gdb: add filename option support

This commit adds support for filename options to GDB's option
sub-system (see cli/cli-option.{c,h}).

The new filename options support quoted and escaped filenames, and tab
completion is fully supported.

This commit adds the new option, and adds these options to the
'maintenance test-options' command as '-filename', along with some
tests that exercise this new option.

I've split the -filename testing into two.  In gdb.base/options.exp we
use the -filename option with some arbitrary strings.  This tests that
GDB can correctly extract the value from a filename option, and that
GDB can complete other options after a filename option.  However,
these tests don't actually pass real filenames, nor do they test
filename completion.

In gdb.base/filename-completion.exp I have added some tests that test
the -filename option with real filenames, and exercise filename tab
completion.

This commit doesn't include any real uses of the new filename options,
that will come in the next commit.

2 files changed, 107 insertions, 3 deletions

diff --git a/gdb/cli/cli-option.c b/gdb/cli/cli-option.c
index 0553928..9eb9ff8 100644
--- a/gdb/cli/cli-option.c
+++ b/gdb/cli/cli-option.c
@@ -43,7 +43,7 @@ union option_value
   /* For var_enum options.  */
   const char *enumeration;
 
-  /* For var_string options.  This is malloc-allocated.  */
+  /* For var_string and var_filename options.  This is allocated with new.  */
   std::string *string;
 };
 
@@ -85,7 +85,7 @@ struct option_def_and_value
   {
     if (value.has_value ())
       {
-	if (option.type == var_string)
+	if (option.type == var_string || option.type == var_filename)
 	  delete value->string;
       }
   }
@@ -102,7 +102,7 @@ private:
   {
     if (value.has_value ())
       {
-	if (option.type == var_string)
+	if (option.type == var_string || option.type == var_filename)
 	  value->string = nullptr;
       }
   }
@@ -452,6 +452,78 @@ parse_option (gdb::array_view<const option_def_group> options_group,
 	return option_def_and_value {*match, match_ctx, val};
       }
 
+    case var_filename:
+      {
+	if (check_for_argument (args, "--"))
+	  {
+	    /* Treat e.g., "maint test-options -filename --" as if there
+	       was no argument after "-filename".  */
+	    error (_("-%s requires an argument"), match->name);
+	  }
+
+	const char *arg_start = *args;
+	std::string str = extract_string_maybe_quoted (args);
+
+	/* If we are performing completion, and extracting STR moved ARGS
+	   to the end of the line, then the user is trying to complete the
+	   filename value.
+
+	   If ARGS didn't make it to the end of the line then the filename
+	   value is already complete and the user is trying to complete
+	   something later on the line.  */
+	if (completion != nullptr && **args == '\0')
+	  {
+	    /* Preserve the current custom word point.  If the call to
+	       advance_to_filename_maybe_quoted_complete_word_point below
+	       skips to the end of the command line then the custom word
+	       point will have been updated even though we generate no
+	       completions.
+
+	       However, *ARGS will also have been updated, and the general
+	       option completion code (which we will return too) also
+	       updates the custom word point based on the adjustment made
+	       to *ARGS.
+
+	       And so, if we don't find any completions, we should restore
+	       the custom word point value, this leaves the generic option
+	       completion code free to make its own adjustments.  */
+	    int prev_word_pt = completion->tracker.custom_word_point ();
+
+	    /* From ARG_START move forward to the start of the completion
+	       word, this will skip over any opening quote if there is
+	       one.
+
+	       If the word to complete is fully quoted, i.e. has an
+	       opening and closing quote, then this will skip over the
+	       word entirely and leave WORD pointing to the end of the
+	       input string.  */
+	    const char *word
+	      = advance_to_filename_maybe_quoted_complete_word_point
+	      (completion->tracker, arg_start);
+
+	    if (word == arg_start || *word != '\0')
+	      {
+		filename_maybe_quoted_completer (nullptr, completion->tracker,
+						 arg_start, word);
+
+		if (completion->tracker.have_completions ())
+		  return {};
+	      }
+
+	    /* No completions.  Restore the custom word point.  See the
+	       comment above for why this is needed.  */
+	    completion->tracker.set_custom_word_point (prev_word_pt);
+	  }
+
+	/* Check we did manage to extract something.  */
+	if (*args == arg_start)
+	  error (_("-%s requires an argument"), match->name);
+
+	option_value val;
+	val.string = new std::string (std::move (str));
+	return option_def_and_value {*match, match_ctx, val};
+      }
+
     default:
       /* Not yet.  */
       gdb_assert_not_reached ("option type not supported");
@@ -612,6 +684,7 @@ save_option_value_in_ctx (std::optional<option_def_and_value> &ov)
 	= ov->value->enumeration;
       break;
     case var_string:
+    case var_filename:
       *ov->option.var_address.string (ov->option, ov->ctx)
 	= std::move (*ov->value->string);
       break;
@@ -701,6 +774,8 @@ get_val_type_str (const option_def &opt, std::string &buffer)
       }
     case var_string:
       return "STRING";
+    case var_filename:
+      return "FILENAME";
     default:
       return nullptr;
     }
@@ -856,6 +931,15 @@ add_setshow_cmds_for_options (command_class cmd_class,
 				  nullptr, option.show_cmd_cb,
 				  set_list, show_list);
 	}
+      else if (option.type == var_filename)
+	{
+	  add_setshow_filename_cmd (option.name, cmd_class,
+				    option.var_address.string (option, data),
+				    option.set_doc, option.show_doc,
+				    option.help_doc,
+				    nullptr, option.show_cmd_cb,
+				    set_list, show_list);
+	}
       else
 	gdb_assert_not_reached ("option type not handled");
     }
diff --git a/gdb/cli/cli-option.h b/gdb/cli/cli-option.h
index bbe281d..26307a5 100644
--- a/gdb/cli/cli-option.h
+++ b/gdb/cli/cli-option.h
@@ -308,6 +308,26 @@ struct string_option_def : option_def
   }
 };
 
+/* A var_filename command line option.  */
+
+template<typename Context>
+struct filename_option_def : option_def
+{
+  filename_option_def (const char *long_option_,
+		       std::string *(*get_var_address_cb_) (Context *),
+		       show_value_ftype *show_cmd_cb_,
+		       const char *set_doc_,
+		       const char *show_doc_ = nullptr,
+		       const char *help_doc_ = nullptr)
+    : option_def (long_option_, var_filename, nullptr,
+		  (erased_get_var_address_ftype *) get_var_address_cb_,
+		  show_cmd_cb_,
+		  set_doc_, show_doc_, help_doc_)
+  {
+    var_address.string = detail::get_var_address<std::string, Context>;
+  }
+};
+
 /* A group of options that all share the same context pointer to pass
    to the options' get-current-value callbacks.  */
 struct option_def_group