Re: obtaining row locking information

> On Fri, Aug 12, 2005 at 02:08:29PM +0900, Tatsuo Ishii wrote:
> > > On Fri, Aug 12, 2005 at 12:27:25PM +0900, Tatsuo Ishii wrote:
> > > 
> > > > However even one of transactions, for example 647 commits, still it
> > > > shows as if 647 is a member of muitixid 3.
> > > > 
> > > > test=# select * from pgrowlocks('t1');
> > > >  locked_row | lock_type | locker | multi |   xids    
> > > > ------------+-----------+--------+-------+-----------
> > > >       (0,1) | Shared    |      3 | t     | {646,647}
> > > > (1 row)
> > > > 
> > > > Am I missing something?
> > > 
> > > By design, a MultiXactId does not change its membership, that is, no
> > > members are added nor deleted.  When this has to happen (for example a
> > > row is locked by another backend), a new MultiXactId is generated.  The
> > > caller is expected to check whether the member transactions are still
> > > running.
> > 
> > But it seems when members are deleted, new multixid is not
> > generated. i.e. I see "locker" column does not change. Is this an
> > expected behavior?
> 
> Yes.  Members are never deleted.  This is for two reasons: first, the
> transaction could theoretically hold millions of MultiXactId, and we
> can't expect it to remember them all; so we don't have a way to find out
> which ones it should clean up when it finishes (a process which would be
> slow and cumbersome anyway).  Second, because the implementation does
> not really allow for shrinking (nor enlarging) an array.  Once created,
> the array is immutable.
> 
> If you locked a tuple with transactions B and C; then transaction B
> committed; then transaction D locked the tuple again, you would see a
> new MultiXactId comprising transactions C and D.

Ok, here is the new version of the function which now checks if the
transactions are still running.

BTW, I think it would be helpfull if the function returns the process
id which runs the transaction. I couldn't find any existing function
which converts an xid to a process id so far, and think inventing
someting like BackendPidGetProc(int pid) would be the way I should
go. Any suggestion?
--
Tatsuo Ishii
/** $PostgreSQL$** Copyright (c) 2005    Tatsuo Ishii** Permission to use, copy, modify, and distribute this software
and*its documentation for any purpose, without fee, and without a* written agreement is hereby granted, provided that
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HEREUNDERIS ON AN "AS* IS" BASIS, AND THE AUTHOR HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,* SUPPORT, UPDATES,
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#include "postgres.h"

#include "funcapi.h"
#include "access/heapam.h"
#include "access/multixact.h"
#include "access/transam.h"
#include "catalog/namespace.h"
#include "catalog/pg_type.h"
#include "storage/procarray.h"
#include "utils/builtins.h"


PG_FUNCTION_INFO_V1(pgrowlocks);

extern Datum pgrowlocks(PG_FUNCTION_ARGS);

/* ----------* pgrowlocks:* returns tids of rows being locked** C FUNCTION definition* pgrowlocks(text) returns set of
pgrowlocks_type*see pgrowlocks.sql for pgrowlocks_type* ----------*/
 

#define DUMMY_TUPLE "public.pgrowlocks_type"
#define NCHARS 32

/** define this if makeRangeVarFromNameList() has two arguments. As far* as I know, this only happens in 8.0.x.*/
#undef MAKERANGEVARFROMNAMELIST_HAS_TWO_ARGS

typedef struct {HeapScanDesc scan;int ncolumns;
} MyData;

Datum
pgrowlocks(PG_FUNCTION_ARGS)
{FuncCallContext *funcctx;HeapScanDesc scan;HeapTuple    tuple;TupleDesc    tupdesc;AttInMetadata *attinmeta;Datum
 result;MyData *mydata;Relation    rel;
 
if (SRF_IS_FIRSTCALL()){    text       *relname;    RangeVar   *relrv;    MemoryContext oldcontext;
    funcctx = SRF_FIRSTCALL_INIT();    oldcontext = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
    tupdesc = RelationNameGetTupleDesc(DUMMY_TUPLE);    attinmeta = TupleDescGetAttInMetadata(tupdesc);
funcctx->attinmeta= attinmeta;
 
    relname = PG_GETARG_TEXT_P(0);
#ifdef MAKERANGEVARFROMNAMELIST_HAS_TWO_ARGS    relrv = makeRangeVarFromNameList(textToQualifiedNameList(relname,
                                         "pgrowlocks"));
 

