Horton

Re: Attempt to consolidate reading of XLOG page

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Antonin Houska <ah@cybertec.at>, Thomas Munro <thomas.munro@gmail.com>, Robert Haas <robertmhaas@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2019-09-27T19:28:38Z
Lists: pgsql-hackers 
Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> On 2019-Sep-27, Antonin Houska wrote:
>>> You placed the errinfo in XLogRead's stack rather than its callers' ...
>>> I don't think that works, because as soon as XLogRead returns that
>>> memory is no longer guaranteed to exist.

>> I was aware of this problem, therefore I defined the field as static:
>> 
>> +XLogReadError *
>> +XLogRead(char *buf, XLogRecPtr startptr, Size count, TimeLineID *tli_p,
>> +                WALOpenSegment *seg, WALSegmentContext *segcxt,
>> +                WALSegmentOpen openSegment)
>> +{
>> +       char       *p;
>> +       XLogRecPtr      recptr;
>> +       Size            nbytes;
>> +       static XLogReadError errinfo;

> I see.

That seems like an absolutely terrible "fix".  We don't really want
XLogRead to be defined in a way that forces it to be non-reentrant do we?

			regards, tom lane

Commits

  1. Remove logical_read_local_xlog_page

  2. Refactor WAL file-reading code into WALRead()

  3. Rework WAL-reading supporting structs

  4. Make XLogReaderInvalReadState static

  5. Use pg_pread() and pg_pwrite() for data files and WAL.