TAP PostgresNode function to gdb stacks and optional cores for all backends

Hi all

I recently wrote a utility that adds a $node->gdb_backends() method to
PostgresNode instances - figured I'd share it here in case anyone finds it
useful, or wants to adopt it into the features of the TAP tools.

This function provides a one-line way to dump stacks for all running
backends to per-pid files or to the main test log, as well as the values of
various global variables that are potentially of interest. A default set of
globals will be dumped for each backend and the caller can specify
additional expressions of interest.

If requested, cores will be dumped for each running backend.

A subset of backends may be passed by pid instead, so you can easily target
specific backends you're interested in.

I initially wrote this to help debug a variety of issues with shutdown,
where I hacked the PostgresNode stop() method to trap failed shutdowns and
report stacks for all surviving processes + the postmaster in my wrapper
class for PostgresNode:

sub stop {
my ($self, $mode) = @_;
local($@);
eval {
PostgresNode::stop($self, $mode);
};
if ($@) {
$node->gdb_backends(want_cores => 1);
die $@;
}
}