gdb: add support for %V to printf command

gdb: add support for %V to printf command

This commit adds a new format for the printf and dprintf commands:
'%V'.  This new format takes any GDB expression and formats it as a
string, just as GDB would for a 'print' command, e.g.:

  (gdb) print a1
  $a = {2, 4, 6, 8, 10, 12, 14, 16, 18, 20}
  (gdb) printf "%V\n", a1
  {2, 4, 6, 8, 10, 12, 14, 16, 18, 20}
  (gdb)

It is also possible to pass the same options to %V as you might pass
to the print command, e.g.:

  (gdb) print -elements 3 -- a1
  $4 = {2, 4, 6...}
  (gdb) printf "%V[-elements 3]\n", a1
  {2, 4, 6...}
  (gdb)

This new feature would effectively replace an existing feature of GDB,
the $_as_string builtin convenience function.  However, the
$_as_string function has a few problems which this new feature solves:

1. $_as_string doesn't currently work when the inferior is not
running, e.g:

  (gdb) printf "%s", $_as_string(a1)
  You can't do that without a process to debug.
  (gdb)

The reason for this is that $_as_string returns a value object with
string type.  When we try to print this we call value_as_address,
which ends up trying to push the string into the inferior's address
space.

Clearly we could solve this problem, the string data exists in GDB, so
there's no reason why we have to push it into the inferior, but this
is an existing problem that would need solving.

2. $_as_string suffers from the fact that C degrades arrays to
pointers, e.g.:

  (gdb) printf "%s\n", $_as_string(a1)
  0x404260 <a1>
  (gdb)

The implementation of $_as_string is passed a gdb.Value object that is
a pointer, it doesn't understand that it's actually an array.  Solving
this would be harder than issue #1 I think.  The whole array to
pointer transformation is part of our expression evaluation.  And in
most cases this is exactly what we want.  It's not clear to me how
we'd (easily) tell GDB that we didn't want this reduction in _some_
cases.  But I'm sure this is solvable if we really wanted to.

3. $_as_string is a gdb.Function sub-class, and as such is passed
gdb.Value objects.  There's no super convenient way to pass formatting
options to $_as_string.  By this I mean that the new %V feature
supports print formatting options.  Ideally, we might want to add this
feature to $_as_string, we might imagine it working something like:

  (gdb) printf "%s\n", $_as_string(a1,
                                   elements = 3,
                                   array_indexes = True)

where the first item is the value to print, while the remaining
options are the print formatting options.  However, this relies on
Python calling syntax, which isn't something that convenience
functions handle.  We could possibly rely on strictly positional
arguments, like:

  (gdb) printf "%s\n", $_as_string(a1, 3, 1)

But that's clearly terrible as there's far more print formatting
options, and if you needed to set the 9th option you'd need to fill in
all the previous options.

And right now, the only way to pass these options to a gdb.Function is
to have GDB first convert them all into gdb.Value objects, which is
really overkill for what we want.

The new %V format solves all these problems: the string is computed
and printed entirely on the GDB side, we are able to print arrays as
actual arrays rather than pointers, and we can pass named format
arguments.

Finally, the $_as_string is sold in the manual as allowing users to
print the string representation of flag enums, so given:

  enum flags
    {
      FLAG_A = (1 << 0),
      FLAG_B = (1 << 1),
      FLAG_C = (1 << 1)
    };

  enum flags ff = FLAG_B;

We can:

  (gdb) printf "%s\n", $_as_string(ff)
  FLAG_B

This works just fine with %V too:

  (gdb) printf "%V\n", ff
  FLAG_B

So all functionality of $_as_string is replaced by %V.  I'm not
proposing to remove $_as_string, there might be users currently
depending on it, but I am proposing that we don't push $_as_string in
the documentation.

As %V is a feature of printf, GDB's dprintf breakpoints naturally gain
access to this feature too.  dprintf breakpoints can be operated in
three different styles 'gdb' (use GDB's printf), 'call' (call a
function in the inferior), or 'agent' (perform the dprintf on the
remote).

The use of '%V' will work just fine when dprintf-style is 'gdb'.

When dprintf-style is 'call' the format string and arguments are
passed to an inferior function (printf by default).  In this case GDB
doesn't prevent use of '%V', but the documentation makes it clear that
support for '%V' will depend on the inferior function being called.

I chose this approach because the current implementation doesn't place
any restrictions on the format string when operating in 'call' style.
That is, the user might already be calling a function that supports
custom print format specifiers (maybe including '%V') so, I claim, it
would be wrong to block use of '%V' in this case.  The documentation
does make it clear that users shouldn't expect this to "just work"
though.

When dprintf-style is 'agent' then GDB does no support the use of
'%V' (right now).  This is handled at the point when GDB tries to
process the format string and send the dprintf command to the remote,
here's an example:

  Reading symbols from /tmp/hello.x...
  (gdb) dprintf call_me, "%V", a1
  Dprintf 1 at 0x401152: file /tmp/hello.c, line 8.
  (gdb) set sysroot /
  (gdb) target remote | gdbserver --once - /tmp/hello.x
  Remote debugging using | gdbserver --once - /tmp/hello.x
  stdin/stdout redirected
  Process /tmp/hello.x created; pid = 3088822
  Remote debugging using stdio
  Reading symbols from /lib64/ld-linux-x86-64.so.2...
  (No debugging symbols found in /lib64/ld-linux-x86-64.so.2)
  0x00007ffff7fd3110 in _start () from /lib64/ld-linux-x86-64.so.2
  (gdb) set dprintf-style agent
  (gdb) c
  Continuing.
  Unrecognized format specifier 'V' in printf
  Command aborted.
  (gdb)

This is exactly how GDB would handle any other invalid format
specifier, for example:

  Reading symbols from /tmp/hello.x...
  (gdb) dprintf call_me, "%Q", a1
  Dprintf 1 at 0x401152: file /tmp/hello.c, line 8.
  (gdb) set sysroot /
  (gdb) target remote | gdbserver --once - /tmp/hello.x
  Remote debugging using | gdbserver --once - /tmp/hello.x
  stdin/stdout redirected
  Process /tmp/hello.x created; pid = 3089193
  Remote debugging using stdio
  Reading symbols from /lib64/ld-linux-x86-64.so.2...
  (No debugging symbols found in /lib64/ld-linux-x86-64.so.2)
  0x00007ffff7fd3110 in _start () from /lib64/ld-linux-x86-64.so.2
  (gdb) set dprintf-style agent
  (gdb) c
  Continuing.
  Unrecognized format specifier 'Q' in printf
  Command aborted.
  (gdb)

The error message isn't the greatest, but improving that can be put
off for another day I hope.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Acked-By: Simon Marchi <simon.marchi@efficios.com>