Re: BUG #15990: PROCEDURE throws "SQL Error [XX000]: ERROR: no known snapshots" with PostGIS geometries

I wrote:
> Yeah, on further thought, the real question to be asking here is
> "what's protecting that in-flight datum from becoming a dangling
> pointer?".  AFAICT, the answer right now is "nothing".  Therefore,
> it is *never* okay for plpgsql to be executing without a registered
> transaction snapshot; and it seems quite unlikely that it'd be any
> safer for any other PL.

Here's a draft patch for this issue.

I concluded that the reason things work for a normal DO block
or plpgsql procedure (without internal transaction management)
is that pquery.c -- in particular, PortalRunUtility -- establishes
a snapshot for the duration of execution of the portal that is
running the CALL or DO.  That allows us to detoast the results of
a SPI query even though SPI has released whatever statement-level
snapshot it was using.  Hence, what we should do to fix this is
to try to restore that state of affairs after an internal commit.
We can't just make a snapshot instantly, because it may be that
the user wants to issue SET command(s) to change the transaction
mode and/or LOCK commands to acquire locks before a new
transaction snapshot is taken.  This means that _SPI_execute_plan
needs to deal with re-creating the Portal-level snapshot, but not
till just before executing a command that has to have a snapshot.

(Of course, fixing it in _SPI_execute_plan only fixes things for
PLs that go through SPI.  But if you don't, you were pretty much
on your own before, and you still are.)

So the attached patch removes PortalRunUtility's local state in
favor of adding a Portal field that tracks the snapshot provided
by the Portal, if any.  Then when we make a new snapshot that
logically substitutes for the portal's original snapshot, we can
store it in that field and it'll be released when we finally
exit the portal.

There's a loose end that this doesn't really resolve, which is
that a procedure could return output data that involves a toast
pointer.  If the procedure's last action was COMMIT, we'll
come back to ExecuteCallStmt with no snapshot that protects the
validity of that toast pointer.  For the moment I just added a
comment that says PLs mustn't do that.  I think that is safe
for plpgsql, because it treats output parameters like local
variables, and it already knows it mustn't store toast pointers
in procedure local variables.  It's probably all right for our
other PLs too, because they don't really deal in direct use of
toasted datums anyway.  But it still seems scary, because there is
really no good way to detect a violation.

A couple of other bits of unfinished business:

* This basically moves all of the snapshot management logic into
_SPI_execute_plan, making "no_snapshots" quite a misnomer.
It kind of was already, because of its contribution to whether
we pass down PROCESS_UTILITY_QUERY_NONATOMIC, but now it's a
flat out lie.  We still need a flag to control what happens with
PROCESS_UTILITY_QUERY_NONATOMIC, so I renamed it to
allow_nonatomic.  For the moment I just did that renaming locally
in _SPI_execute_plan.  In HEAD, we ought to propagate that name
into SPIExecuteOptions, but I haven't done that here.

* I think we could remove the PLPGSQL_STMT_SET infrastructure
and go back to treating SET/RESET the same as other utility
commands so far as plpgsql is concerned.  I haven't done that
here either, because it wouldn't be appropriate to do in the
back branches (if only for fear of breaking pldebugger).

Note that we still need the other patch to discourage plpgsql
from leaving not-yet-detoasted datums hanging around in a
FOR loop's fetch queue.

            regards, tom lane

diff --git a/src/backend/commands/functioncmds.c b/src/backend/commands/functioncmds.c
index 9548287217..4c12aa33df 100644
--- a/src/backend/commands/functioncmds.c
+++ b/src/backend/commands/functioncmds.c
@@ -64,6 +64,7 @@
 #include "parser/parse_func.h"
 #include "parser/parse_type.h"
 #include "pgstat.h"
+#include "tcop/pquery.h"
 #include "tcop/utility.h"
 #include "utils/acl.h"
 #include "utils/builtins.h"
@@ -2319,6 +2320,20 @@ ExecuteCallStmt(CallStmt *stmt, ParamListInfo params, bool atomic, DestReceiver
         if (fcinfo->isnull)
             elog(ERROR, "procedure returned null record");

