Fix btmarkpos/btrestrpos array key wraparound bug.

nbtree's mark/restore processing failed to correctly handle an edge case
involving array key advancement and related search-type scan key state.
Scans with ScalarArrayScalarArrayOpExpr quals requiring mark/restore
processing (for a merge join) could incorrectly conclude that an
affected array/scan key must not have advanced during the time between
marking and restoring the scan's position.

As a result of all this, array key handling within btrestrpos could skip
a required call to _bt_preprocess_keys().  This confusion allowed later
primitive index scans to overlook tuples matching the true current array
keys.  The scan's search-type scan keys would still have spurious values
corresponding to the final array element(s) -- not values matching the
first/now-current array element(s).

To fix, remember that "array key wraparound" has taken place during the
ongoing btrescan in a flag variable stored in the scan's state, and use
that information at the point where btrestrpos decides if another call
to _bt_preprocess_keys is required.

Oversight in commit 70bc5833, which taught nbtree to handle array keys
during mark/restore processing, but missed this subtlety.  That commit
was itself a bug fix for an issue in commit 9e8da0f7, which taught
nbtree to handle ScalarArrayOpExpr quals natively.

Author: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-WzkgP3DDRJxw6DgjCxo-cu-DKrvjEv_ArkP2ctBJatDCYg@mail.gmail.com
Backpatch: 11- (all supported branches).

src/backend/access/nbtree/nbtree.c

@@ -360,6 +360,7 @@ btbeginscan(Relation rel, int nkeys, int norderbys)
 		so->keyData = NULL;
 
 	so->arrayKeyData = NULL;	/* assume no array keys for now */
+	so->arraysStarted = false;
 	so->numArrayKeys = 0;
 	so->arrayKeys = NULL;
 	so->arrayContext = NULL;

src/backend/access/nbtree/nbtutils.c

@@ -532,6 +532,8 @@ _bt_start_array_keys(IndexScanDesc scan, ScanDirection dir)
 			curArrayKey->cur_elem = 0;
 		skey->sk_argument = curArrayKey->elem_values[curArrayKey->cur_elem];
 	}
+
+	so->arraysStarted = true;
 }
 
 /*
@@ -591,6 +593,14 @@ _bt_advance_array_keys(IndexScanDesc scan, ScanDirection dir)
 	if (scan->parallel_scan != NULL)
 		_bt_parallel_advance_array_keys(scan);
 
+	/*
+	 * When no new array keys were found, the scan is "past the end" of the
+	 * array keys.  _bt_start_array_keys can still "restart" the array keys if
+	 * a rescan is required.
+	 */
+	if (!found)
+		so->arraysStarted = false;
+
 	return found;
 }
 
@@ -644,8 +654,13 @@ _bt_restore_array_keys(IndexScanDesc scan)
 	 * If we changed any keys, we must redo _bt_preprocess_keys.  That might
 	 * sound like overkill, but in cases with multiple keys per index column
 	 * it seems necessary to do the full set of pushups.
+	 *
+	 * Also do this whenever the scan's set of array keys "wrapped around" at
+	 * the end of the last primitive index scan.  There won't have been a call
+	 * to _bt_preprocess_keys from some other place following wrap around, so
+	 * we do it for ourselves.
 	 */
-	if (changed)
+	if (changed || !so->arraysStarted)
 	{
 		_bt_preprocess_keys(scan);
 		/* The mark should have been set on a consistent set of keys... */

src/include/access/nbtree.h

@@ -1029,8 +1029,10 @@ typedef struct BTArrayKeyInfo
 
 typedef struct BTScanOpaqueData
 {
-	/* these fields are set by _bt_preprocess_keys(): */
+	/* all fields (except arraysStarted) are set by _bt_preprocess_keys(): */
 	bool		qual_ok;		/* false if qual can never be satisfied */
+	bool		arraysStarted;	/* Started array keys, but have yet to "reach
+								 * past the end" of all arrays? */
 	int			numberOfKeys;	/* number of preprocessed scan keys */
 	ScanKey		keyData;		/* array of preprocessed scan keys */