Fix several weaknesses in slot and logical replication on-disk serialization.

Heikki noticed in 544E23C0.8090605@vmware.com that slot.c and
snapbuild.c were missing the FIN_CRC32 call when computing/checking
checksums of on disk files. That doesn't lower the the error detection
capabilities of the checksum, but is inconsistent with other usages.

In a followup mail Heikki also noticed that, contrary to a comment,
the 'version' and 'length' struct fields of replication slot's on disk
data where not covered by the checksum. That's not likely to lead to
actually missed corruption as those fields are cross checked with the
expected version and the actual file length. But it's wrong
nonetheless.

As fixing these issues makes existing on disk files unreadable, bump
the expected versions of on disk files for both slots and logical
decoding historic catalog snapshots.  This means that loading old
files will fail with
ERROR: "replication slot file ... has unsupported version 1"
and
ERROR: "snapbuild state file ... has unsupported version 1 instead of
2" respectively. Given the low likelihood of anybody already using
these new features in a production setup that seems acceptable.

Fixing these issues made me notice that there's no regression test
covering the loading of historic snapshot from disk - so add one.

Backpatch to 9.4 where these features were introduced.

1 files changed, 43 insertions, 0 deletions

diff --git a/contrib/test_decoding/expected/ondisk_startup.out b/contrib/test_decoding/expected/ondisk_startup.out
new file mode 100644
index 00000000000..65115c830a4
--- /dev/null
+++ b/contrib/test_decoding/expected/ondisk_startup.out
@@ -0,0 +1,43 @@
+Parsed test spec with 3 sessions
+
+starting permutation: s2txid s1init s3txid s2alter s2c s1insert s1checkpoint s1start s1insert s1alter s1insert s1start
+step s2txid: BEGIN ISOLATION LEVEL REPEATABLE READ; SELECT txid_current() IS NULL;
+?column?       
+
+f              
+step s1init: SELECT 'init' FROM pg_create_logical_replication_slot('isolation_slot', 'test_decoding'); <waiting ...>
+step s3txid: BEGIN ISOLATION LEVEL REPEATABLE READ; SELECT txid_current() IS NULL;
+?column?       
+
+f              
+step s2alter: ALTER TABLE do_write ADD COLUMN addedbys2 int;
+step s2c: COMMIT;
+step s1init: <... completed>
+?column?       
+
+init           
+step s1insert: INSERT INTO do_write DEFAULT VALUES;
+step s1checkpoint: CHECKPOINT;
+step s1start: SELECT data FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', 'false');
+data           
+
+BEGIN          
+table public.do_write: INSERT: id[integer]:1 addedbys2[integer]:null
+COMMIT         
+step s1insert: INSERT INTO do_write DEFAULT VALUES;
+step s1alter: ALTER TABLE do_write ADD COLUMN addedbys1 int;
+step s1insert: INSERT INTO do_write DEFAULT VALUES;
+step s1start: SELECT data FROM pg_logical_slot_get_changes('isolation_slot', NULL, NULL, 'include-xids', 'false');
+data           
+
+BEGIN          
+table public.do_write: INSERT: id[integer]:2 addedbys2[integer]:null
+COMMIT         
+BEGIN          
+COMMIT         
+BEGIN          
+table public.do_write: INSERT: id[integer]:3 addedbys2[integer]:null addedbys1[integer]:null
+COMMIT         
+?column?       
+
+stop