Fix race in SSI interaction with gin fast path.

The ginfast.c code previously checked for conflicts in before locking
the relevant buffer, leaving a window where a RW conflict could be
missed.  Re-order.

There was also a place where buffer ID and block number were confused
while trying to predicate-lock a page, noted by visual inspection.

Back-patch to all supported releases.  Fixes one more problem discovered
with the reproducer from bug #17949, in this case when Dmitry tried
other index types.

Reported-by: Artem Anisimov <artem.anisimov.255@gmail.com>
Reported-by: Dmitry Dolgov <9erthalion6@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/17949-a0f17035294a55e2%40postgresql.org

diff --git a/src/backend/access/gin/ginfast.c b/src/backend/access/gin/ginfast.c
index 4d98d61658ed0249558035aba7b3d7d064cebb54..6dcc2dabafa3db3d93ffc3603c707988e99e2e9b 100644 (file)
--- a/src/backend/access/gin/ginfast.c
+++ b/src/backend/access/gin/ginfast.c
@@ -244,9 +244,10 @@ ginHeapTupleFastInsert(GinState *ginstate, GinTupleCollector *collector)
    /*
     * An insertion to the pending list could logically belong anywhere in the
     * tree, so it conflicts with all serializable scans.  All scans acquire a
-    * predicate lock on the metabuffer to represent that.
+    * predicate lock on the metabuffer to represent that.  Therefore we'll
+    * check for conflicts in, but not until we have the page locked and are
+    * ready to modify the page.
     */
-   CheckForSerializableConflictIn(index, NULL, metabuffer);
 
    if (collector->sumsize + collector->ntuples * sizeof(ItemIdData) > GinListPageSize)
    {
@@ -290,6 +291,8 @@ ginHeapTupleFastInsert(GinState *ginstate, GinTupleCollector *collector)
        LockBuffer(metabuffer, GIN_EXCLUSIVE);
        metadata = GinPageGetMeta(metapage);
 
+       CheckForSerializableConflictIn(index, NULL, metabuffer);
+
        if (metadata->head == InvalidBlockNumber)
        {
            /*
@@ -352,6 +355,8 @@ ginHeapTupleFastInsert(GinState *ginstate, GinTupleCollector *collector)
        char       *ptr;
        char       *collectordata;
 
+       CheckForSerializableConflictIn(index, NULL, metabuffer);
+
        buffer = ReadBuffer(index, metadata->tail);
        LockBuffer(buffer, GIN_EXCLUSIVE);
        page = BufferGetPage(buffer);

diff --git a/src/backend/access/gin/ginget.c b/src/backend/access/gin/ginget.c
index edfa7fe85263287afba5ad6f4e38cb5997faf3e3..f03fa57d9580920e58cda390af5bfaaa09b7c6c8 100644 (file)
--- a/src/backend/access/gin/ginget.c
+++ b/src/backend/access/gin/ginget.c
@@ -139,7 +139,9 @@ collectMatchBitmap(GinBtreeData *btree, GinBtreeStack *stack,
     * Predicate lock entry leaf page, following pages will be locked by
     * moveRightIfItNeeded()
     */
-   PredicateLockPage(btree->index, stack->buffer, snapshot);
+   PredicateLockPage(btree->index,
+                     BufferGetBlockNumber(stack->buffer),
+                     snapshot);
 
    for (;;)
    {