+        /*
+         * Ensure there's an active snapshot whilst we execute whatever's
+         * involved here.  Note that this is *not* sufficient to make the
+         * world safe for TOAST pointers to be included in the returned data:
+         * the referenced data could have gone away while we didn't hold a
+         * snapshot.  Hence, it's incumbent on PLs that can do COMMIT/ROLLBACK
+         * to not return TOAST pointers, unless those pointers were fetched
+         * after the last COMMIT/ROLLBACK in the procedure.
+         *
+         * XXX that is a really nasty, hard-to-test requirement.  Is there a
+         * way to remove it?
+         */
+        EnsurePortalSnapshotExists();
+
         td = DatumGetHeapTupleHeader(retval);
         tupType = HeapTupleHeaderGetTypeId(td);
         tupTypmod = HeapTupleHeaderGetTypMod(td);
diff --git a/src/backend/executor/spi.c b/src/backend/executor/spi.c
index 00aa78ea53..0912967fe3 100644
--- a/src/backend/executor/spi.c
+++ b/src/backend/executor/spi.c
@@ -66,7 +66,7 @@ static void _SPI_prepare_oneshot_plan(const char *src, SPIPlanPtr plan);

 static int    _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
                               Snapshot snapshot, Snapshot crosscheck_snapshot,
-                              bool read_only, bool no_snapshots,
+                              bool read_only, bool allow_nonatomic,
                               bool fire_triggers, uint64 tcount,
                               DestReceiver *caller_dest,
                               ResourceOwner plan_owner);
@@ -260,12 +260,8 @@ _SPI_commit(bool chain)
     /* Start the actual commit */
     _SPI_current->internal_xact = true;

-    /*
-     * Before committing, pop all active snapshots to avoid error about
-     * "snapshot %p still active".
-     */
-    while (ActiveSnapshotSet())
-        PopActiveSnapshot();
+    /* Release snapshots associated with portals */
+    ForgetPortalSnapshots();

     if (chain)
         SaveTransactionCharacteristics();
@@ -322,6 +318,9 @@ _SPI_rollback(bool chain)
     /* Start the actual rollback */
     _SPI_current->internal_xact = true;

+    /* Release snapshots associated with portals */
+    ForgetPortalSnapshots();
+
     if (chain)
         SaveTransactionCharacteristics();

@@ -2264,7 +2263,7 @@ _SPI_prepare_oneshot_plan(const char *src, SPIPlanPtr plan)
  *        behavior of taking a new snapshot for each query.
  * crosscheck_snapshot: for RI use, all others pass InvalidSnapshot
  * read_only: true for read-only execution (no CommandCounterIncrement)
- * no_snapshots: true to skip snapshot management
+ * allow_nonatomic: true to allow nonatomic CALL/DO execution
  * fire_triggers: true to fire AFTER triggers at end of query (normal case);
  *        false means any AFTER triggers are postponed to end of outer query
  * tcount: execution tuple-count limit, or 0 for none
@@ -2275,7 +2274,7 @@ _SPI_prepare_oneshot_plan(const char *src, SPIPlanPtr plan)
 static int
 _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
                   Snapshot snapshot, Snapshot crosscheck_snapshot,
-                  bool read_only, bool no_snapshots,
+                  bool read_only, bool allow_nonatomic,
                   bool fire_triggers, uint64 tcount,
                   DestReceiver *caller_dest, ResourceOwner plan_owner)
 {
@@ -2318,11 +2317,12 @@ _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
      * In the first two cases, we can just push the snap onto the stack once
      * for the whole plan list.
      *
-     * But if no_snapshots is true, then don't manage snapshots at all here.
-     * The caller must then take care of that.
+     * Note that snapshot != InvalidSnapshot implies an atomic execution
+     * context.
      */
-    if (snapshot != InvalidSnapshot && !no_snapshots)
+    if (snapshot != InvalidSnapshot)
     {
+        Assert(!allow_nonatomic);
         if (read_only)
         {
             PushActiveSnapshot(snapshot);
@@ -2408,15 +2408,39 @@ _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
         stmt_list = cplan->stmt_list;

