Here is a new version of the patch incorporating most of Noah's
suggestions. The patch now also adds documentation. Since I couldn't
really find a suitable section to document the two new functions, I
added a new one for now. Any better ideas where it should go?
On Thu, Jan 20, 2011 at 1:37 AM, Noah Misch <[email protected]> wrote:
> Just to clarify, I was envisioning something like:
>
> typedef struct { bool valid; char value[16]; } snapshotChksum;
>
> I don't object to the way you've done it, but someone else might not like the
> extra marshalling between text and binary. Your call.
I didn't like it in the beginning but later on I adopted your
proposal. I have also changed the locking to be more natural. Even
though we don't really need it, I am now grabbing shared ProcArrayLock
for any reads of shared memory and exclusive lock for writes. Of
course no additional lock is taken if the feature is not used.
> You're right. Then consider "VALUES (pg_import_snapshot('...'), (SELECT
> count(*) from t))" at READ COMMITTED. It works roughly as I'd guess; the
> subquery uses the imported snapshot. If I flip the two expressions and do
> "VALUES ((SELECT count(*) from t), pg_import_snapshot('...'))", the subquery
> uses the normal snapshot. That makes sense, but I can't really see a use case
> for it. A warning, like your code has today, seems appropriate.
Yeah, that would do what you wanted to illustrate but it truely cannot
be considered the standard use case :-)
>> > 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.
> Okay. Had no special reason to believe otherwise, just didn't know.
This is one part where I'd like someone more experienced than me look into it.
> Thanks; that was handy. One thing I noticed is that the second "SELECT * FROM
> kidseen" yields zero rows instead of yielding "ERROR: relation "kidseen" does
> not exist". I changed things around per the attached "test-drop.pl", such
> that
> the table is already gone in the ordinary snapshot, but still visible to the
> imported snapshot. Note how the pg_class row is visible, but an actual
> attempt
> to query the table fails. Must some kind of syscache invalidation follow the
> snapshot alteration to make this behave normally?
As discussed with Noah off-list this is just not an MVCC safe
operation. You could hit on this in a regular SERIALIZABLE transaction
as well: Somebody else can delete a table and you won't be able to
access it anymore. This is also the reason why pg_dump in the
beginning gets a shared lock on every table that it will dump later
on.
Thanks for the review Noah,
Joachim
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index ed2039c..4169594 100644
*** a/doc/src/sgml/func.sgml
--- b/doc/src/sgml/func.sgml
*************** FOR EACH ROW EXECUTE PROCEDURE suppress_
*** 14761,14764 ****
--- 14761,14860 ----
<xref linkend="SQL-CREATETRIGGER">.
</para>
</sect1>
+
+ <sect1 id="functions-snapshotsync">
+ <title>Snapshot Synchronization Functions</title>
+
+ <indexterm>
+ <primary>pg_export_snapshot</primary>
+ </indexterm>
+ <indexterm>
+ <primary>pg_import_snapshot</primary>
+ </indexterm>
+
+ <para>
+ <productname>PostgreSQL</> allows different sessions to synchronize their
+ snapshots. A database snapshot determines which data is visible to
+ the client that is using this snapshot. Synchronized snapshots are necessary if
+ two clients need to see the same content in the database. If these two clients
+ just connected to the database and opened their transactions, then they could
+ never be sure that there was no data modification right between both
+ connections. To solve this, <productname>PostgreSQL</> offers the two functions
+ <function>pg_export_snapshot</> and <function>pg_import_snapshot</>.
+ </para>
+ <para>
+ The idea is that one client retrieves the information about the snapshot that it
+ is currently using and then the second client passes this information back to
+ the server. The database server will then modify the second client's snapshot
+ to match the snapshot of the first and from then on both can see the identical
+ data in the database.
+ </para>
+ <para>
+ Note that a snapshot can only be imported as long as the transaction that
+ exported it originally is held open. Also note that even after the
+ synchronization both clients still run their own independent transactions.
+ As a consequence, even though synchronized with respect to reading pre-existing
+ data, both transactions won't be able to see each other's uncommitted data.
