Factor out final completion match string building

We have several places doing essentially the same thing; factor them
out to a central place.  Some of the places overallocate for no good
reason, or use strcat unnecessarily.  The centralized version is more
precise and to the point.

(I considered making the gdb::unique_xmalloc_ptr overload version of
make_completer_match_str try to realloc (not xrealloc) probably
avoiding an allocation in most cases, but that'd be probably overdoing
it, and also, now that I'm writing this I thought I'd try to see how
could we ever get to filename_completer with "text != word", but I
couldn't figure it out.  Running the testsuite with 'gdb_assert (text
== word);' never tripped on the assertion either.  So post gdb 8.1,
I'll probably propose a patch to simplify filename_completer a bit,
and the gdb::unique_xmalloc_str overload can be removed then.)

gdb/ChangeLog:
2017-12-13  Pedro Alves  <palves@redhat.com>

	* cli/cli-decode.c (complete_on_cmdlist, complete_on_enum): Use
	make_completion_match_str.
	* completer.c: Use gdb::unique_xmalloc_ptr and
	make_completion_match_str.
	(make_completion_match_str_1): New.
	(make_completion_match_str(const char *, const char *,
	const char *)): New.
	(make_completion_match_str(gdb::unique_xmalloc_ptr<char> &&,
	const char *, const char *)): New.
	* completer.h (make_completion_match_str(const char *,
	const char *, const char *)): New.
	(make_completion_match_str(gdb::unique_xmalloc_ptr<char> &&,
	const char *, const char *)): New.
	* interps.c (interpreter_completer): Use make_completion_match_str.
	* symtab.c (completion_list_add_name, add_filename_to_list): Use
	make_completion_match_str.

1 files changed, 2 insertions, 18 deletions

diff --git a/gdb/interps.c b/gdb/interps.c
index b177a89..fa71bb4 100644
--- a/gdb/interps.c
+++ b/gdb/interps.c
@@ -439,24 +439,8 @@ interpreter_completer (struct cmd_list_element *ignore,
     {
       if (strncmp (interp.name, text, textlen) == 0)
 	{
-	  char *match;
-
-	  match = (char *) xmalloc (strlen (word) + strlen (interp.name) + 1);
-	  if (word == text)
-	    strcpy (match, interp.name);
-	  else if (word > text)
-	    {
-	      /* Return some portion of interp->name.  */
-	      strcpy (match, interp.name + (word - text));
-	    }
-	  else
-	    {
-	      /* Return some of text plus interp->name.  */
-	      strncpy (match, word, text - word);
-	      match[text - word] = '\0';
-	      strcat (match, interp.name);
-	    }
-	  tracker.add_completion (gdb::unique_xmalloc_ptr<char> (match));
+	  tracker.add_completion
+	    (make_completion_match_str (interp.name, text, word));
 	}
     }
 }