         /*
-         * In the default non-read-only case, get a new snapshot, replacing
-         * any that we pushed in a previous cycle.
+         * If we weren't given a specific snapshot to use, and the statement
+         * list requires a snapshot, set that up.
          */
-        if (snapshot == InvalidSnapshot && !read_only && !no_snapshots)
+        if (snapshot == InvalidSnapshot &&
+            (list_length(stmt_list) > 1 ||
+             (list_length(stmt_list) == 1 &&
+              PlannedStmtRequiresSnapshot(linitial_node(PlannedStmt,
+                                                        stmt_list)))))
         {
-            if (pushed_active_snap)
-                PopActiveSnapshot();
-            PushActiveSnapshot(GetTransactionSnapshot());
-            pushed_active_snap = true;
+            /*
+             * First, ensure there's a Portal-level snapshot.  This back-fills
+             * the snapshot stack in case the previous operation was a COMMIT
+             * or ROLLBACK inside a procedure or DO block.  (We can't put back
+             * the Portal snapshot any sooner, or we'd break cases like doing
+             * SET or LOCK just after COMMIT.)  It's enough to check once per
+             * statement list, since COMMIT/ROLLBACK/CALL/DO can't appear
+             * within a multi-statement list.
+             */
+            EnsurePortalSnapshotExists();
+
+            /*
+             * In the default non-read-only case, get a new per-statement-list
+             * snapshot, replacing any that we pushed in a previous cycle.
+             * Skip it when doing non-atomic execution, though (we rely
+             * entirely on the Portal snapshot in that case).
+             */
+            if (!read_only && !allow_nonatomic)
+            {
+                if (pushed_active_snap)
+                    PopActiveSnapshot();
+                PushActiveSnapshot(GetTransactionSnapshot());
+                pushed_active_snap = true;
+            }
         }

         foreach(lc2, stmt_list)
@@ -2434,6 +2458,7 @@ _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
             _SPI_current->processed = 0;
             _SPI_current->tuptable = NULL;

+            /* Check for unsupported cases. */
             if (stmt->utilityStmt)
             {
                 if (IsA(stmt->utilityStmt, CopyStmt))
@@ -2462,9 +2487,10 @@ _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,

             /*
              * If not read-only mode, advance the command counter before each
-             * command and update the snapshot.
+             * command and update the snapshot.  (But skip it if the snapshot
+             * isn't under our control.)
              */
-            if (!read_only && !no_snapshots)
+            if (!read_only && pushed_active_snap)
             {
                 CommandCounterIncrement();
                 UpdateActiveSnapshotCommandId();
@@ -2507,13 +2533,11 @@ _SPI_execute_plan(SPIPlanPtr plan, ParamListInfo paramLI,
                 QueryCompletion qc;

                 /*
-                 * If the SPI context is atomic, or we are asked to manage
-                 * snapshots, then we are in an atomic execution context.
-                 * Conversely, to propagate a nonatomic execution context, the
-                 * caller must be in a nonatomic SPI context and manage
-                 * snapshots itself.
+                 * If the SPI context is atomic, or we were not told to allow
+                 * nonatomic operations, tell ProcessUtility this is an atomic
+                 * execution context.
                  */
-                if (_SPI_current->atomic || !no_snapshots)
+                if (_SPI_current->atomic || !allow_nonatomic)
                     context = PROCESS_UTILITY_QUERY;
                 else
                     context = PROCESS_UTILITY_QUERY_NONATOMIC;
diff --git a/src/backend/replication/logical/worker.c b/src/backend/replication/logical/worker.c
index 1432554d5a..d92a512839 100644
--- a/src/backend/replication/logical/worker.c
+++ b/src/backend/replication/logical/worker.c
@@ -349,6 +349,13 @@ create_estate_for_relation(LogicalRepRelMapEntry *rel,
     EState       *estate;
     RangeTblEntry *rte;

+    /*
+     * Input functions may need an active snapshot, as may AFTER triggers
+     * invoked during finish_estate.  For safety, ensure an active snapshot
+     * exists throughout all our usage of the executor.
+     */
+    PushActiveSnapshot(GetTransactionSnapshot());
+
     estate = CreateExecutorState();

     rte = makeNode(RangeTblEntry);
@@ -400,6 +407,7 @@ finish_estate(EState *estate)
     /* Cleanup. */
     ExecResetTupleTable(estate->es_tupleTable, false);
     FreeExecutorState(estate);
+    PopActiveSnapshot();
 }