+ </para>
+ <table id="functions-snapshot-synchronization">
+ <title>Snapshot Synchronization Functions</title>
+ <tgroup cols="3">
+ <thead>
+ <row><entry>Name</entry> <entry>Return Type</entry> <entry>Description</entry>
+ </row>
+ </thead>
+
+ <tbody>
+ <row>
+ <entry>
+ <literal><function>pg_export_snapshot()</function></literal>
+ </entry>
+ <entry><type>bytea</type></entry>
+ <entry>Export the snapshot and return its textual representation</entry>
+ </row>
+ <row>
+ <entry>
+ <literal><function>pg_import_snapshot(<parameter>snapshotData</> <type>bytea</>)</function></literal>
+ </entry>
+ <entry><type>boolean</type></entry>
+ <entry>Import a previously exported snapshot</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <para>
+ The function <function>pg_export_snapshot</> does not take an argument
+ and returns the current snapshot information as <type>bytea</type> data.
+ You should not assume any maximal length for this data. Also make sure that
+ the transaction that exports the snapshot will continue to live at least until
+ the snapshot has been imported into another connection. As soon as the
+ transaction ends, the exported snapshot becomes invalid and cannot be imported
+ anymore. You should not rely on what exactly the function returns. If you need
+ to access any transaction IDs of the current transaction, please refer to <xref
+ linkend="functions-txid-snapshot"> instead.
+ </para>
+ <programlisting>
+ BEGIN TRANSACTION READ ONLY;
+ SELECT pg_export_snapshot();
+ </programlisting>
+ <para>
+ <function>pg_import_snapshot</> takes an argument of type <type>bytea</type>
+ and returns a <type>boolean</type> value. Actually the function will
+ always either return true or throw an error. As the argument you need to pass
+ the exact value that has been returned by <function>pg_export_snapshot</>.
+ The function can only be used reasonably if your <literal>TRANSACTION ISOLATION
+ LEVEL</> (see <xref
+ linkend="transaction-iso">) is set to <literal>SERIALIZABLE</> (or
+ <literal>REPEATABLE READ</> which will implicitly use <literal>SERIALIZABLE</>).
+ The reason is that if your transaction is set to <literal>READ COMMITTED</>,
+ it will acquire a new snapshot for the next query and will not use the one that
+ has just been imported.
+ </para>
+ <programlisting>
+ BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
+ SELECT pg_import_snapshot('\x6469643a31206269643a3320786d693a31303133313420786d613a3130313331342078636e743a30207378636e743a3020');
+ </programlisting>
+ </sect1>
</chapter>
+
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..5caddec 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
*** 158,163 ****
--- 161,175 ----
TransactionId xmax);
static TransactionId KnownAssignedXidsGetOldestXmin(void);
static void KnownAssignedXidsDisplay(int trace_level);
+ static void InvalidateExportedSnapshot(void);
+
+ typedef char snapshotChksumPlain[16];
+ typedef struct {
+ snapshotChksumPlain chksum;
+ bool valid;
+ } snapshotChksum;
+ static snapshotChksum *syncSnapshotChksums;
+ static Snapshot exportedSnapshot;
/*
* Report shared-memory space needed by CreateSharedProcArray.
*************** ProcArrayRemove(PGPROC *proc, Transactio
*** 350,355 ****
--- 362,373 ----
void
ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
{
+ /*
+ * Release any exported snapshots.
+ */
+ if (exportedSnapshot)
+ InvalidateExportedSnapshot();
+
if (TransactionIdIsValid(latestXid))
{
/*
*************** void
*** 415,420 ****
--- 433,445 ----
ProcArrayClearTransaction(PGPROC *proc)
{
/*
+ * Release any exported snapshots (Note that this will take exclusive
+ * ProcArrayLock if the feature has been used).
