Noah, thank you for this excellent review. I have taken over most
(allmost all) of your suggestions (except for the documentation which
is still missing). Please check the updated & attached patch for
details.
On Fri, Jan 14, 2011 at 10:13 PM, Noah Misch <n...@leadboat.com> wrote:
> "00000000000000000000" is a valid md5 message digest. To avoid always
> accepting
> a snapshot with that digest, we would need to separately store a flag
> indicating
> whether a given syncSnapshotChksums member is currently valid. Maybe that
> hole
> is acceptable, though.
In the end I decided to store md5 checksums as printable characters in
shmem. We now need a few more bytes for each checksum but as soon as a
byte is NUL we know that it is not a valid checksum.
>> +
>> + if (!XactReadOnly)
>> + elog(WARNING, "A snapshot exporting function should be
>> readonly.");
>
> There are legitimate use cases for copying the snapshot of a read-write
> transaction. Consider a task like calculating some summaries for insert into
> a
> table. To speed this up, you might notionally partition the source data and
> have each of several workers calculate each partition. I'd vote for removing
> this warning entirely.
Warning removed, adding this fact to the documentation only is fine with me.
> InvalidBackendId is -1. Is InvalidOid (0) in fact also invalid as a backend
> ID?
Yes, there is a check in utils/init/postinit.c that makes sure that 0
is never a valid backendId. But anyway, I have now made this more
explicit by adding an own parse routine for ints.
>> + while ((xid = parseXactFromText(&s, "xip:")) != InvalidTransactionId)
>> + {
>> + if (xid == GetTopTransactionIdIfAny())
>> + continue;
>> + snapshot->xip[i++] = xid;
>
> This works on a virgin GetSnapshotData() snapshot, which always has enough
> space
> (sized according to max_connections). A snapshot from CopySnapshot(),
> however,
> has just enough space for the original usage. I get a SIGSEGV at this line
> with
> the attached test script. If I change things around so xip starts non-NULL, I
> get messages like these as the code scribbles past the end of the allocation:
This has been fixed. xip/subxip are now allocated separately if necessary.
> Though unlikely, the other backend may not even exist by now. Indeed, that
> could have happened and RecentGlobalXmin advanced since we compared the
> checksum. Not sure what the right locking is here, but something is needed.
Good catch. What I have done now is a second check at the end of
ImportSnapshot(). At that point we have set up the new snapshot and
adjusted MyProc->xmin. If we succeed, then we are fine. If not we
throw an error and invalidate the whole transaction.
>> + * Instead we must check to not go forward (we might have opened a
>> cursor
>> + * in this transaction and still have its snapshot registered)
>> + */
>
> I'm thinking this case should yield an ERROR. Otherwise, our net result would
> be to silently adopt a snapshot that is neither our old self nor the target.
> Maybe there's a use case for that, but none comes to my mind.
This can happen when you do:
DECLARE foo CURSOR FOR SELECT * FROM bar;
import snapshot...
FETCH 1 FROM foo;
> I guess the use case for pg_import_snapshot from READ COMMITTED would be
> something like "DO $$BEGIN PERFORM pg_import_snapshot('...'); stuff; END$$;".
> First off, would that work ("stuff" use the imported snapshot)? If it does
> work, we should take the precedent of SET LOCAL and issue no warning. If it
> doesn't work, then we should document that the snapshot does take effect until
> the next command and probably keep this warning or something similar.
No, this will also give you a new snapshot for every query, no matter
if it is executed within or outside of a DO-Block.
>> + Datum
>> + pg_import_snapshot(PG_FUNCTION_ARGS)
>> + {
>> + bytea *snapshotData = PG_GETARG_BYTEA_P(0);
>> + bool ret = true;
>> +
>> + if (ActiveSnapshotSet())
>> + ret = ImportSnapshot(snapshotData, GetActiveSnapshot());
>> +
>> + ret &= ImportSnapshot(snapshotData, GetTransactionSnapshot());
>
> Is it valid to scribble directly on snapshots like this?
I figured that previously executed code still has references pointing
to the snapshots and so we cannot just push a new snapshot on top but
really need to change the memory where they are pointing to.
I am also adding a test script that shows the difference of READ
COMMITTED and SERIALIZABLE in an importing transaction in a DO-Block.
It's based on the script you sent.