#else    relrv = makeRangeVarFromNameList(textToQualifiedNameList(relname));
#endif    rel = heap_openrv(relrv, AccessShareLock);    scan = heap_beginscan(rel, SnapshotNow, 0, NULL);    mydata =
palloc(sizeof(*mydata));   mydata->scan = scan;    mydata->ncolumns = tupdesc->natts;    funcctx->user_fctx = mydata;
 
    MemoryContextSwitchTo(oldcontext);}
funcctx = SRF_PERCALL_SETUP();attinmeta = funcctx->attinmeta;mydata = (MyData *)funcctx->user_fctx;scan =
mydata->scan;
/* scan the relation */while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL){    /* must hold a buffer
lockto call HeapTupleSatisfiesUpdate */    LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
 
    if (HeapTupleSatisfiesUpdate(tuple->t_data, GetCurrentCommandId(), scan->rs_cbuf)        == HeapTupleBeingUpdated)
 {
 
        char **values;        int i;
        values = (char **) palloc(mydata->ncolumns * sizeof(char *));
        i = 0;        values[i++] = (char *)DirectFunctionCall1(tidout, PointerGetDatum(&tuple->t_self));

#ifdef HEAP_XMAX_SHARED_LOCK        if (tuple->t_data->t_infomask & HEAP_XMAX_SHARED_LOCK)            values[i++] =
pstrdup("Shared");       else            values[i++] = pstrdup("Exclusive");
 
#else        values[i++] = pstrdup("Exclusive");
#endif        values[i] = palloc(NCHARS*sizeof(char));        snprintf(values[i++], NCHARS, "%d",
HeapTupleHeaderGetXmax(tuple->t_data));
#ifdef HEAP_XMAX_SHARED_LOCK        if (tuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI)        {
TransactionId*xids;            int nxids;            int j;            int isValidXid = 0;        /* any valid xid ever
exists?*/
 
            values[i++] = pstrdup("true");            nxids =
GetMultiXactIdMembers(HeapTupleHeaderGetXmax(tuple->t_data),&xids);            if (nxids == -1)            {
   elog(ERROR, "GetMultiXactIdMembers returns error");            }
 
            values[i] = palloc(NCHARS*nxids);            strcpy(values[i], "{");
            for (j=0;j<nxids;j++)            {                char buf[NCHARS];
                if (TransactionIdIsInProgress(xids[j]))                {                    if (isValidXid)
      {                        strcat(values[i], ",");                    }                    snprintf(buf, NCHARS,
"%d",xids[j]);                    strcat(values[i], buf);                    isValidXid = 1;                }
}
 
            strcat(values[i], "}");            i++;        }        else        {            values[i++] =
pstrdup("false");           values[i++] = pstrdup("{}");        }
 
#else        values[i++] = pstrdup("false");        values[i++] = pstrdup("{}");
#endif
        LockBuffer(scan->rs_cbuf, BUFFER_LOCK_UNLOCK);
        /* build a tuple */        tuple = BuildTupleFromCStrings(attinmeta, values);
        /* make the tuple into a datum */        result = HeapTupleGetDatum(tuple);
        /* Clean up */        for (i = 0; i < mydata->ncolumns; i++)            pfree(values[i]);
pfree(values);
        SRF_RETURN_NEXT(funcctx, result);    }    else    {        LockBuffer(scan->rs_cbuf, BUFFER_LOCK_UNLOCK);
}}
heap_endscan(scan);heap_close(scan->rs_rd, AccessShareLock);
SRF_RETURN_DONE(funcctx);
}