 /*
@@ -1212,9 +1220,6 @@ apply_handle_insert(StringInfo s)
                                         RelationGetDescr(rel->localrel),
                                         &TTSOpsVirtual);

-    /* Input functions may need an active snapshot, so get one */
-    PushActiveSnapshot(GetTransactionSnapshot());
-
     /* Process and store remote tuple in the slot */
     oldctx = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));
     slot_store_data(remoteslot, rel, &newtup);
@@ -1229,8 +1234,6 @@ apply_handle_insert(StringInfo s)
         apply_handle_insert_internal(resultRelInfo, estate,
                                      remoteslot);

-    PopActiveSnapshot();
-
     finish_estate(estate);

     logicalrep_rel_close(rel, NoLock);
@@ -1358,8 +1361,6 @@ apply_handle_update(StringInfo s)
     /* Also populate extraUpdatedCols, in case we have generated columns */
     fill_extraUpdatedCols(target_rte, rel->localrel);

-    PushActiveSnapshot(GetTransactionSnapshot());
-
     /* Build the search tuple. */
     oldctx = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));
     slot_store_data(remoteslot, rel,
@@ -1374,8 +1375,6 @@ apply_handle_update(StringInfo s)
         apply_handle_update_internal(resultRelInfo, estate,
                                      remoteslot, &newtup, rel);

-    PopActiveSnapshot();
-
     finish_estate(estate);

     logicalrep_rel_close(rel, NoLock);
@@ -1482,8 +1481,6 @@ apply_handle_delete(StringInfo s)
                                         RelationGetDescr(rel->localrel),
                                         &TTSOpsVirtual);

-    PushActiveSnapshot(GetTransactionSnapshot());
-
     /* Build the search tuple. */
     oldctx = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));
     slot_store_data(remoteslot, rel, &oldtup);
@@ -1497,8 +1494,6 @@ apply_handle_delete(StringInfo s)
         apply_handle_delete_internal(resultRelInfo, estate,
                                      remoteslot, &rel->remoterel);

-    PopActiveSnapshot();
-
     finish_estate(estate);

     logicalrep_rel_close(rel, NoLock);
diff --git a/src/backend/tcop/pquery.c b/src/backend/tcop/pquery.c
index 44f5fe8fc9..f7f08e6c05 100644
--- a/src/backend/tcop/pquery.c
+++ b/src/backend/tcop/pquery.c
@@ -476,6 +476,13 @@ PortalStart(Portal portal, ParamListInfo params,
                 else
                     PushActiveSnapshot(GetTransactionSnapshot());

+                /*
+                 * We could remember the snapshot in portal->portalSnapshot,
+                 * but presently there seems no need to, as this code path
+                 * cannot be used for non-atomic execution.  Hence there can't
+                 * be any commit/abort that might destroy the snapshot.
+                 */
+
                 /*
                  * Create QueryDesc in portal's context; for the moment, set
                  * the destination to DestNone.
@@ -1116,45 +1123,26 @@ PortalRunUtility(Portal portal, PlannedStmt *pstmt,
                  bool isTopLevel, bool setHoldSnapshot,
                  DestReceiver *dest, QueryCompletion *qc)
 {
-    Node       *utilityStmt = pstmt->utilityStmt;
-    Snapshot    snapshot;
-
     /*
-     * Set snapshot if utility stmt needs one.  Most reliable way to do this
-     * seems to be to enumerate those that do not need one; this is a short
-     * list.  Transaction control, LOCK, and SET must *not* set a snapshot
-     * since they need to be executable at the start of a transaction-snapshot
-     * mode transaction without freezing a snapshot.  By extension we allow
-     * SHOW not to set a snapshot.  The other stmts listed are just efficiency
-     * hacks.  Beware of listing anything that can modify the database --- if,
-     * say, it has to update an index with expressions that invoke
-     * user-defined functions, then it had better have a snapshot.
+     * Set snapshot if utility stmt needs one.
      */
-    if (!(IsA(utilityStmt, TransactionStmt) ||
-          IsA(utilityStmt, LockStmt) ||
-          IsA(utilityStmt, VariableSetStmt) ||
-          IsA(utilityStmt, VariableShowStmt) ||
-          IsA(utilityStmt, ConstraintsSetStmt) ||
-    /* efficiency hacks from here down */
-          IsA(utilityStmt, FetchStmt) ||
-          IsA(utilityStmt, ListenStmt) ||
-          IsA(utilityStmt, NotifyStmt) ||
-          IsA(utilityStmt, UnlistenStmt) ||
-          IsA(utilityStmt, CheckPointStmt)))
+    if (PlannedStmtRequiresSnapshot(pstmt))
     {
-        snapshot = GetTransactionSnapshot();
+        Snapshot    snapshot = GetTransactionSnapshot();
+
         /* If told to, register the snapshot we're using and save in portal */
         if (setHoldSnapshot)
         {
             snapshot = RegisterSnapshot(snapshot);
             portal->holdSnapshot = snapshot;
         }
+        /* In any case, make the snapshot active and remember it in portal */
         PushActiveSnapshot(snapshot);
         /* PushActiveSnapshot might have copied the snapshot */
-        snapshot = GetActiveSnapshot();
+        portal->portalSnapshot = GetActiveSnapshot();
     }
     else
-        snapshot = NULL;
+        portal->portalSnapshot = NULL;