So thanks again and I'm looking forward to your next review... :-)
Joachim
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index 2dbac56..c96942a 100644
*** a/src/backend/storage/ipc/ipci.c
--- b/src/backend/storage/ipc/ipci.c
*************** CreateSharedMemoryAndSemaphores(bool mak
*** 124,129 ****
--- 124,130 ----
size = add_size(size, BTreeShmemSize());
size = add_size(size, SyncScanShmemSize());
size = add_size(size, AsyncShmemSize());
+ size = add_size(size, SyncSnapshotShmemSize());
#ifdef EXEC_BACKEND
size = add_size(size, ShmemBackendArraySize());
#endif
*************** CreateSharedMemoryAndSemaphores(bool mak
*** 228,233 ****
--- 229,235 ----
BTreeShmemInit();
SyncScanShmemInit();
AsyncShmemInit();
+ SyncSnapshotInit();
#ifdef EXEC_BACKEND
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index 8b36df4..29c9426 100644
*** a/src/backend/storage/ipc/procarray.c
--- b/src/backend/storage/ipc/procarray.c
***************
*** 50,60 ****
--- 50,63 ----
#include "access/transam.h"
#include "access/xact.h"
#include "access/twophase.h"
+ #include "libpq/md5.h"
#include "miscadmin.h"
#include "storage/procarray.h"
#include "storage/spin.h"
#include "storage/standby.h"
#include "utils/builtins.h"
+ #include "utils/bytea.h"
+ #include "utils/memutils.h"
#include "utils/snapmgr.h"
*************** static int KnownAssignedXidsGetAndSetXmi
*** 159,164 ****
--- 162,171 ----
static TransactionId KnownAssignedXidsGetOldestXmin(void);
static void KnownAssignedXidsDisplay(int trace_level);
+ typedef char snapshotChksum[33];
+ static snapshotChksum *syncSnapshotChksums;
+ static Snapshot exportedSnapshot;
+
/*
* Report shared-memory space needed by CreateSharedProcArray.
*/
*************** KnownAssignedXidsDisplay(int trace_level
*** 3065,3067 ****
--- 3072,3424 ----
pfree(buf.data);
}
+
+
+ /*
+ * Report space needed for our shared memory area, which is basically an
+ * md5 checksum per connection.
+ */
+ Size
+ SyncSnapshotShmemSize(void)
+ {
+ return PROCARRAY_MAXPROCS * sizeof(snapshotChksum);
+ }
+
+ void
+ SyncSnapshotInit(void)
+ {
+ Size size;
+ bool found;
+
+ size = SyncSnapshotShmemSize();
+
+ syncSnapshotChksums = (snapshotChksum*) ShmemInitStruct("SyncSnapshotChksums",
+ size, &found);
+ if (!found)
+ memset(syncSnapshotChksums[0], 0, sizeof(snapshotChksum) * PROCARRAY_MAXPROCS);
+ }
+
+ void
+ InvalidateExportedSnapshot(void)
+ {
+ if (syncSnapshotChksums[MyBackendId][0] == '\0')
+ return;
+
+ memset(syncSnapshotChksums[MyBackendId], 0, sizeof(snapshotChksum));
+
+ UnregisterSnapshotFromOwner(exportedSnapshot, TopTransactionResourceOwner);
+ exportedSnapshot = NULL;
+ }
+
+ static void
+ snapshot_appendf(char ** buffer, int *bufsize_filled, int *bufsize_left, char *fmt, ...)
+ {
+ va_list ap;
+ int ret;
+
+ if (*bufsize_left < 64)
+ {
+ /* enlarge buffer by 1024 bytes */
+ *buffer = repalloc(*buffer, *bufsize_filled + 1024);
+ *bufsize_left = *bufsize_filled + 1024;
+ }
+
+ va_start(ap, fmt);
+ ret = vsnprintf(*buffer + *bufsize_filled, *bufsize_left, fmt, ap);
+ va_end(ap);
+
+ /* There shouldn't be any error, we leave enough room for the data that we
+ * are writing (only numbers basically). */
+ Assert(ret < *bufsize_left && ret > 0);
+
+ *bufsize_left -= ret;
+ *bufsize_filled += ret;
+
+ Assert(strlen(*buffer) == *bufsize_filled);
+ }
+
+ bytea *
+ ExportSnapshot(Snapshot snapshot)
+ {
+ int bufsize = 1024;
+ int bufsize_filled = 0; /* doesn't include NUL byte */
+ int bufsize_left = bufsize - bufsize_filled;
+ char *buf = (char *) palloc(bufsize);
+ int i;
+ TransactionId *children;
+ int nchildren;
+
+ /* In a subtransaction we don't see our open subxip values in the snapshot
+ * so they would be missing in the backend applying it. */
+ /* XXX is ERRCODE_INVALID_TRANSACTION_STATE more appropriate or less?
