Re: refactoring basebackup.c

> On Jul 8, 2021, at 8:56 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> The interesting
> patches in terms of functionality are 0006 and 0007;

The difficulty in v3-0007 with pg_basebackup only knowing how to parse tar archives seems to be a natural consequence of not sufficiently abstracting out the handling of the tar format. If the bbsink and bbstreamer abstractions fully encapsulated a set of parsing callbacks, then pg_basebackup wouldn't contain things like:

streamer = bbstreamer_tar_parser_new(streamer);

but instead would use the parser callbacks without knowledge of whether they were parsing tar vs. cpio vs. whatever. It just seems really odd that pg_basebackup is using the extensible abstraction layer and then defeating the purpose by knowing too much about the format. It might even be a useful exercise to write cpio support into this patch set rather than waiting until v16, just to make sure the abstraction layer doesn't have tar-specific assumptions left over.

printf(_(" -F, --format=p|t output format (plain (default), tar)\n"));

printf(_(" -z, --gzip compress tar output\n"));
printf(_(" -Z, --compress=0-9 compress tar output with given compression level\n"));

This is the pre-existing --help output, not changed by your patch, but if you anticipate that other output formats will be supported in future releases, perhaps it's better not to write the --help output in such a way as to imply that -z and -Z are somehow connected with the choice of tar format? Would changing the --help now make for less confusion later? I'm just asking...

The new options to pg_basebackup should have test coverage in src/bin/pg_basebackup/t/010_pg_basebackup.pl, though I expect you are waiting to hammer out the interface before writing the tests.

> the rest is
> preparatory refactoring.

patch v3-0001:

The new function AppendPlainCommandOption writes too many spaces, which does no harm, but seems silly, resulting in lines like:

LOG: received replication command: BASE_BACKUP ( LABEL 'pg_basebackup base backup', PROGRESS, WAIT 0, MANIFEST 'yes')

patch v3-0003:

The introduction of the sink abstraction seems incomplete, as basebackup.c still has knowledge of things like tar headers. Calls like _tarWriteHeader(sink, ...) feel like an abstraction violation. I expected perhaps this would get addressed in later patches, but it doesn't.

+ * 'bbs_buffer' is the buffer into which data destined for the bbsink
+ * should be stored. It must be a multiple of BLCKSZ.
+ *
+ * 'bbs_buffer_length' is the allocated length of the buffer.

The length must be a multiple of BLCKSZ, not the pointer.

patch-v3-0005:

+ * 'copystream' sends a starts a single COPY OUT operation and transmits

too many verbs.

+ * Regardless of which method is used, we sent a result set with

"is used" vs. "sent" verb tense mismatch.

+ * So we only check it after the number of bytes sine the last check reaches

typo. s/sine/since/

- * (2) we need to inject backup_manifest or recovery configuration into it.
+ * (2) we need to inject backup_manifest or recovery configuration into
+ * it.

src/bin/pg_basebackup/pg_basebackup.c contains word wrap changes like the above which would better be left to a different commit, if done at all.

+ if (state.manifest_file !=NULL)

Need a space after !=

—
Mark Dilger
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