     ProcessUtility(pstmt,
                    portal->sourceText,
@@ -1168,13 +1156,17 @@ PortalRunUtility(Portal portal, PlannedStmt *pstmt,
     MemoryContextSwitchTo(portal->portalContext);

     /*
-     * Some utility commands may pop the ActiveSnapshot stack from under us,
-     * so be careful to only pop the stack if our snapshot is still at the
-     * top.
+     * Some utility commands (e.g., VACUUM) pop the ActiveSnapshot stack from
+     * under us, so don't complain if it's now empty.  Otherwise, our snapshot
+     * should be the top one; pop it.  Note that this could be a different
+     * snapshot from the one we made above; see EnsurePortalSnapshotExists.
      */
-    if (snapshot != NULL && ActiveSnapshotSet() &&
-        snapshot == GetActiveSnapshot())
+    if (portal->portalSnapshot != NULL && ActiveSnapshotSet())
+    {
+        Assert(portal->portalSnapshot == GetActiveSnapshot());
         PopActiveSnapshot();
+    }
+    portal->portalSnapshot = NULL;
 }

 /*
@@ -1256,6 +1248,12 @@ PortalRunMulti(Portal portal,
                  * from what holdSnapshot has.)
                  */
                 PushCopiedSnapshot(snapshot);
+
+                /*
+                 * As for PORTAL_ONE_SELECT portals, it does not seem
+                 * necessary to maintain portal->portalSnapshot here.
+                 */
+
                 active_snapshot_set = true;
             }
             else
@@ -1692,3 +1690,78 @@ DoPortalRewind(Portal portal)
     portal->atEnd = false;
     portal->portalPos = 0;
 }
+
+/*
+ * PlannedStmtRequiresSnapshot - what it says on the tin
+ */
+bool
+PlannedStmtRequiresSnapshot(PlannedStmt *pstmt)
+{
+    Node       *utilityStmt = pstmt->utilityStmt;
+
+    /* If it's not a utility statement, it definitely needs a snapshot */
+    if (utilityStmt == NULL)
+        return true;
+
+    /*
+     * Most utility statements need a snapshot, and the default presumption
+     * about new ones should be that they do too.  Hence, enumerate those that
+     * do not need one.
+     *
+     * Transaction control, LOCK, and SET must *not* set a snapshot, since
+     * they need to be executable at the start of a transaction-snapshot-mode
+     * transaction without freezing a snapshot.  By extension we allow SHOW
+     * not to set a snapshot.  The other stmts listed are just efficiency
+     * hacks.  Beware of listing anything that can modify the database --- if,
+     * say, it has to update an index with expressions that invoke
+     * user-defined functions, then it had better have a snapshot.
+     */
+    if (IsA(utilityStmt, TransactionStmt) ||
+        IsA(utilityStmt, LockStmt) ||
+        IsA(utilityStmt, VariableSetStmt) ||
+        IsA(utilityStmt, VariableShowStmt) ||
+        IsA(utilityStmt, ConstraintsSetStmt) ||
+    /* efficiency hacks from here down */
+        IsA(utilityStmt, FetchStmt) ||
+        IsA(utilityStmt, ListenStmt) ||
+        IsA(utilityStmt, NotifyStmt) ||
+        IsA(utilityStmt, UnlistenStmt) ||
+        IsA(utilityStmt, CheckPointStmt))
+        return false;
+
+    return true;
+}
+
+/*
+ * EnsurePortalSnapshotExists - recreate Portal-level snapshot, if needed
+ *
+ * Generally, we will have an active snapshot whenever we are executing
+ * inside a Portal, unless the Portal's query is one of the utility
+ * statements exempted from that rule (see PlannedStmtRequiresSnapshot).
+ * However, procedures and DO blocks can commit or abort the transaction,
+ * and thereby destroy all snapshots.  This function can be called to
+ * re-establish the Portal-level snapshot when none exists.
+ */
+void
+EnsurePortalSnapshotExists(void)