+ * It's a valid transaction state but invalid for the requested operation. */
+ if (GetCurrentTransactionNestLevel() != 1)
+ ereport(ERROR,
+ (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ errmsg("can only export a snapshot from a top level transaction")));
+
+ /* We do however see our already committed subxip values and add them to
+ * the subxip array. */
+ nchildren = xactGetCommittedChildren(&children);
+
+ /* Write up all the data that we return */
+ snapshot_appendf(&buf, &bufsize_filled, &bufsize_left,
+ "did:%d ", MyDatabaseId);
+ snapshot_appendf(&buf, &bufsize_filled, &bufsize_left,
+ "bid:%d ", MyBackendId);
+ snapshot_appendf(&buf, &bufsize_filled, &bufsize_left,
+ "xmi:%d ", snapshot->xmin);
+ snapshot_appendf(&buf, &bufsize_filled, &bufsize_left,
+ "xma:%d ", snapshot->xmax);
+ /* Include our own transaction ID into the count if any. */
+ snapshot_appendf(&buf, &bufsize_filled, &bufsize_left,
+ "xcnt:%d ", TransactionIdIsValid(GetTopTransactionIdIfAny()) ?
+ snapshot->xcnt + 1 : snapshot->xcnt);
+ for (i = 0; i < snapshot->xcnt; i++)
+ snapshot_appendf(&buf, &bufsize_filled, &bufsize_left,
+ "xip:%d ", snapshot->xip[i]);
+ /*
+ * Finally add our own XID if we have one, since by definition we will
+ * still be running when the other transaction takes over the snapshot.
+ */
+ if (TransactionIdIsValid(GetTopTransactionIdIfAny()))
+ snapshot_appendf(&buf, &bufsize_filled, &bufsize_left,
+ "xip:%d ", GetTopTransactionIdIfAny());
+ if (snapshot->suboverflowed || snapshot->subxcnt + nchildren > TOTAL_MAX_CACHED_SUBXIDS)
+ snapshot_appendf(&buf, &bufsize_filled, &bufsize_left,
+ "sof:1 ");
+ else
+ {
+ snapshot_appendf(&buf, &bufsize_filled, &bufsize_left,
+ "sxcnt:%d ", snapshot->subxcnt + nchildren);
+ for (i = 0; i < snapshot->subxcnt; i++)
+ snapshot_appendf(&buf, &bufsize_filled, &bufsize_left,
+ "sxp:%d ", snapshot->subxip[i]);
+ /* Add already committed subtransactions. */
+ for (i = 0; i < nchildren; i++)
+ snapshot_appendf(&buf, &bufsize_filled, &bufsize_left,
+ "sxp:%d ", children[i]);
+ }
+
+ /*
+ * buf ends with a trailing space but we leave it in for simplicity. The
+ * parsing routines also depend on it.
+ */
+
+ /* Register the snapshot */
+ snapshot = RegisterSnapshotOnOwner(snapshot, TopTransactionResourceOwner);
+ exportedSnapshot = snapshot;
+
+ pg_md5_hash(buf, bufsize_filled, syncSnapshotChksums[MyBackendId]);
+
+ return DatumGetByteaP(DirectFunctionCall1(byteain, CStringGetDatum(buf)));
+ }
+
+ static Oid
+ parseOidFromText(char **s, const char *prefix)
+ {
+ char *n, *p = strstr(*s, prefix);
+ Oid oid;
+
+ if (!p)
+ return InvalidOid;
+ p += strlen(prefix);
+ n = strchr(p, ' ');
+ if (!n)
+ return InvalidOid;
+ *n = '\0';
+ oid = DatumGetObjectId(DirectFunctionCall1(oidin, CStringGetDatum(p)));
+ *s = n + 1;
+ return oid;
+ }
+
+ static int
+ parseIntFromText(char **s, const char *prefix, int notfound)
+ {
+ char *n, *p = strstr(*s, prefix);
+ int i;
+
+ if (!p)
+ return notfound;
+ p += strlen(prefix);
+ n = strchr(p, ' ');
+ if (!n)
+ return notfound;
+ *n = '\0';
+ i = DatumGetObjectId(DirectFunctionCall1(int4in, CStringGetDatum(p)));
+ *s = n + 1;
+ return i;
+ }
+
+ static bool
+ parseBoolFromText(char **s, const char *prefix)
+ {
+ return (bool) parseIntFromText(s, prefix, 0);
+ }
+
+ static TransactionId
+ parseXactFromText(char **s, const char *prefix)
+ {
+ char *n, *p = strstr(*s, prefix);
+ TransactionId xid;
+
+ if (!p)
+ return InvalidTransactionId;
+ p += strlen(prefix);
+ n = strchr(p, ' ');
+ if (!n)
+ return InvalidTransactionId;
+ *n = '\0';
+ xid = DatumGetTransactionId(DirectFunctionCall1(xidin, CStringGetDatum(p)));
+ *s = n + 1;
+ return xid;
+ }
+
+ bool
+ ImportSnapshot(bytea *snapshotData, Snapshot snapshot)
+ {
+ snapshotChksum cksum;
+ Oid databaseId;
+ BackendId backendId;
+ int i;
+ TransactionId xid;
+ int len = VARSIZE_ANY_EXHDR(snapshotData);
+ char *s = palloc(len + 1);
+ MemoryContext oldctx;
+
+ /* XXX is ERRCODE_INVALID_TRANSACTION_STATE more appropriate or less?
