gdb: Allow GDB to _not_ load a previous command history

gdb: Allow GDB to _not_ load a previous command history

This commit aims to give a cleaner mechanism by which the user can
prevent GDB from trying to load any previous command history.

Currently the user can change the path to the history file, either
using a command line flag, or by setting the GDBHISTFILE environment
variable, and if the path is set to a non-existent file, then
obviously GDB wont load any command history.  However, this feels like
a bit of a bodge, I'd like to add an official mechanism by which we
can disable command history loading.

Why would we want to prevent command history loading?  The specific
use case I have is GDB starting with a CWD that is a network mounted
directory, and there is no command history present.  Still GDB will
access the network in order to check for the file.  In my particular
use case I'm actually starting a large number of GDB instances in
parallel, all in the same network mounted directory, the large number
of network accesses looking for this file introduces a noticeable
delay at GDB startup.

The approach I'm proposing here is a slight adjustment to the current
rules for setting up the history filename.  Currently, if a user does
this, they see an error:

  (gdb) set history filename
  Argument required (filename to set it to.).

However, if a user does this:

  $ GDBHISTFILE= gdb --quiet
  (gdb) set history save on
  (gdb) q
  warning: Could not rename -gdb18416~ to : No such file or directory

So, we already have a bug in this area.  My plan is to allow the empty
filename to be accepted, and for this to mean, neither load, nor save
the command history.

This does mean that we now have two mechanisms to prevent saving the
command history:

  (gdb) set history filename

or

  (gdb) set history save off

But the only way to prevent loading the command history is to set the
filename to the empty string _before_ you get to a GDB prompt, either
using a command line option, or the environment variable.

I've updated some of the show commands, for example this session:

  (gdb) set history filename
  (gdb) show history filename
  There is no filename currently set for recording the command history in.
  (gdb) show history save
  Saving of the history record on exit is off.
  (gdb) set history save on
  (gdb) show history save
  Saving of the history is disabled due to the value of 'history filename'.
  (gdb) set history filename /tmp/hist
  (gdb) show history save
  Saving of the history record on exit is on.

I've updated the manual, and added some tests.

gdb/ChangeLog:

	* NEWS: Mention new behaviour of the history filename.
	* top.c (write_history_p): Add comment.
	(show_write_history_p): Add header comment, give a different
	message when history writing is on, but the history filename is
	empty.
	(history_filename): Add comment.
	(history_filename_empty): New function.
	(show_history_filename): Add header comment, give a different
	message when the filename is empty.
	(init_history): Compare history_filename against nullptr, and only
	read history if the filename is not empty.
	(set_history_filename): Add header comment, and only make
	non-empty filenames absolute.
	(init_main): Make the filename argument to 'set history filename'
	optional.

gdb/doc/ChangeLog:

	* gdb.texinfo (Command History): Extend description for
	GDBHISTFILE and GDBHISTSIZE, add detail about the filename for
	'set history filename' being optional.  Describe the effect of an
	empty history filename on 'set history save on'.

gdb/testsuite/ChangeLog:

	* gdb.base/default.exp: Remove test of 'set history filename'.
	* gdb.base/gdbinit-history.exp: Add tests for setting the history
	filename to the empty string.
	* lib/gdb.exp (gdb_init): Unset environment variables GDBHISTFILE
	and GDBHISTSIZE.

Change-Id: Ia586e4311182fac99113b60f11ef8a11fbd5450b