+ */
+ if (exportedSnapshot)
+ InvalidateExportedSnapshot();
+
+ /*
* We can skip locking ProcArrayLock here, because this action does not
* actually change anyone's view of the set of running XIDs: our entry is
* duplicate with the gxact that has already been inserted into the
*************** KnownAssignedXidsDisplay(int trace_level
*** 3065,3067 ****
--- 3090,3469 ----
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;
+ int i;
+
+ size = SyncSnapshotShmemSize();
+
+ syncSnapshotChksums = (snapshotChksum*) ShmemInitStruct("SyncSnapshotChksums",
+ size, &found);
+ if (!found)
+ for (i = 0; i < PROCARRAY_MAXPROCS; i++)
+ syncSnapshotChksums[i].valid = false;
+ }
+
+ static void
+ InvalidateExportedSnapshot(void)
+ {
+ /* See lock considerations at ImportSnapshot() */
+ LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
+ syncSnapshotChksums[MyBackendId].valid = false;
+ LWLockRelease(ProcArrayLock);
+
+ 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)
+ {
+ #define SNAPSHOT_APPEND(x, y) \
+ (snapshot_appendf(&buf, &bufsize_filled, &bufsize_left, (x), (y)))
+ int bufsize = 1024;
+ int bufsize_filled = 0; /* doesn't include NUL byte */
+ int bufsize_left = bufsize - bufsize_filled;
+ char *buf = (char *) palloc(bufsize);
+ TransactionId *children;
+ int i;
+ 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. */
+ if (GetCurrentTransactionNestLevel() != 1)
+ ereport(ERROR,
+ (errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
+ errmsg("cannot export a snapshot from a subtransaction")));
+
+ /* 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_APPEND("did:%d ", MyDatabaseId);
+ SNAPSHOT_APPEND("bid:%d ", MyBackendId);
+ SNAPSHOT_APPEND("xmi:%d ", snapshot->xmin);
+ SNAPSHOT_APPEND("xma:%d ", snapshot->xmax);
+ /* Include our own transaction ID into the count if any. */
+ SNAPSHOT_APPEND("xcnt:%d ", TransactionIdIsValid(GetTopTransactionIdIfAny()) ?
+ snapshot->xcnt + 1 : snapshot->xcnt);
+ for (i = 0; i < snapshot->xcnt; i++)
+ SNAPSHOT_APPEND("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_APPEND("xip:%d ", GetTopTransactionIdIfAny());
+ if (snapshot->suboverflowed || snapshot->subxcnt + nchildren > TOTAL_MAX_CACHED_SUBXIDS)
+ SNAPSHOT_APPEND("sof:%d ", 1);
+ else
+ {
+ SNAPSHOT_APPEND("sxcnt:%d ", snapshot->subxcnt + nchildren);
+ for (i = 0; i < snapshot->subxcnt; i++)
+ SNAPSHOT_APPEND("sxp:%d ", snapshot->subxip[i]);
+ /* Add already committed subtransactions. */
+ for (i = 0; i < nchildren; i++)
+ SNAPSHOT_APPEND("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;
+
+ /* See lock considerations at ImportSnapshot() */
+ LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
+ if (!pg_md5_binary(buf, bufsize_filled, syncSnapshotChksums[MyBackendId].chksum))
+ {
+ LWLockRelease(ProcArrayLock);
+ ereport(ERROR,
+ (errcode(ERRCODE_OUT_OF_MEMORY),
+ errmsg("out of memory")));
+ }
+ syncSnapshotChksums[MyBackendId].valid = true;
+ LWLockRelease(ProcArrayLock);
+
+ return DatumGetByteaP(DirectFunctionCall1(byteain, CStringGetDatum(buf)));
+ #undef SNAPSHOT_APPEND
+ }
+
+ 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 = DatumGetInt32(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;
+ }
+
+ /*
+ * Import a previously exported snapshot: First check the data, then compare
+ * the checksum with the exported snapshots. Finally update the snapshot that
+ * we get passed in.
+ *
+ * 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.
+ *
+ * We'd not really need to take a lock because of the access pattern. Instead we
+ * need to take one because we might run on an architecture with weak memory
+ * ordering: Usually we first invalidate our exported snapshot and then release
+ * our ProcArray entry. A different backend however could see the updated
+ * ProcArray entry but still see our checksum as valid. This can be solved with
+ * a few shared locks but in order to make the access least awkward, we take
+ * exclusive lock on writes and shared lock on reads.
+ */
+ bool
+ ImportSnapshot(bytea *snapshotData, Snapshot snapshot)
+ {
+ BackendId backendId;
+ snapshotChksumPlain cksum;
+ Oid databaseId;
+ int i;
+ int len = VARSIZE_ANY_EXHDR(snapshotData);
+ char *s = palloc(len + 1);
+ int sxcnt, xcnt;
+ TransactionId xid;
+
+ if (GetCurrentTransactionNestLevel() != 1)
+ ereport(ERROR,
+ (errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
+ errmsg("cannot import a snapshot into a subtransaction")));
+
+ 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';
+
+ if (!pg_md5_binary(s, len, (void *) &cksum))
+ ereport(ERROR,
+ (errcode(ERRCODE_OUT_OF_MEMORY),
+ errmsg("out of memory")));
+
+ databaseId = parseOidFromText(&s, "did:");
+ backendId = (BackendId) parseIntFromText(&s, "bid:", (int) InvalidBackendId);
+
+ if (databaseId != MyDatabaseId)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("cannot import snapshot from a different database")));
+
+ if (backendId == InvalidBackendId
+ /*
+ * Make sure backendId is in a reasonable range before using it as an
+ * array subscript.
