amcheck: Report an error when the next page to a leaf is not a leaf

This is a very unlikely condition during checking a B-tree unique constraint,
meaning that the index structure is violated badly, and we shouldn't continue
checking to avoid endless loops, etc.  So it's worth immediately throwing an
error.

Reported-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-Wzk%2B2116uOXdOViA27SHcr31WKPgmjsxXLBs_aTxAeThzg%40mail.gmail.com
Author: Pavel Borisov

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 5354b393b11513dc82bb0827d3630d0e398568ba..4347f9594c9c95940da9604253d9a05a7b24cbce 100644 (file)
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -1831,7 +1831,6 @@ bt_target_page_check(BtreeCheckState *state)
                if (offset == max)
                {
                        BTScanInsert rightkey;
-                       BlockNumber rightblock_number;
 
                        /* first offset on a right index page (log only) */
                        OffsetNumber rightfirstoffset = InvalidOffsetNumber;
@@ -1876,10 +1875,11 @@ bt_target_page_check(BtreeCheckState *state)
                         * If index has unique constraint make sure that no more than one
                         * found equal items is visible.
                         */
-                       rightblock_number = topaque->btpo_next;
                        if (state->checkunique && state->indexinfo->ii_Unique &&
-                               rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+                               rightkey && P_ISLEAF(topaque) && !P_RIGHTMOST(topaque))
                        {
+                               BlockNumber rightblock_number = topaque->btpo_next;
+
                                elog(DEBUG2, "check cross page unique condition");
 
                                /*
@@ -1899,9 +1899,19 @@ bt_target_page_check(BtreeCheckState *state)
                                                                                                  rightblock_number);
                                        topaque = BTPageGetOpaque(rightpage);
 
-                                       if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
-                                               break;
-
+                                       if (P_IGNORE(topaque))
+                                       {
+                                               if (unlikely(!P_ISLEAF(topaque)))
+                                                       ereport(ERROR,
+                                                                       (errcode(ERRCODE_INDEX_CORRUPTED),
+                                                                        errmsg("right block of leaf block is non-leaf for index \"%s\"",
+                                                                                       RelationGetRelationName(state->rel)),
+                                                                        errdetail_internal("Block=%u page lsn=%X/%X.",
+                                                                                                               state->targetblock,
+                                                                                                               LSN_FORMAT_ARGS(state->targetlsn))));
+                                               else
+                                                       break;
+                                       }
                                        itemid = PageGetItemIdCareful(state, rightblock_number,
                                                                                                  rightpage,
                                                                                                  rightfirstoffset);