gdb: add setting to disable reading source code files

In some situations it is possible that a user might not want GDB to
try and access source code files, for example, the source code might
be stored on a slow to access network file system.

It is almost certainly possible that using some combination of 'set
directories' and/or 'set substitute-path' a user can trick GDB into
being unable to find the source files, but this feels like a rather
crude way to solve the problem.

In this commit a new option is add that stops GDB from opening and
reading the source files.  A user can run with source code reading
disabled if this is required, then re-enable later if they decide
that they now want to view the source code.

gdb/NEWS

@@ -3,6 +3,8 @@
 
 *** Changes since GDB 11
 
+* New commands
+
 maint set backtrace-on-fatal-signal on|off
 maint show backtrace-on-fatal-signal
   This setting is 'on' by default.  When 'on' GDB will print a limited
@@ -10,6 +12,13 @@ maint show backtrace-on-fatal-signal
   fatal signal.  This only supported on some platforms where the
   backtrace and backtrace_symbols_fd functions are available.
 
+set source open on|off
+show source open
+  This setting, which is on by default, controls whether GDB will try
+  to open source code files.  Switching this off will stop GDB trying
+  to open and read source code files, which can be useful if the files
+  are located over a slow network connection.
+
 * Python API
 
   ** New function gdb.add_history(), which takes a gdb.Value object

gdb/cli/cli-cmds.c

@@ -168,6 +168,10 @@ struct cmd_list_element *setchecklist;
 
 struct cmd_list_element *showchecklist;
 
+struct cmd_list_element *setsourcelist;
+
+struct cmd_list_element *showsourcelist;
+
 /* Command tracing state.  */
 
 int source_verbose = 0;

gdb/cli/cli-cmds.h

@@ -137,6 +137,14 @@ extern struct cmd_list_element *showchecklist;
 
 extern struct cmd_list_element *save_cmdlist;
 
+/* Chain containing all defined "set source" subcommands.  */
+
+extern struct cmd_list_element *setsourcelist;
+
+/* Chain containing all defined "show source" subcommands.  */
+
+extern struct cmd_list_element *showsourcelist;
+
 /* Limit the call depth of user-defined commands */
 
 extern unsigned int max_user_call_depth;

gdb/doc/gdb.texinfo

@@ -8919,6 +8919,7 @@ prefer to use Emacs facilities to view source; see @ref{Emacs, ,Using
 * Search::                      Searching source files
 * Source Path::                 Specifying source directories
 * Machine Code::                Source and machine code
+* Disable Reading Source::	Disable Reading Source Code
 @end menu
 
 @node List
@@ -9926,6 +9927,31 @@ OFF, which means never display the disassembly of the next line or
 instruction.
 @end table
 
+@node Disable Reading Source
+@section Disable Reading Source Code
+@cindex source code, disable access
+
+In some cases it can be desirable to prevent @value{GDBN} from
+accessing source code files.  One case where this might be desirable
+is if the source code files are located over a slow network
+connection.
+
+The following command can be used to control whether @value{GDBN}
+should access source code files or not:
+
+@table @code
+@kindex set source open
+@kindex show source open
+@item set source open @r{[}on@r{|}off@r{]}
+@itemx show source open
+When this option is @code{on}, which is the default, @value{GDBN} will
+access source code files when needed, for example to print source
+lines when @value{GDBN} stops, or in response to the @code{list}
+command.
+
+When this option is @code{off}, @value{GDBN} will not access source
+code files.
+@end table
 
 @node Data
 @chapter Examining Data

gdb/source.c

@@ -148,7 +148,21 @@ show_filename_display_string (struct ui_file *file, int from_tty,
 {
   fprintf_filtered (file, _("Filenames are displayed as \"%s\".\n"), value);
 }
- 
+
+/* When true GDB will stat and open source files as required, but when
+   false, GDB will avoid accessing source files as much as possible.  */
+
+static bool source_open = true;
+
+/* Implement 'show source open'.  */
+
+static void
+show_source_open (struct ui_file *file, int from_tty,
+		  struct cmd_list_element *c, const char *value)
+{
+  fprintf_filtered (file, _("Source opening is \"%s\".\n"), value);
+}
+
 /* Line number of last line printed.  Default for various commands.
    current_source_line is usually, but not always, the same as this.  */
 
@@ -1048,8 +1062,13 @@ find_and_open_source (const char *filename,
   const char *p;
   int result;
 