+ */
+ || backendId >= PROCARRAY_MAXPROCS
+ || backendId < 0)
+ {
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("invalid data in snapshot information")));
+ }
+
+ if (backendId == MyBackendId)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("cannot export and import a snapshot in the same connection")));
+
+ /*
+ * Verify that the checksum matches before doing any more work. We will
+ * later verify again to make sure that the exporting transaction has not
+ * yet terminated by then. We don't want to optimize this into just one
+ * verification call at the very end because the instructions that follow
+ * this comment rely on a sane format of the textual snapshot data. In
+ * particular they assume that there are not more XactIds than
+ * advertised...
+ */
+ if (syncSnapshotChksums[backendId].valid == false
+ || memcmp(cksum, syncSnapshotChksums[backendId].chksum,
+ sizeof(syncSnapshotChksums[backendId].chksum)) != 0)
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("snapshot not found exported by any running transaction")));
+
+ Assert(databaseId != InvalidOid);
+
+ snapshot->xmin = parseXactFromText(&s, "xmi:");
+ Assert(snapshot->xmin != InvalidTransactionId);
+ snapshot->xmax = parseXactFromText(&s, "xma:");
+ Assert(snapshot->xmax != InvalidTransactionId);
+
+ 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->copied && snapshot->xcnt < xcnt)
+ snapshot->xip = MemoryContextAlloc(TopTransactionContext,
+ xcnt * sizeof(TransactionId));
+
+ i = 0;
+ while ((xid = parseXactFromText(&s, "xip:")) != InvalidTransactionId)
+ {
+ if (xid == GetTopTransactionIdIfAny())
+ continue;
+ snapshot->xip[i++] = xid;
+ }
+ snapshot->xcnt = i;
+ Assert(snapshot->xcnt == xcnt);
+
+ /*
+ * 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)
+ {
+ sxcnt = parseIntFromText(&s, "sxcnt:", 0);
+ if (snapshot->copied && snapshot->subxcnt < sxcnt)
+ snapshot->subxip = MemoryContextAlloc(TopTransactionContext,
+ sxcnt * sizeof(TransactionId));
+
+ i = 0;
+ while ((xid = parseXactFromText(&s, "sxp:")) != InvalidTransactionId)
+ snapshot->subxip[i++] = xid;
+ snapshot->subxcnt = i;
+ Assert(snapshot->subxcnt == sxcnt);
+ }
+
+ /* Leave snapshot->curcid as is. */
+
+ LWLockAcquire(ProcArrayLock, LW_SHARED);
+
+ /*
+ * 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 then we fail here
+ * and error out, thus invalidating the snapshot we've built up.
+ */
+ if (syncSnapshotChksums[backendId].valid == false
+ || memcmp(cksum, syncSnapshotChksums[backendId].chksum,
+ sizeof(syncSnapshotChksums[backendId].chksum)) != 0)
+ {
+ LWLockRelease(ProcArrayLock);
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("invalid data in snapshot information")));
+ }
+ LWLockRelease(ProcArrayLock);
+
+ return true;
+ }
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
index 45b92a0..c5ef1ab 100644
*** a/src/backend/utils/time/snapmgr.c
--- b/src/backend/utils/time/snapmgr.c
*************** AtEOXact_Snapshot(bool isCommit)
*** 559,561 ****
--- 559,593 ----
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/REPEATABLE READ"),
+ errhint("A transaction of isolation level READ COMMITTED/READ UNCOMMITED 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..9f47a2e 100644
*** a/src/include/storage/procarray.h
--- b/src/include/storage/procarray.h
*************** extern void XidCacheRemoveRunningXids(Tr
*** 71,74 ****
--- 71,80 ----
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);
+
#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 */
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