+{
+    Portal        portal;
+
+    /*
+     * Nothing to do if a snapshot is set.  (We take it on faith that the
+     * outermost active snapshot belongs to some Portal; or if there is no
+     * Portal, it's somebody else's responsibility to manage things.)
+     */
+    if (ActiveSnapshotSet())
+        return;
+
+    /* Otherwise, we'd better have an active Portal */
+    portal = ActivePortal;
+    Assert(portal != NULL);
+    Assert(portal->portalSnapshot == NULL);
+
+    /* Create a new snapshot and make it active */
+    PushActiveSnapshot(GetTransactionSnapshot());
+    /* PushActiveSnapshot might have copied the snapshot */
+    portal->portalSnapshot = GetActiveSnapshot();
+}
diff --git a/src/backend/utils/mmgr/portalmem.c b/src/backend/utils/mmgr/portalmem.c
index 66e3181815..5c30e141f5 100644
--- a/src/backend/utils/mmgr/portalmem.c
+++ b/src/backend/utils/mmgr/portalmem.c
@@ -502,6 +502,9 @@ PortalDrop(Portal portal, bool isTopCommit)
         portal->cleanup = NULL;
     }

+    /* There shouldn't be an active snapshot anymore, except after error */
+    Assert(portal->portalSnapshot == NULL || !isTopCommit);
+
     /*
      * Remove portal from hash table.  Because we do this here, we will not
      * come back to try to remove the portal again if there's any error in the
@@ -709,6 +712,8 @@ PreCommit_Portals(bool isPrepare)
                 portal->holdSnapshot = NULL;
             }
             portal->resowner = NULL;
+            /* Clear portalSnapshot too, for cleanliness */
+            portal->portalSnapshot = NULL;
             continue;
         }

@@ -1278,3 +1283,54 @@ HoldPinnedPortals(void)
         }
     }
 }
+
+/*
+ * Drop the outer active snapshots for all portals, so that no snapshots
+ * remain active.
+ *
+ * Like HoldPinnedPortals, this must be called when initiating a COMMIT or
+ * ROLLBACK inside a procedure.  This has to be separate from that since it
+ * should not be run until we're done with steps that are likely to fail.
+ *
+ * It's tempting to fold this into PreCommit_Portals, but to do so, we'd
+ * need to clean up snapshot management in VACUUM and perhaps other places.
+ */
+void
+ForgetPortalSnapshots(void)
+{
+    HASH_SEQ_STATUS status;
+    PortalHashEnt *hentry;
+    int            numPortalSnaps = 0;
+    int            numActiveSnaps = 0;
+
+    /* First, scan PortalHashTable and clear portalSnapshot fields */
+    hash_seq_init(&status, PortalHashTable);
+
+    while ((hentry = (PortalHashEnt *) hash_seq_search(&status)) != NULL)
+    {
+        Portal        portal = hentry->portal;
+
+        if (portal->portalSnapshot != NULL)
+        {
+            portal->portalSnapshot = NULL;
+            numPortalSnaps++;
+        }
+        /* portal->holdSnapshot will be cleaned up in PreCommit_Portals */
+    }
+
+    /*
+     * Now, pop all the active snapshots, which should be just those that were
+     * portal snapshots.  Ideally we'd drive this directly off the portal
+     * scan, but there's no good way to visit the portals in the correct
+     * order.  So just cross-check after the fact.
+     */
+    while (ActiveSnapshotSet())
+    {
+        PopActiveSnapshot();
+        numActiveSnaps++;
+    }
+
+    if (numPortalSnaps != numActiveSnaps)
+        elog(ERROR, "portal snapshots (%d) did not account for all active snapshots (%d)",
+             numPortalSnaps, numActiveSnaps);
+}
diff --git a/src/include/tcop/pquery.h b/src/include/tcop/pquery.h
index ed2c4d374b..2318f04ff0 100644
--- a/src/include/tcop/pquery.h
+++ b/src/include/tcop/pquery.h
@@ -17,6 +17,8 @@
 #include "nodes/parsenodes.h"
 #include "utils/portal.h"

