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

2 files changed, 53 insertions, 15 deletions

diff --git a/gdbsupport/pathstuff.cc b/gdbsupport/pathstuff.cc
index ac65651..5b5a8ee 100644
--- a/gdbsupport/pathstuff.cc
+++ b/gdbsupport/pathstuff.cc
@@ -119,10 +119,7 @@ gdb_realpath_keepfile (const char *filename)
      directory separator, avoid doubling it.  */
   gdb::unique_xmalloc_ptr<char> path_storage = gdb_realpath (dir_name);
   const char *real_path = path_storage.get ();
-  if (IS_DIR_SEPARATOR (real_path[strlen (real_path) - 1]))
-    return string_printf ("%s%s", real_path, base_name);
-  else
-    return string_printf ("%s/%s", real_path, base_name);
+  return path_join (real_path, base_name);
 }
 
 /* See gdbsupport/pathstuff.h.  */
@@ -138,12 +135,7 @@ gdb_abspath (const char *path)
   if (IS_ABSOLUTE_PATH (path) || current_directory == NULL)
     return path;
 
-  /* Beware the // my son, the Emacs barfs, the botch that catch...  */
-  return string_printf
-    ("%s%s%s", current_directory,
-     (IS_DIR_SEPARATOR (current_directory[strlen (current_directory) - 1])
-      ? "" : SLASH_STRING),
-     path);
+  return path_join (current_directory, path);
 }
 
 /* See gdbsupport/pathstuff.h.  */
@@ -198,6 +190,29 @@ child_path (const char *parent, const char *child)
 
 /* See gdbsupport/pathstuff.h.  */
 
+std::string
+path_join (gdb::array_view<const gdb::string_view> paths)
+{
+  std::string ret;
+
+  for (int i = 0; i < paths.size (); ++i)
+    {
+      const gdb::string_view path = paths[i];
+
+      if (i > 0)
+	gdb_assert (!IS_ABSOLUTE_PATH (path));
+
+      if (!ret.empty () && !IS_DIR_SEPARATOR (ret.back ()))
+	  ret += '/';
+
+      ret.append (path.begin (), path.end ());
+    }
+
+  return ret;
+}
+
+/* See gdbsupport/pathstuff.h.  */
+
 bool
 contains_dir_separator (const char *path)
 {
@@ -227,7 +242,7 @@ get_standard_cache_dir ()
     {
       /* Make sure the path is absolute and tilde-expanded.  */
       std::string abs = gdb_abspath (xdg_cache_home);
-      return string_printf ("%s/gdb", abs.c_str ());
+      return path_join (abs.c_str (), "gdb");
     }
 #endif
 
@@ -236,7 +251,7 @@ get_standard_cache_dir ()
     {
       /* Make sure the path is absolute and tilde-expanded.  */
       std::string abs = gdb_abspath (home);
-      return string_printf ("%s/" HOME_CACHE_DIR "/gdb", abs.c_str ());
+      return path_join (abs.c_str (), HOME_CACHE_DIR,  "gdb");
     }
 
 #ifdef WIN32
@@ -245,7 +260,7 @@ get_standard_cache_dir ()
     {
       /* Make sure the path is absolute and tilde-expanded.  */
       std::string abs = gdb_abspath (win_home);
-      return string_printf ("%s/gdb", abs.c_str ());
+      return path_join (abs.c_str (), "gdb");
     }
 #endif
 
@@ -294,7 +309,7 @@ get_standard_config_dir ()
     {
       /* Make sure the path is absolute and tilde-expanded.  */
       std::string abs = gdb_abspath (xdg_config_home);
-      return string_printf ("%s/gdb", abs.c_str ());
+      return path_join (abs.c_str (), "gdb");
     }
 #endif
 
@@ -303,7 +318,7 @@ get_standard_config_dir ()
     {
       /* Make sure the path is absolute and tilde-expanded.  */
       std::string abs = gdb_abspath (home);
-      return string_printf ("%s/" HOME_CONFIG_DIR "/gdb", abs.c_str ());
+      return path_join (abs.c_str (), HOME_CONFIG_DIR, "gdb");
     }
 
   return {};
diff --git a/gdbsupport/pathstuff.h b/gdbsupport/pathstuff.h
index f6c51e9..c859279 100644
--- a/gdbsupport/pathstuff.h
+++ b/gdbsupport/pathstuff.h
@@ -21,6 +21,7 @@
 #define COMMON_PATHSTUFF_H
 
 #include "gdbsupport/byte-vector.h"
+#include "gdbsupport/array-view.h"
 
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -60,6 +61,28 @@ extern std::string gdb_abspath (const char *path);
 
 extern const char *child_path (const char *parent, const char *child);
 
+/* Join elements in PATHS into a single path.
+
+   The first element can be absolute or relative.  All the others must be
+   relative.  */
+
+extern std::string path_join (gdb::array_view<const gdb::string_view> paths);
+
+/* Same as the above, but accept paths as distinct parameters.  */
+
+template<typename ...Args>
+std::string
+path_join (Args... paths)
+{
+  /* It doesn't make sense to join less than two paths.  */
+  gdb_static_assert (sizeof... (Args) >= 2);
+
+  std::array<gdb::string_view, sizeof... (Args)> views
+    { gdb::string_view (paths)... };
+
+  return path_join (gdb::array_view<const gdb::string_view> (views));
+}
+
 /* Return whether PATH contains a directory separator character.  */
 
 extern bool contains_dir_separator (const char *path);