "foo");
+  test_one ("foo/bar", "foo", "", "bar");
+  test_one ("foo/", "foo", "");
+  test_one ("foo/", "foo/", "");
+
+  test_one ("D:/foo/bar", "D:/foo", "bar");
+  test_one ("D:/foo/bar", "D:/foo/", "bar");
+
+#if defined(_WIN32)
+  /* The current implementation doesn't recognize backslashes as directory
+     separators on Unix-like systems, so only run these tests on Windows.  If
+     we ever switch our implementation to use std::filesystem::path, they
+     should work anywhere, in theory.  */
+  test_one ("D:\\foo/bar", "D:\\foo", "bar");
+  test_one ("D:\\foo\\bar", "D:\\foo\\", "bar");
+  test_one ("\\\\server\\dir\\file", "\\\\server\\dir\\", "file");
+  test_one ("\\\\server\\dir/file", "\\\\server\\dir", "file");
+#endif /* _WIN32 */
+}
+
+}
+}
+
+void _initialize_path_join_selftests ();
+void
+_initialize_path_join_selftests ()
+{
+  selftests::register_test ("path_join",
+			    selftests::path_join::test);
+}