+ * It's a valid transaction state but invalid for the requested operation. */
+ if (GetCurrentTransactionNestLevel() != 1)
+ ereport(ERROR,
+ (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ errmsg("can only import a snapshot to a top level transaction")));
+
+ if (len == 0)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("invalid data in snapshot information")));
+
+ strncpy(s, VARDATA_ANY(snapshotData), len);
+ s[len] = '\0';
+
+ pg_md5_hash(s, len, (void *) &cksum);
+
+ databaseId = parseOidFromText(&s, "did:");
+ backendId = (BackendId) parseIntFromText(&s, "bid:", (int) InvalidBackendId);
+
+ if (databaseId != MyDatabaseId
+ || backendId == InvalidBackendId
+ /*
+ * Make sure backendId is in a reasonable range before using it as an
+ * array subscript.
+ */
+ || backendId >= PROCARRAY_MAXPROCS
+ || backendId < 0
+ || backendId == MyBackendId
+ /*
+ * Lock considerations:
+ *
+ * syncSnapshotChksums[backendId] is only changed by the backend with ID
+ * backendID and read by another backend that is asked to import a
+ * snapshot.
+ * syncSnapshotChksums[backendId] contains either NUL bytes or the checksum
+ * of a valid snapshot.
+ * Every comparision to the checksum while it is being written or
+ * deleted is okay to fail, so we don't need to take a lock at all.
+ */
+ || strcmp(cksum, syncSnapshotChksums[backendId]) != 0)
+ {
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("invalid data in snapshot information")));
+ }
+
+ Assert(databaseId != InvalidOid);
+
+ oldctx = MemoryContextSwitchTo(TopTransactionContext);
+
+ snapshot->xmin = parseXactFromText(&s, "xmi:");
+ Assert(snapshot->xmin != InvalidTransactionId);
+ snapshot->xmax = parseXactFromText(&s, "xma:");
+ Assert(snapshot->xmax != InvalidTransactionId);
+
+ if (snapshot->copied)
+ {
+ int xcnt = parseIntFromText(&s, "xcnt:", 0);
+ /*
+ * Unfortunately CopySnapshot allocates just one large chunk of memory,
+ * and makes snapshot->xip then point past the end of the fixed-size
+ * snapshot data. That way we cannot just repalloc(snapshot->xip, ...).
+ * Neither can we just change the base address of the snapshot because
+ * this address might still be saved somewhere.
+ */
+ if (snapshot->xcnt < xcnt)
+ snapshot->xip = palloc(xcnt * sizeof(TransactionId));
+ }
+
+ i = 0;
+ while ((xid = parseXactFromText(&s, "xip:")) != InvalidTransactionId)
+ {
+ if (xid == GetTopTransactionIdIfAny())
+ continue;
+ snapshot->xip[i++] = xid;
+ }
+ snapshot->xcnt = i;
+
+ /*
+ * We only write "sof:1" if the snapshot overflowed. If not, then there is
+ * no "sof:x" entry at all and parseBoolFromText() will just return false.
