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, 21 insertions, 24 deletions

diff --git a/gdb/dwarf2/line-header.c b/gdb/dwarf2/line-header.c
index 77e7e55..8f4eb4f 100644
--- a/gdb/dwarf2/line-header.c
+++ b/gdb/dwarf2/line-header.c
@@ -24,6 +24,7 @@
 #include "dwarf2/read.h"
 #include "complaints.h"
 #include "filenames.h"
+#include "gdbsupport/pathstuff.h"
 
 void
 line_header::add_include_dir (const char *include_dir)
@@ -73,7 +74,7 @@ line_header::file_file_name (int file) const
 	{
 	  const char *dir = fe->include_dir (this);
 	  if (dir != NULL)
-	    return string_printf ("%s/%s", dir, fe->name);
+	    return path_join (dir, fe->name);
 	}
 
       return fe->name;
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index bb72e0e..8586463 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -1280,7 +1280,7 @@ static const char *compute_include_file_name
      (const struct line_header *lh,
       const file_entry &fe,
       const file_and_directory &cu_info,
-      gdb::unique_xmalloc_ptr<char> *name_holder);
+      std::string &name_holder);
 
 static htab_up allocate_signatured_type_table ();
 
@@ -2780,9 +2780,9 @@ dw2_get_file_names_reader (const struct die_reader_specs *reader,
     {
       for (const auto &entry : lh->file_names ())
 	{
-	  gdb::unique_xmalloc_ptr<char> name_holder;
+	  std::string name_holder;
 	  const char *include_name =
-	    compute_include_file_name (lh.get (), entry, fnd, &name_holder);
+	    compute_include_file_name (lh.get (), entry, fnd, name_holder);
 	  if (include_name != nullptr)
 	    {
 	      include_name = per_objfile->objfile->intern (include_name);
@@ -11098,14 +11098,13 @@ open_dwo_file (dwarf2_per_objfile *per_objfile,
 
   if (comp_dir != NULL)
     {
-      gdb::unique_xmalloc_ptr<char> path_to_try
-	(concat (comp_dir, SLASH_STRING, file_name, (char *) NULL));
+      std::string path_to_try = path_join (comp_dir, file_name);
 
       /* NOTE: If comp_dir is a relative path, this will also try the
 	 search path, which seems useful.  */
-      gdb_bfd_ref_ptr abfd (try_open_dwop_file (per_objfile, path_to_try.get (),
-						0 /*is_dwp*/,
-						1 /*search_cwd*/));
+      gdb_bfd_ref_ptr abfd (try_open_dwop_file
+	(per_objfile, path_to_try.c_str (), 0 /*is_dwp*/, 1 /*search_cwd*/));
+
       if (abfd != NULL)
 	return abfd;
     }
@@ -19697,14 +19696,14 @@ dwarf_decode_line_header (sect_offset sect_off, struct dwarf2_cu *cu)
 static const char *
 compute_include_file_name (const struct line_header *lh, const file_entry &fe,
 			   const file_and_directory &cu_info,
-			   gdb::unique_xmalloc_ptr<char> *name_holder)
+			   std::string &name_holder)
 {
   const char *include_name = fe.name;
   const char *include_name_to_compare = include_name;
 
   const char *dir_name = fe.include_dir (lh);
 
-  gdb::unique_xmalloc_ptr<char> hold_compare;
+  std::string hold_compare;
   if (!IS_ABSOLUTE_PATH (include_name)
       && (dir_name != nullptr || cu_info.get_comp_dir () != nullptr))
     {
@@ -19731,27 +19730,24 @@ compute_include_file_name (const struct line_header *lh, const file_entry &fe,
 
       if (dir_name != NULL)
 	{
-	  name_holder->reset (concat (dir_name, SLASH_STRING,
-				      include_name, (char *) NULL));
-	  include_name = name_holder->get ();
+	  name_holder = path_join (dir_name, include_name);
+	  include_name = name_holder.c_str ();
 	  include_name_to_compare = include_name;
 	}
       if (!IS_ABSOLUTE_PATH (include_name)
 	  && cu_info.get_comp_dir () != nullptr)
 	{
-	  hold_compare.reset (concat (cu_info.get_comp_dir (), SLASH_STRING,
-				      include_name, (char *) NULL));
-	  include_name_to_compare = hold_compare.get ();
+	  hold_compare = path_join (cu_info.get_comp_dir (), include_name);
+	  include_name_to_compare = hold_compare.c_str ();
 	}
     }
 
-  gdb::unique_xmalloc_ptr<char> copied_name;
+  std::string copied_name;
   const char *cu_filename = cu_info.get_name ();
   if (!IS_ABSOLUTE_PATH (cu_filename) && cu_info.get_comp_dir () != nullptr)
     {
-      copied_name.reset (concat (cu_info.get_comp_dir (), SLASH_STRING,
-				 cu_filename, (char *) NULL));
-      cu_filename = copied_name.get ();
+      copied_name = path_join (cu_info.get_comp_dir (), cu_filename);
+      cu_filename = copied_name.c_str ();
     }
 
   if (FILENAME_CMP (include_name_to_compare, cu_filename) == 0)
@@ -20486,7 +20482,7 @@ static void
 dwarf2_start_subfile (struct dwarf2_cu *cu, const char *filename,
 		      const char *dirname)
 {
-  gdb::unique_xmalloc_ptr<char> copy;
+  std::string copy;
 
   /* In order not to lose the line information directory,
      we concatenate it to the filename when it makes sense.
@@ -20497,8 +20493,8 @@ dwarf2_start_subfile (struct dwarf2_cu *cu, const char *filename,
 
   if (!IS_ABSOLUTE_PATH (filename) && dirname != NULL)
     {
-      copy.reset (concat (dirname, SLASH_STRING, filename, (char *) NULL));
-      filename = copy.get ();
+      copy = path_join (dirname, filename);
+      filename = copy.c_str ();
     }
 
   cu->get_builder ()->start_subfile (filename);