In the original design for parallel dump, worker processes reported errors
by sending them up to the master process, which would print the messages.
This is unworkably fragile for a couple of reasons: it risks deadlock if a
worker sends an error at an unexpected time, and if the master has already
died for some reason, the user will never get to see the error at all.
Revert that idea and go back to just always printing messages to stderr.
This approach means that if all the workers fail for similar reasons (eg,
bad password or server shutdown), the user will see N copies of that
message, not only one as before. While that's slightly annoying, it's
certainly better than not seeing any message; not to mention that we
shouldn't assume that only the first failure is interesting.
An additional problem in the same area was that the master failed to
disable SIGPIPE (at least until much too late), which meant that sending a
command to an already-dead worker would cause the master to crash silently.
That was bad enough in itself but was made worse by the total reliance on
the master to print errors: even if the worker had reported an error, you
would probably not see it, depending on timing. Instead disable SIGPIPE
right after we've forked the workers, before attempting to send them
anything.
Additionally, the master relies on seeing socket EOF to realize that a
worker has exited prematurely --- but on Windows, there would be no EOF
since the socket is attached to the process that includes both the master
and worker threads, so it remains open. Make archive_close_connection()
close the worker end of the sockets so that this acts more like the Unix
case. It's not perfect, because if a worker thread exits without going
through exit_nicely() the closures won't happen; but that's not really
supposed to happen.
This has been wrong all along, so back-patch to 9.3 where parallel dump
was introduced.
Report: <2458.1450894615@sss.pgh.pa.us>
Rather than passing around DumpOptions and RestoreOptions as separate
arguments, add fields to struct Archive to carry pointers to these objects,
and access them through those fields when needed. There already was a
RestoreOptions pointer in Archive, though for no obvious reason it was part
of the "private" struct rather than out where pg_dump.c could see it.
Doing this allows reversion of quite a lot of parameter-addition changes
made in commit 0eea8047bf, which is a good thing IMO because this will
reduce the code delta between 9.4 and 9.5, probably easing a few future
back-patch efforts. Moreover, the previous commit only added a DumpOptions
argument to functions that had to have it at the time, which means we could
anticipate still more code churn (and more back-patch hazard) as the
requirement spread further. I'd hit exactly that problem in my upcoming
patch to fix extension membership marking, which is what motivated me to
do this.
Most pg_dump.c global variables, which were passed down individually to
dumping routines, are now grouped as members of the new DumpOptions
struct, which is used as a local variable and passed down into routines
that need it. This helps future development efforts; in particular it
is said to enable a mode in which a parallel pg_dump run can output
multiple streams, and have them restored in parallel.
Also take the opportunity to clean up the pg_dump header files somewhat,
to avoid circularity.
Author: Joachim Wieland, revised by Álvaro Herrera
Reviewed by Peter Eisentraut
Move functions used only by pg_dump and pg_restore from dumputils.c to a new
file, pg_backup_utils.c. dumputils.c is linked into psql and some programs
in bin/scripts, so it seems good to keep it slim. The parallel functionality
is moved to parallel.c, as is exit_horribly, because the interesting code in
exit_horribly is parallel-related.
This refactoring gets rid of the on_exit_msg_func function pointer. It was
problematic, because a modern gcc version with -Wmissing-format-attribute
complained if it wasn't marked with PF_PRINTF_ATTRIBUTE, but the ancient gcc
version that Tom Lane's old HP-UX box has didn't accept that attribute on a
function pointer, and gave an error. We still use a similar function pointer
trick for getLocalPQBuffer() function, to use a thread-local version of that
in parallel mode on Windows, but that dodges the problem because it doesn't
take printf-like arguments.
New infrastructure is added which creates a set number of workers
(threads on Windows, forked processes on Unix). Jobs are then
handed out to these workers by the master process as needed.
pg_restore is adjusted to use this new infrastructure in place of the
old setup which created a new worker for each step on the fly. Parallel
dumps acquire a snapshot clone in order to stay consistent, if
available.
The parallel option is selected by the -j / --jobs command line
parameter of pg_dump.
Joachim Wieland, lightly editorialized by Andrew Dunstan.
Tom Lane
Bruce Momjian
Alvaro Herrera
Heikki Linnakangas
Andrew Dunstan