-  /* Quick way out if we already know its full name.  */
+  /* If reading of source files is disabled then return a result indicating
+     the attempt to read this source file failed.  GDB will then display
+     the filename and line number instead.  */
+  if (!source_open)
+    return scoped_fd (-1);
 
+  /* Quick way out if we already know its full name.  */
   if (*fullname)
     {
       /* The user may have requested that source paths be rewritten
@@ -1062,6 +1081,7 @@ find_and_open_source (const char *filename,
 	*fullname = std::move (rewritten_fullname);
 
       result = gdb_open_cloexec (fullname->get (), OPEN_MODE, 0);
+
       if (result >= 0)
 	{
 	  *fullname = gdb_realpath (fullname->get ());
@@ -1300,7 +1320,7 @@ print_source_lines_base (struct symtab *s, int line, int stopline,
 
   /* If printing of source lines is disabled, just print file and line
      number.  */
-  if (uiout->test_flags (ui_source_list))
+  if (uiout->test_flags (ui_source_list) && source_open)
     {
       /* Only prints "No such file or directory" once.  */
       if (s == last_source_visited)
@@ -1603,6 +1623,9 @@ search_command_helper (const char *regex, int from_tty, bool forward)
   if (loc->symtab () == nullptr)
     select_source_symtab (0);
 
+  if (!source_open)
+    error (_("source code access disabled"));
+
   scoped_fd desc (open_source_file (loc->symtab ()));
   if (desc.get () < 0)
     perror_with_name (symtab_to_filename_for_display (loc->symtab ()));
@@ -1927,6 +1950,22 @@ source_lines_range::source_lines_range (int startline,
     }
 }
 
+/* Handle the "set source" base command.  */
+
+static void
+set_source (const char *arg, int from_tty)
+{
+  help_list (setsourcelist, "set source ", all_commands, gdb_stdout);
+}
+
+/* Handle the "show source" base command.  */
+
+static void
+show_source (const char *args, int from_tty)
+{
+  help_list (showsourcelist, "show source ", all_commands, gdb_stdout);
+}
+
 
 void _initialize_source ();
 void
@@ -2050,4 +2089,25 @@ By default, relative filenames are displayed."),
 			show_filename_display_string,
 			&setlist, &showlist);
 
+  add_prefix_cmd ("source", no_class, set_source,
+                  _("Generic command for setting how sources are handled."),
+                  &setsourcelist, 0, &setlist);
+
+  add_prefix_cmd ("source", no_class, show_source,
+                  _("Generic command for showing source settings."),
+                  &showsourcelist, 0, &showlist);
+
+  add_setshow_boolean_cmd ("open", class_files, &source_open, _("\
+Set whether GDB should open source files."), _("\
+Show whether GDB should open source files."), _("\
+When this option is on GDB will open source files and display the\n\
+contents when appropriate, for example, when GDB stops, or the list\n\
+command is used.\n\
+When this option is off GDB will not try to open source files, instead\n\
+GDB will print the file and line number that would have been displayed.\n\
+This can be useful if access to source code files is slow, for example\n\
+due to the source being located over a slow network connection."),
+                           NULL,
+                           show_source_open,
+                           &setsourcelist, &showsourcelist);
 }

gdb/testsuite/gdb.base/source-open.c

@@ -0,0 +1,25 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2021 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+
+int
+main (int argc, char* argv[])
+{
+  printf ("source code line.");
+  return 0;
+}

gdb/testsuite/gdb.base/source-open.exp

@@ -0,0 +1,44 @@
+# Copyright 2021 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test the 'set source open on|off' command.
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile debug]} {
+    return -1
+}
+
+gdb_test "show source open" "Source opening is \"on\"\\." \
+    "source opening is initially on"
+gdb_test_no_output "set listsize 1"
+gdb_test_no_output "set source open on"
+gdb_test "list 18" "18\t#include <stdio.h>" "source reading on"
+
+# File opening is turned off therefore source lines are not printed.
+gdb_test_no_output "set source open off"
+gdb_test "show source open" "Source opening is \"off\"\\." \
+    "source opening is now off"
+gdb_test "list 18" "18\tin .*/${srcfile}" "source reading off"
+
+gdb_test "forward-search main" "source code access disabled"
+gdb_test "reverse-search main" "source code access disabled"
+
+# And finally, turn source code reading back on.
+gdb_test_no_output "set source open on" \
+    "turn source reading back on"
+gdb_test "show source open" "Source opening is \"on\"\\." \
+    "source opening is on again"
+gdb_test "list 18" "18\t#include <stdio.h>" "source reading on again"