+struct PlannedStmt;                /* avoid including plannodes.h here */
+

 extern PGDLLIMPORT Portal ActivePortal;

@@ -42,4 +44,8 @@ extern uint64 PortalRunFetch(Portal portal,
                              long count,
                              DestReceiver *dest);

+extern bool PlannedStmtRequiresSnapshot(struct PlannedStmt *pstmt);
+
+extern void EnsurePortalSnapshotExists(void);
+
 #endif                            /* PQUERY_H */
diff --git a/src/include/utils/portal.h b/src/include/utils/portal.h
index 3c17b039cc..2e5bbdd0ec 100644
--- a/src/include/utils/portal.h
+++ b/src/include/utils/portal.h
@@ -160,6 +160,14 @@ typedef struct PortalData
     /* and these are the format codes to use for the columns: */
     int16       *formats;        /* a format code for each column */

+    /*
+     * Outermost ActiveSnapshot for execution of the portal's queries.  For
+     * all but a few utility commands, we require such a snapshot to exist.
+     * This ensures that TOAST references in query results can be detoasted,
+     * and helps to reduce thrashing of the process's exposed xmin.
+     */
+    Snapshot    portalSnapshot; /* active snapshot, or NULL if none */
+
     /*
      * Where we store tuples for a held cursor or a PORTAL_ONE_RETURNING or
      * PORTAL_UTIL_SELECT query.  (A cursor held past the end of its
@@ -237,5 +245,6 @@ extern void PortalCreateHoldStore(Portal portal);
 extern void PortalHashTableDeleteAll(void);
 extern bool ThereAreNoReadyPortals(void);
 extern void HoldPinnedPortals(void);
+extern void ForgetPortalSnapshots(void);

 #endif                            /* PORTAL_H */
diff --git a/src/pl/plpgsql/src/pl_exec.c b/src/pl/plpgsql/src/pl_exec.c
index b4c70aaa7f..5b0f698ac7 100644
--- a/src/pl/plpgsql/src/pl_exec.c
+++ b/src/pl/plpgsql/src/pl_exec.c
@@ -2159,7 +2159,6 @@ exec_stmt_call(PLpgSQL_execstate *estate, PLpgSQL_stmt_call *stmt)
     PLpgSQL_expr *expr = stmt->expr;
     LocalTransactionId before_lxid;
     LocalTransactionId after_lxid;
-    bool        pushed_active_snap = false;
     ParamListInfo paramLI;
     SPIExecuteOptions options;
     int            rc;
@@ -2189,22 +2188,13 @@ exec_stmt_call(PLpgSQL_execstate *estate, PLpgSQL_stmt_call *stmt)

     before_lxid = MyProc->lxid;

-    /*
-     * The procedure call could end transactions, which would upset the
-     * snapshot management in SPI_execute*, so handle snapshots here instead.
-     * Set a snapshot only for non-read-only procedures, similar to SPI
-     * behavior.
-     */
-    if (!estate->readonly_func)
-    {
-        PushActiveSnapshot(GetTransactionSnapshot());
-        pushed_active_snap = true;
-    }
-
     /*
      * If we have a procedure-lifespan resowner, use that to hold the refcount
      * for the plan.  This avoids refcount leakage complaints if the called
      * procedure ends the current transaction.
+     *
+     * Also, disable SPI's snapshot management; this is a hacky way of telling
+     * it to allow non-atomicness to propagate to the callee.
      */
     memset(&options, 0, sizeof(options));
     options.params = paramLI;
@@ -2220,17 +2210,7 @@ exec_stmt_call(PLpgSQL_execstate *estate, PLpgSQL_stmt_call *stmt)

     after_lxid = MyProc->lxid;

