Avoid race when reading dwz file

PR gdb/31260 points out a race introduced by the background reading
changes.  If a given objfile is re-opened when it is already being
read, dwarf2_initialize_objfile will call dwarf2_read_dwz_file again,
causing the 'dwz_file' to be reset.

This patch fixes the problem by arranging to open the dwz just once:
when the dwarf2_per_bfd object is created.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31260

gdb/dwarf2/dwz.c

@@ -188,6 +188,8 @@ dwarf2_read_dwz_file (dwarf2_per_objfile *per_objfile)
      only be run in the main thread.  */
   gdb_assert (is_main_thread ());
 
+  /* This should only be called once.  */
+  gdb_assert (!per_bfd->dwz_file.has_value ());
   /* Set this early, so that on error it remains NULL.  */
   per_bfd->dwz_file.emplace (nullptr);
 
@@ -281,14 +283,9 @@ dwarf2_read_dwz_file (dwarf2_per_objfile *per_objfile)
 struct dwz_file *
 dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd, bool require)
 {
-  gdb_assert (!require || per_bfd->dwz_file.has_value ());
-
-  dwz_file *result = nullptr;
-  if (per_bfd->dwz_file.has_value ())
-    {
-      result = per_bfd->dwz_file->get ();
-      if (require && result == nullptr)
-	error (_("could not read '.gnu_debugaltlink' section"));
-    }
+  gdb_assert (per_bfd->dwz_file.has_value ());
+  dwz_file *result = per_bfd->dwz_file->get ();
+  if (require && result == nullptr)
+    error (_("could not read '.gnu_debugaltlink' section"));
   return result;
 }

gdb/dwarf2/read.c

@@ -1366,6 +1366,7 @@ dwarf2_has_info (struct objfile *objfile,
   if (per_objfile == NULL)
     {
       dwarf2_per_bfd *per_bfd;
+      bool just_created = false;
 
       /* We can share a "dwarf2_per_bfd" with other objfiles if the
 	 BFD doesn't require relocations.
@@ -1385,6 +1386,7 @@ dwarf2_has_info (struct objfile *objfile,
 	      per_bfd = new dwarf2_per_bfd (objfile->obfd.get (), names,
 					    can_copy);
 	      dwarf2_per_bfd_bfd_data_key.set (objfile->obfd.get (), per_bfd);
+	      just_created = true;
 	    }
 	}
       else
@@ -1392,9 +1394,25 @@ dwarf2_has_info (struct objfile *objfile,
 	  /* No sharing possible, create one specifically for this objfile.  */
 	  per_bfd = new dwarf2_per_bfd (objfile->obfd.get (), names, can_copy);
 	  dwarf2_per_bfd_objfile_data_key.set (objfile, per_bfd);
+	  just_created = true;
 	}
 
       per_objfile = dwarf2_objfile_data_key.emplace (objfile, objfile, per_bfd);
+
+      if (just_created)
+	{
+	  /* Try to fetch any potential dwz file early, while still on
+	     the main thread.  Also, be sure to do it just once per
+	     BFD, to avoid races.  */
+	  try
+	    {
+	      dwarf2_read_dwz_file (per_objfile);
+	    }
+	  catch (const gdb_exception_error &err)
+	    {
+	      warning (_("%s"), err.what ());
+	    }
+	}
     }
 
   return (!per_objfile->per_bfd->info.is_virtual
@@ -3202,17 +3220,6 @@ dwarf2_initialize_objfile (struct objfile *objfile,
 
   dwarf_read_debug_printf ("called");
 
-  /* Try to fetch any potential dwz file early, while still on the
-     main thread.  */
-  try
-    {
-      dwarf2_read_dwz_file (per_objfile);
-    }
-  catch (const gdb_exception_error &err)
-    {
-      warning (_("%s"), err.what ());
-    }
-
   /* If we're about to read full symbols, don't bother with the
      indices.  In this case we also don't care if some other debug
      format is making psymtabs, because they are all about to be