[WIP] Patches to enable extraction state of query execution from external session

Hi, hackers!

Now I complete extension that provides facility to see the current state
of query execution working on external session in form of EXPLAIN
ANALYZE output. This extension works on 9.5 version, for 9.6 and later
it doesn't support detailed statistics for parallel nodes yet.

I want to present patches to the latest version of PostgreSQL core to
enable this extension.

1. Patch that provides facility to send user signal to external backend
(custom_signals.patch).

This patch transparently extends process signal interface to enable
sending user defined signals (I call them - custom signals) and defining
callbacks to them. Formally it will appear in following manner:

/* Register custom signal and define signal callback */

Reason = RegisterCustomProcSignalHandler(sighandler);

void
sighandler(void)
{
...
}

/* On other session we can send process signal to the first backend */
SendProcSignal(pid, Reason, backendId)

The InterruptPending flag is set under process signal handling and
sighandler is called in CHECK_FOR_INTERRUPTS.
I use this patch to notice other backend to send its state to caller.

2. Patch that enables to interrupt the query executor
(executor_hooks.patch).
This patch enables to hang up hooks on executor function of each node
(ExecProcNode). I define hooks before any node execution and after
execution.
I use this patch to add possibility of query tracing by emitted rows
from any node. I interrupt query executor after any node delivers one or
zero rows to upper node. And after execution of specific number trace
steps I can get the query state of traceable backend which will be
somewhat deterministic. I use this possibility for regression tests of
my extension.

3. Patch that enables to output runtime explain statistics
(runtime_explain.patch).
This patch extends the regular explain functionality. The problem is in
the point that regular explain call makes result output after query
execution performing InstrEndLoop on nodes where necessary. My patch
introduces specific flag *runtime* that indicates whether we explain
running query and does some insertions in source code dedicated to
output the statistics of running query.
I want to notice that this patch is completed only for 9.5 postgres
version. For later versions there is not implemented detailed statistics
for parellel nodes. Now, I'm working at this feature.

That's all. CC welcome!

--
Maksim Milyutin
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company