pgsql: Don't get confused if a WAL partial record header has xl_tot_len

Don't get confused if a WAL partial record header has xl_tot_len == 0.

If a WAL record header was split across pages, but xl_tot_len was 0, we
would get confused and conclude that we had already read the whole record,
and proceed to CRC check it. That can lead to a crash in RecordIsValid(),
which isn't careful to not read beyond end-of-record, as defined by
xl_tot_len.

Add an explicit sanity check for xl_tot_len <= SizeOfXlogRecord. Also,
make RecordIsValid() more robust by checking in each step that it doesn't
try to access memory beyond end of record, even if a length field in the
record's or a backup block's header is bogus.

Per report and analysis by Tom Lane.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/51fed14d73ed3acd2282b531fb1396877e44e86a

Modified Files
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src/backend/access/transam/xlog.c | 38 +++++++++++++++++++++++++++++++++++-
1 files changed, 36 insertions(+), 2 deletions(-)