+ */
+ snapshot->suboverflowed = parseBoolFromText(&s, "sof:");
+
+ if (!snapshot->suboverflowed)
+ {
+ if (snapshot->copied)
+ {
+ int sxcnt = parseIntFromText(&s, "sxcnt:", 0);
+ if (snapshot->subxcnt < sxcnt)
+ snapshot->subxip = palloc(sxcnt * sizeof(TransactionId));
+ }
+
+ i = 0;
+ while ((xid = parseXactFromText(&s, "sxp:")) != InvalidTransactionId)
+ snapshot->subxip[i++] = xid;
+ snapshot->subxcnt = i;
+ }
+
+ MemoryContextSwitchTo(oldctx);
+
+ /* Leave snapshot->curcid as is. */
+
+ /*
+ * No ProcArrayLock held here, we assume that a write is atomic. Also note
+ * that MyProc->xmin can go backwards here. However this is safe because
+ * the xmin we set here is the same as in the backend's proc->xmin whose
+ * snapshot we are copying. At this very moment, anybody computing a
+ * minimum will calculate at least this xmin as the overall xmin with or
+ * without us setting MyProc->xmin to this value.
+ * Instead we must check to not go forward (we might have opened a cursor
+ * in this transaction and still have its snapshot registered)
+ */
+ if (TransactionIdPrecedes(snapshot->xmin, MyProc->xmin))
+ MyProc->xmin = snapshot->xmin;
+
+ /*
+ * Check the checksum again to prevent a race condition. If the exporting
+ * backend invalidated its snapshot since we last checked or before we
+ * finish with this second check, then we fail here and error out, thus
+ * invalidating the snapshot we've built up.
+ * We could succeed here even though the exporting transaction is at the
+ * same time invalidating the checksum while we are checking it here for
+ * the second time. But even then we are good, because we have set up our
+ * MyProc record already.
+ */
+ if (strcmp(cksum, syncSnapshotChksums[backendId]) != 0)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("invalid data in snapshot information")));
+
+ return true;
+ }
+
+
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 45b92a0..f74ea76 100644
*** a/src/backend/utils/time/snapmgr.c
--- b/src/backend/utils/time/snapmgr.c
***************
*** 29,34 ****
--- 29,35 ----
#include "access/xact.h"
#include "storage/proc.h"
#include "storage/procarray.h"
+ #include "utils/builtins.h"
#include "utils/memutils.h"
#include "utils/memutils.h"
#include "utils/resowner.h"
*************** AtEarlyCommit_Snapshot(void)
*** 523,528 ****
--- 524,530 ----
TopTransactionResourceOwner);
registered_xact_snapshot = false;
+ InvalidateExportedSnapshot();
}
/*
*************** AtEOXact_Snapshot(bool isCommit)
*** 559,561 ****
--- 561,595 ----
FirstSnapshotSet = false;
registered_xact_snapshot = false;
}
+
+ Datum
+ pg_export_snapshot(PG_FUNCTION_ARGS)
+ {
+ bytea *snapshotData = ExportSnapshot(GetTransactionSnapshot());
+ PG_RETURN_BYTEA_P(snapshotData);
+ }
+
+ Datum
+ pg_import_snapshot(PG_FUNCTION_ARGS)
+ {
+ bytea *snapshotData = PG_GETARG_BYTEA_P(0);
+ bool ret = true;
+
+ if (ActiveSnapshotSet())
+ ret = ImportSnapshot(snapshotData, GetActiveSnapshot());
+
+ ret &= ImportSnapshot(snapshotData, GetTransactionSnapshot());
+
+ /*
+ * If we are in read committed mode then the next query will execute with a
+ * new snapshot making this function call quite useless.
