Date: Thu, 5 Jun 2014 21:39:43 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Serge Negodyuck <petr@petrovich.kiev.ua>
Cc: pgsql-bugs@postgresql.org
Subject: Re: BUG #8673: Could not open file
 "pg_multixact/members/xxxx" on slave during hot_standby
Message-ID: <20140606013943.GT5146@eldon.alvh.no-ip.org>
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Serge Negodyuck wrote:

> 2014-06-02 08:20:55 EEST 172.18.10.4 db PANIC: could not access status of
> transaction 2080547
> 2014-06-02 08:20:55 EEST 172.18.10.4 db DETAIL: Could not open file
> "pg_multixact/members/14078": No such file or directory.
> 2014-06-02 08:20:55 EEST 172.18.10.4 db CONTEXT: SQL statement "  UPDATE
> ....."

So as it turns out, this was caused because the arithmetic to handle the
wraparound case neglected to handle multixacts with more members than
the number that fit in the last page(s) of the last segment, leading to
a number of pages in the 14078 segment (or whatever the last segment is
for a given BLCKSZ) to fail to be initialized.  This patch is a rework
of that arithmetic, although it seems little bit too obscure.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services

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*** a/src/backend/access/transam/multixact.c
--- b/src/backend/access/transam/multixact.c
***************
*** 133,138 ****
--- 133,142 ----
  #define MULTIXACT_MEMBERS_PER_PAGE	\
  	(MULTIXACT_MEMBERGROUPS_PER_PAGE * MULTIXACT_MEMBERS_PER_MEMBERGROUP)
  
+ /* The number of members in the last page of the last segment. */
+ #define MAX_MEMBERS_IN_LAST_MEMBERS_PAGE \
+ 		((uint32) ((0xFFFFFFFF % MULTIXACT_MEMBERS_PER_PAGE) + 1))
+ 
  /* page in which a member is to be found */
  #define MXOffsetToMemberPage(xid) ((xid) / (TransactionId) MULTIXACT_MEMBERS_PER_PAGE)
  
***************
*** 2278,2283 **** ExtendMultiXactMember(MultiXactOffset offset, int nmembers)
--- 2282,2288 ----
  	{
  		int			flagsoff;
  		int			flagsbit;
+ 		uint32		difference;
  
  		/*
  		 * Only zero when at first entry of a page.
***************
*** 2302,2322 **** ExtendMultiXactMember(MultiXactOffset offset, int nmembers)
  		 * Advance to next page, taking care to properly handle the wraparound
  		 * case.  OK if nmembers goes negative.
  		 */
- 		if ((unsigned int) (offset + nmembers) < offset)
- 		{
- 			uint32		difference = offset + MULTIXACT_MEMBERS_PER_PAGE;
  
! 			nmembers -= (unsigned int) (MULTIXACT_MEMBERS_PER_PAGE - difference);
! 			offset = 0;
  		}
  		else
- 		{
- 			int			difference;
- 
  			difference = MULTIXACT_MEMBERS_PER_PAGE - offset % MULTIXACT_MEMBERS_PER_PAGE;
! 			nmembers -= difference;
! 			offset += difference;
! 		}
  	}
  }
  
--- 2307,2332 ----
  		 * Advance to next page, taking care to properly handle the wraparound
  		 * case.  OK if nmembers goes negative.
  		 */
  
! 		if ((unsigned int) (offset + MAX_MEMBERS_IN_LAST_MEMBERS_PAGE) < offset)
! 		{
! 			/*
! 			 * This is the last page of the last segment; we compute the number
! 			 * of members to store in it noting that all previous pages have
! 			 * MULTIXACT_MEMBERS_PER_PAGE members, but this one has a smaller
! 			 * number of them.  Note that while in general the (offset %
! 			 * MULTIXACT_MEMBERS_PER_PAGE) expression might be larger than
! 			 * MAX_MEMBERS_IN_LAST_MEMBERS_PAGE, we know that it cannot be in
! 			 * this case, so the difference is always a positive number.
! 			 */
! 			difference = MAX_MEMBERS_IN_LAST_MEMBERS_PAGE -
! 				offset % MULTIXACT_MEMBERS_PER_PAGE;
  		}
  		else
  			difference = MULTIXACT_MEMBERS_PER_PAGE - offset % MULTIXACT_MEMBERS_PER_PAGE;
! 
! 		nmembers -= difference;
! 		offset += difference;
  	}
  }
  

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