Re: [PATCH] Teach pg_waldump to extract FPIs from the WAL

On Wed, Dec 14, 2022 at 04:44:34PM -0600, David Christensen wrote:
> I can get one sent in tomorrow.

-XLogRecordHasFPW(XLogReaderState *record)
+XLogRecordHasFPI(XLogReaderState *record)
This still refers to a FPW, so let's leave that out as well as any
renamings of this kind..

+ if (config.save_fpi_path != NULL)
+ {
+ /* Create the dir if it doesn't exist */
+ if (pg_mkdir_p(config.save_fpi_path, pg_dir_create_mode) < 0)
+ {
+ pg_log_error("could not create output directory \"%s\": %m",
+ config.save_fpi_path);
+ goto bad_argument;
+ }
+ }
It seems to me that you could allow things to work even if the
directory exists and is empty. See for example
verify_dir_is_empty_or_create() in pg_basebackup.c.

+my $file_re =
+ qr/^([0-9A-F]{8})-([0-9A-F]{8})[.][0-9]+[.][0-9]+[.][0-9]+[.][0-9]+(?:_vm|_init|_fsm|_main)?$/;
This is artistic to parse for people not used to regexps (I do, a
little). Perhaps this could use a small comment with an example of
name or a reference describing this format?

+# Set umask so test directories and files are created with default permissions
+umask(0077);
Incorrect copy-paste coming from elsewhere like the TAP tests of
pg_basebackup with group permissions? Doesn't
PostgreSQL::Test::Utils::tempdir give already enough protection in
terms of umask() and permissions?

+ if (config.save_fpi_path != NULL)
+ {
+ /* We fsync our output directory only; since these files are not part
+ * of the production database we do not require the performance hit
+ * that fsyncing every FPI would entail, so are doing this as a
+ * compromise. */
+
+ fsync_fname(config.save_fpi_path, true);
+ }
Why is it necessary to flush anything at all, then?

+my $relation = $node->safe_psql('postgres',
+ q{SELECT format('%s/%s/%s', CASE WHEN reltablespace = 0 THEN
dattablespace ELSE reltablespace END, pg_database.oid,
pg_relation_filenode(pg_class.oid)) FROM pg_class, pg_database WHERE
relname = 'test_table' AND datname = current_database()}
Could you rewrite that to be on multiple lines?

+diag "using walfile: $walfile";
You should avoid the use of "diag", as this would cause extra output
noise with a simple make check.

+$node->safe_psql('postgres', "SELECT pg_drop_replication_slot('regress_pg_waldump_slot')")
That's not really necessary, the nodes are wiped out at the end of the
test.

+$node->safe_psql('postgres', <<EOF);
+SELECT 'init' FROM pg_create_physical_replication_slot('regress_pg_waldump_slot', true, false);
+CREATE TABLE test_table AS SELECT generate_series(1,100) a;
+CHECKPOINT; -- required to force FPI for next writes
+UPDATE test_table SET a = a + 1;
Using an EOF to execute a multi-line query would be a first. Couldn't
you use the same thing as anywhere else? 009_twophase.pl just to
mention one. (Mentioned by Bharath upthread, where he asked for an
extra opinion so here it is.)
--
Michael