[PATCH] Incremental backup: add backup profile to base backup

Hi Hackers,

This is the first piece of file level incremental backup support, as
described on wiki page https://wiki.postgresql.org/wiki/Incremental_backup

It is not yet complete, but I wish to share it on the list to receive
comments and suggestions.

The point of the patch is adding an option to pg_basebackup and
replication protocol BASE_BACKUP command to generate a backup_profile file.

When taking a full backup with pg_basebackup, the user can request
Postgres to generate a backup_profile file through the --profile option
(-B short option, which I've arbitrarily picked up because both -P and
-p was already taken)

At the moment the backup profile consists of a file with one line per
file detailing modification time, md5, size, tablespace and path
relative to tablespace root (PGDATA or the tablespace)

To calculate the md5 checksum I've used the md5 code present in pgcrypto
contrib as the code in src/include/libpq/md5.h is not suitable for large
files. Since a core feature cannot depend on a piece of contrib, I've
moved the files

contrib/pgcrypto/md5.c
contrib/pgcrypto/md5.h

to

src/backend/utils/hash/md5.c
src/include/utils/md5.h

changing the pgcrypto extension to use them.

There are still some TODOs:

* User documentation

* Remove the pg_basebackup code duplication I've introduced with the
ReceiveAndUnpackTarFileToDir function, which is almost the same of
ReceiveAndUnpackTarFile but does not expect to handle a tablespace. It
instead simply extract a tar stream in a destination directory. The
latter could probably be rewritten using the former as component, but it
needs some adjustment to the "progress reporting" part, which is not
present at the moment in ReceiveAndUnpackTarFileToDir.

* Add header section to backup_profile file which at the moment contains
only the body part. I'm thinking to change the original design and put
the whole backup label as header, which is IMHO clearer and well known.
I would use something like:

START WAL LOCATION: 0/E000028 (file 00000001000000000000000E)
CHECKPOINT LOCATION: 0/E000060
BACKUP METHOD: streamed
BACKUP FROM: master
START TIME: 2014-08-14 18:54:01 CEST
LABEL: pg_basebackup base backup
END LABEL

I've attached the current patch based on master.

Any comment will be appreciated.

Regards,
Marco

--
Marco Nenciarini - 2ndQuadrant Italy
PostgreSQL Training, Services and Support
marco(dot)nenciarini(at)2ndQuadrant(dot)it | www.2ndQuadrant.it