Add basic Python API for convenience variables

This adds a basic Python API for accessing convenience variables.
With this, convenience variables can be read and set from Python.
Although gdb supports convenience variables whose value changes at
each call, this is not exposed to Python; it could be, but I think
it's just as good to write a convenience function in this situation.

This is PR python/23080.

Tested on x86-64 Fedora 26.

2018-04-22  Tom Tromey  <tom@tromey.com>

	PR python/23080:
	* NEWS: Update for new functions.
	* python/py-value.c (gdbpy_set_convenience_variable)
	(gdbpy_convenience_variable): New functions.
	* python/python-internal.h (gdbpy_convenience_variable)
	(gdbpy_set_convenience_variable): Declare.
	* python/python.c (python_GdbMethods): Add convenience_variable,
	set_convenience_variable.

doc/ChangeLog
2018-04-22  Tom Tromey  <tom@tromey.com>

	PR python/23080:
	* python.texi (Basic Python): Document gdb.convenience_variable,
	gdb.set_convenience_variable.

testsuite/ChangeLog
2018-04-22  Tom Tromey  <tom@tromey.com>

	PR python/23080:
	* gdb.python/python.exp: Add convenience variable tests.

1 files changed, 21 insertions, 2 deletions

diff --git a/gdb/doc/python.texi b/gdb/doc/python.texi
index 4182b60..60e016a 100644
--- a/gdb/doc/python.texi
+++ b/gdb/doc/python.texi
@@ -288,6 +288,26 @@ If no exception is raised, the return value is always an instance of
 @code{gdb.Value} (@pxref{Values From Inferior}).
 @end defun
 
+@findex gdb.convenience_variable
+@defun gdb.convenience_variable (name)
+Return the value of the convenience variable (@pxref{Convenience
+Vars}) named @var{name}.  @var{name} must be a string.  The name
+should not include the @samp{$} that is used to mark a convenience
+variable in an expression.  If the convenience variable does not
+exist, then @code{None} is returned.
+@end defun
+
+@findex gdb.set_convenience_variable
+@defun gdb.set_convenience_variable (name, value)
+Set the value of the convenience variable (@pxref{Convenience Vars})
+named @var{name}.  @var{name} must be a string.  The name should not
+include the @samp{$} that is used to mark a convenience variable in an
+expression.  If @var{value} is @code{None}, then the convenience
+variable is removed.  Otherwise, if @var{value} is not a
+@code{gdb.Value} (@pxref{Values From Inferior}), it is is converted
+using the @code{gdb.Value} constructor.
+@end defun
+
 @findex gdb.parse_and_eval
 @defun gdb.parse_and_eval (expression)
 Parse @var{expression}, which must be a string, as an expression in
@@ -297,8 +317,7 @@ the current language, evaluate it, and return the result as a
 This function can be useful when implementing a new command
 (@pxref{Commands In Python}), as it provides a way to parse the
 command's argument as an expression.  It is also useful simply to
-compute values, for example, it is the only way to get the value of a
-convenience variable (@pxref{Convenience Vars}) as a @code{gdb.Value}.
+compute values.
 @end defun
 
 @findex gdb.find_pc_line