[PATCH 01/10] gdb/doc: spring clean the Python unwinders documentation

[Andrew Burgess <aburgess@redhat.com>]

The documentation for the Python Unwinders API could do with some
improvement.  The 'Unwinder Skeleton Code' has an error: it says
'unwinders' when it should say 'unwinder' in one case.

Additionally, by placing the 'Unwinder Skeleton Code' before the
section 'Registering an Unwinder' we have skipping including the
registration line in the skeleton code.  But this is confusion for
users (I think) as the skeleton code is almost complete, except for
one missing line which the user has to figure out for themselves.  By
reordering the sections, it is now obvious that the registration
should be included in the skeleton code, and the example is therefore
almost complete.

Additionally, in the example skeleton code the way in which the
frame-id was being built (using the current stack point and program
counter is (a) not correct, and (b) counter to what is laid out in the
'Unwinder Input' section when describing building a frame-id.

I've removed the incorrect code and replaced it with more generic
comments indicating what needs to be done.  As the actual actions that
need to be performed are both architecture specific, and dependent on
the function being unwound, it's almost impossible to include more
exact code here, but I think what I'm proposing is less misleading
than what we had before.

I've also added more cross references.
---
 gdb/doc/python.texi | 82 ++++++++++++++++++++++++---------------------
 1 file changed, 44 insertions(+), 38 deletions(-)

diff --git a/gdb/doc/python.texi b/gdb/doc/python.texi
index b04f1de2ddf..9ab0aca7d57 100644
--- a/gdb/doc/python.texi
+++ b/gdb/doc/python.texi
@@ -2784,9 +2784,10 @@
 
 @defun PendingFrame.create_unwind_info (frame_id)
 Returns a new @code{gdb.UnwindInfo} instance identified by given
-@var{frame_id}.  The argument is used to build @value{GDBN}'s frame ID
-using one of functions provided by @value{GDBN}.  @var{frame_id}'s attributes
-determine which function will be used, as follows:
+@var{frame_id}.  The @var{frame_id} is used internally by @value{GDBN}
+to identify the frames within the current thread's stack.  The
+attributes of @var{frame_id} determine what type of frame ID is
+created within @value{GDBN}:
 
 @table @code
 @item sp, pc
@@ -2841,6 +2842,34 @@
 @var{value} is a register value (a @code{gdb.Value} object).
 @end defun
 
+@subheading Registering an Unwinder
+
+Object files and program spaces can have unwinders registered with
+them.  In addition, you can also register unwinders globally.
+
+The @code{gdb.unwinders} module provides the function to register an
+unwinder:
+
+@defun gdb.unwinder.register_unwinder (locus, unwinder, replace=False)
+@var{locus} specifies to which unwinder list to prepend the
+@var{unwinder}.  It can be either an object file (@pxref{Objfiles In
+Python}), a program space (@pxref{Progspaces In Python}), or
+@code{None}, in which case the unwinder is registered globally.  The
+newly added @var{unwinder} will be called before any other unwinder
+from the same locus.  Two unwinders in the same locus cannot have the
+same name.  An attempt to add an unwinder with an already existing
+name raises an exception unless @var{replace} is @code{True}, in which
+case the old unwinder is deleted and the new unwinder is registered in
+its place.
+@end defun
+
+@subheading Unwinder Precedence
+
+@value{GDBN} first calls the unwinders from all the object files in no
+particular order, then the unwinders from the current program space,
+then the globally registered unwinders, and finally the unwinders
+builtin to @value{GDBN}.
+
 @subheading Unwinder Skeleton Code
 
 @value{GDBN} comes with the module containing the base @code{Unwinder}
@@ -2848,7 +2877,7 @@
 follows:
 
 @smallexample
-from gdb.unwinders import Unwinder
+from gdb.unwinder import Unwinder
 
 class FrameId(object):
     def __init__(self, sp, pc):
@@ -2857,53 +2886,30 @@
 
 
 class MyUnwinder(Unwinder):
-    def __init__(....):
-        super(MyUnwinder, self).__init___(<expects unwinder name argument>)
+    def __init__(self):
+        super().__init___("MyUnwinder_Name")
 
-    def __call__(pending_frame):
+    def __call__(self, pending_frame):
         if not <we recognize frame>:
             return None
-        # Create UnwindInfo.  Usually the frame is identified by the stack 
-        # pointer and the program counter.
-        sp = pending_frame.read_register(<SP number>)
-        pc = pending_frame.read_register(<PC number>)
+
+        # Create a FrameID.  Usually the frame is identified by a
+        # stack pointer and the function address.
+        sp = ... compute a stack address ...
+        pc = ... compute function address ...
         unwind_info = pending_frame.create_unwind_info(FrameId(sp, pc))
 
-        # Find the values of the registers in the caller's frame and 
+        # Find the values of the registers in the caller's frame and
         # save them in the result:
-        unwind_info.add_saved_register(<register>, <value>)
+        unwind_info.add_saved_register(<register-number>, <register-value>)
         ....
 
         # Return the result:
         return unwind_info
 
+gdb.unwinder.register_unwinder(<locus>, MyUnwinder(), <replace>)
 @end smallexample
 
-@subheading Registering an Unwinder
-
-Object files and program spaces can have unwinders registered with
-them.  In addition, you can also register unwinders globally.
-
-The @code{gdb.unwinders} module provides the function to register an
-unwinder:
-
-@defun gdb.unwinder.register_unwinder (locus, unwinder, replace=False)
-@var{locus} specifies to which unwinder list to prepend the
-@var{unwinder}.  It can be either an object file, a program space, or
-@code{None}, in which case the unwinder is registered globally.  The
-newly added @var{unwinder} will be called before any other unwinder from the
-same locus.  Two unwinders in the same locus cannot have the same
-name.  An attempt to add an unwinder with an already existing name raises
-an exception unless @var{replace} is @code{True}, in which case the
-old unwinder is deleted.
-@end defun
-
-@subheading Unwinder Precedence
-
-@value{GDBN} first calls the unwinders from all the object files in no
-particular order, then the unwinders from the current program space,
-and finally the unwinders from @value{GDBN}.
-
 @node Xmethods In Python
 @subsubsection Xmethods In Python
 @cindex xmethods in Python
-- 
2.25.4