-    if (before_lxid == after_lxid)
-    {
-        /*
-         * If we are still in the same transaction after the call, pop the
-         * snapshot that we might have pushed.  (If it's a new transaction,
-         * then all the snapshots are gone already.)
-         */
-        if (pushed_active_snap)
-            PopActiveSnapshot();
-    }
-    else
+    if (before_lxid != after_lxid)
     {
         /*
          * If we are in a new transaction after the call, we need to build new
@@ -4954,6 +4934,7 @@ exec_stmt_rollback(PLpgSQL_execstate *estate, PLpgSQL_stmt_rollback *stmt)
  *
  * We just parse and execute the statement normally, but we have to do it
  * without setting a snapshot, for things like SET TRANSACTION.
+ * XXX spi.c now handles this correctly, so we no longer need a special case.
  */
 static int
 exec_stmt_set(PLpgSQL_execstate *estate, PLpgSQL_stmt_set *stmt)
@@ -4967,7 +4948,6 @@ exec_stmt_set(PLpgSQL_execstate *estate, PLpgSQL_stmt_set *stmt)

     memset(&options, 0, sizeof(options));
     options.read_only = estate->readonly_func;
-    options.no_snapshots = true;    /* disable SPI's snapshot management */

     rc = SPI_execute_plan_extended(expr->plan, &options);

diff --git a/src/test/isolation/expected/plpgsql-toast.out b/src/test/isolation/expected/plpgsql-toast.out
index fc557da5e7..61a33c113c 100644
--- a/src/test/isolation/expected/plpgsql-toast.out
+++ b/src/test/isolation/expected/plpgsql-toast.out
@@ -192,3 +192,30 @@ pg_advisory_unlock
 t
 s1: NOTICE:  length(r) = 6002
 step assign5: <... completed>
+
+starting permutation: fetch-after-commit
+pg_advisory_unlock_all
+
+
+pg_advisory_unlock_all
+
+
+s1: NOTICE:  length(t) = 6000
+s1: NOTICE:  length(t) = 9000
+s1: NOTICE:  length(t) = 12000
+step fetch-after-commit:
+do $$
+  declare
+    r record;
+    t text;
+  begin
+    insert into test1 values (2, repeat('bar', 3000));
+    insert into test1 values (3, repeat('baz', 4000));
+    for r in select test1.a from test1 loop
+      commit;
+      select b into t from test1 where a = r.a;
+      raise notice 'length(t) = %', length(t);
+    end loop;
+  end;
+$$;
+
diff --git a/src/test/isolation/specs/plpgsql-toast.spec b/src/test/isolation/specs/plpgsql-toast.spec
index fe7090addb..8806e3e6dd 100644
--- a/src/test/isolation/specs/plpgsql-toast.spec
+++ b/src/test/isolation/specs/plpgsql-toast.spec
@@ -112,6 +112,26 @@ do $$
 $$;
 }

+# Check that the results of a query can be detoasted just after committing
+# (there's no interaction with VACUUM here)
+step "fetch-after-commit"
+{
+do $$
+  declare
+    r record;
+    t text;
+  begin
+    insert into test1 values (2, repeat('bar', 3000));
+    insert into test1 values (3, repeat('baz', 4000));
+    for r in select test1.a from test1 loop
+      commit;
+      select b into t from test1 where a = r.a;
+      raise notice 'length(t) = %', length(t);
+    end loop;
+  end;
+$$;
+}
+
 session "s2"
 setup
 {
@@ -135,3 +155,4 @@ permutation "lock" "assign2" "vacuum" "unlock"
 permutation "lock" "assign3" "vacuum" "unlock"
 permutation "lock" "assign4" "vacuum" "unlock"
 permutation "lock" "assign5" "vacuum" "unlock"
+permutation "fetch-after-commit"