+ */
+ if (!IsolationUsesXactSnapshot())
+ ereport(WARNING,
+ (errcode(ERRCODE_INAPPROPRIATE_ISOLATION_LEVEL_FOR_BRANCH_TRANSACTION),
+ errmsg("a snapshot importing transaction should be ISOLATION LEVEL SERIALIZABLE"),
+ errhint("A transaction of isolation level READ COMMITTED gives you a new snapshot for each query.")));
+
+ PG_RETURN_BOOL(ret);
+ }
+
diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index f8b5d4d..3a4bc6d 100644
*** a/src/include/catalog/pg_proc.h
--- b/src/include/catalog/pg_proc.h
*************** DATA(insert OID = 2171 ( pg_cancel_backe
*** 3391,3396 ****
--- 3391,3400 ----
DESCR("cancel a server process' current query");
DATA(insert OID = 2096 ( pg_terminate_backend PGNSP PGUID 12 1 0 0 f f f t f v 1 0 16 "23" _null_ _null_ _null_ _null_ pg_terminate_backend _null_ _null_ _null_ ));
DESCR("terminate a server process");
+ DATA(insert OID = 3115 ( pg_export_snapshot PGNSP PGUID 12 1 0 0 f f f t f v 0 0 17 "" _null_ _null_ _null_ _null_ pg_export_snapshot _null_ _null_ _null_ ));
+ DESCR("export a snapshot");
+ DATA(insert OID = 3116 ( pg_import_snapshot PGNSP PGUID 12 1 0 0 f f f t f v 1 0 16 "17" _null_ _null_ _null_ _null_ pg_import_snapshot _null_ _null_ _null_ ));
+ DESCR("import a snapshot");
DATA(insert OID = 2172 ( pg_start_backup PGNSP PGUID 12 1 0 0 f f f t f v 2 0 25 "25 16" _null_ _null_ _null_ _null_ pg_start_backup _null_ _null_ _null_ ));
DESCR("prepare for taking an online backup");
DATA(insert OID = 2173 ( pg_stop_backup PGNSP PGUID 12 1 0 0 f f f t f v 0 0 25 "" _null_ _null_ _null_ _null_ pg_stop_backup _null_ _null_ _null_ ));
diff --git a/src/include/storage/procarray.h b/src/include/storage/procarray.h
index 334f9a2..ee94451 100644
*** a/src/include/storage/procarray.h
--- b/src/include/storage/procarray.h
*************** extern void XidCacheRemoveRunningXids(Tr
*** 71,74 ****
--- 71,81 ----
int nxids, const TransactionId *xids,
TransactionId latestXid);
+ extern Size SyncSnapshotShmemSize(void);
+ extern void SyncSnapshotInit(void);
+
+ extern bytea *ExportSnapshot(Snapshot snapshot);
+ extern bool ImportSnapshot(bytea *snapshotData, Snapshot snapshot);
+ extern void InvalidateExportedSnapshot(void);
+
#endif /* PROCARRAY_H */
diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
index 6d784d2..3dcb924 100644
*** a/src/include/utils/snapmgr.h
--- b/src/include/utils/snapmgr.h
*************** extern void AtSubAbort_Snapshot(int leve
*** 43,46 ****
--- 43,49 ----
extern void AtEarlyCommit_Snapshot(void);
extern void AtEOXact_Snapshot(bool isCommit);
+ extern Datum pg_export_snapshot(PG_FUNCTION_ARGS);
+ extern Datum pg_import_snapshot(PG_FUNCTION_ARGS);
+
#endif /* SNAPMGR_H */
use DBI;
use Data::Dumper;
$dad = DBI->connect('dbi:Pg:dbname=template1;host=localhost', '', '', {AutoCommit => 0, RaiseError => 1});
$kid = DBI->connect('dbi:Pg:dbname=template1;host=localhost', '', '', {AutoCommit => 0, RaiseError => 1});
# Cleanup...
$kid->do("DROP TABLE IF EXISTS kidseen");
$kid->do("DROP TABLE IF EXISTS kidtest");
$kid->commit();
# Dad opens a transaction...
$dad->rollback();
$dad->do("SET bytea_output = 'escape'");
# Dad exports its snapshot for the kid to take it over...
($snap) = $dad->selectrow_array('SELECT pg_export_snapshot()');
print "Dad: " . Dumper($dad->selectall_arrayref("SELECT * FROM pg_class where relname = 'kidtest'"));
# The kid creates a table that will disappear when it takes over dad's snapshot...
$kid->do("CREATE TABLE kidtest (a int);");
$kid->commit();
# READ COMMITTED
$kid->do("set default_transaction_isolation to 'read committed'");
$kid->commit();
$kid->do("DO \$\$BEGIN
PERFORM pg_import_snapshot('$snap');
CREATE TABLE kidseen AS SELECT * FROM pg_class WHERE relname = 'kidtest';
END\$\$");
print "Kid RC: " . Dumper($kid->selectall_arrayref("SELECT * FROM kidseen"));
$kid->rollback();
# SERIALIZABLE
$kid->do("set default_transaction_isolation to serializable");
$kid->commit();
$kid->do("DO \$\$BEGIN
PERFORM pg_import_snapshot('$snap');
CREATE TABLE kidseen AS SELECT * FROM pg_class WHERE relname = 'kidtest';
END\$\$");
print "Kid SERIAL: " . Dumper($kid->selectall_arrayref("SELECT * FROM kidseen"));
$kid->rollback();
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