[PATCH] Add PROMPT_COMMAND and dynamic prompt support to psql

Hello Folks,

This patch adds optional, bash-like dynamic prompt support to psql so the
interactive prompt can be regenerated before each readline() call. The
motivation is to let users plug in external prompt renderers (status lines,
connection context, last-command exit status) without forking psql.

Picture attached to see it in action.

Comments of all sorts welcome.

Summary
-------

* PROMPT_COMMAND — if set, run a shell command before each interactive
prompt;
capture the first line of stdout (truncated to the existing prompt buffer
limit).

* %D — new PROMPT1/PROMPT2 escape; substitute the most recent PROMPT_COMMAND
stdout. Intended for embedding renderer output in the prompt string.

* SHELL_EXIT — new psql variable holding the exit status of the last user
command: 0 on success, 1 on SQL failure, or the real exit code for \! and
similar shell invocations. PROMPT_COMMAND does not update SHELL_EXIT, so
renderers can reflect the prior command's outcome.

* PROMPT_SESSION_EXPORT — new boolean psql variable, off by default. When
on,
export current-session PG* and PSQL_* environment variables to the
PROMPT_COMMAND subprocess (connection target, transaction state, row
count,
PostgreSQL superuser flag, optional txid from a user-defined :txid
variable).
This is separate from PROMPT_COMMAND so users who only need %D without
touching the subprocess environment can leave it off.

* Prompt refresh — call run_prompt_command() immediately before readline()
in
input.c (replacing an rl_pre_input_hook approach that left the prompt
blank
until the first keypress).

* \connect housekeeping — after a successful \connect, reset ROW_COUNT and
SHELL_EXIT and optionally refresh :txid when that variable exists.

Backward compatibility
----------------------

No behavior change unless the user configures the new facilities in .psqlrc
(or equivalent). PROMPT_COMMAND, %D, and PROMPT_SESSION_EXPORT are all
inert
until set. Existing scripts and non-interactive use are unaffected.

I am happy to rename variables, change defaults, add further gating, or
split
the patch if reviewers prefer a narrower initial commit.

Testing
-------

Manual testing on Linux with readline:

* PROMPT_COMMAND + %D rendering before each prompt
* \c with spaces preserved in readline (e.g. \c postgres)
* SHELL_EXIT after successful SQL, failed SQL, and \!
* PROMPT_SESSION_EXPORT on/off
* \connect reset of ROW_COUNT / SHELL_EXIT and txid refresh

I have not added a TAP test yet; I can add coverage in src/bin/psql/t/ if
that is desired before commit.

Companion work (not part of this patch)
---------------------------------------

A reference implementation for the Powerline prompt utility is maintained
separately:

https://github.com/powerline/powerline/pull/2287

Integration notes for that companion (example .psqlrc, exported variables,
readline marker requirements) are in the patch as:

src/bin/psql/powerline-integration.md

That file is documentation only and is not installed. The two projects can
be adopted independently; full functionality requires both plus user config.

References
----------

Fork branch (for convenience):

https://github.com/bithead2k/postgres/tree/psql-prompt-command

Tracking PR on the mirror (not upstream merge):

https://github.com/bithead2k/postgres/pull/1

Patch is attached (git format-patch output). A screenshot of the companion
powerline prompt in use is also attached as powerline-prompt-example.png and
checked in as src/bin/psql/powerline-prompt-example.png (see
powerline-integration.md).

Comments welcome.

Regards,
Kirk Roybal