storage: CheckSSTConflicts: fix instance of iterator mismatch

Previously, in one case, we'd let the sst iterator advance
ahead of the engine iterator, which violates an invariant
in this function. This change fixes that case.

Fixes #99566.
Fixes #99010.

Epic: none

Release note: None

pkg/kv/kvserver/batcheval/cmd_add_sstable_test.go

@@ -1083,6 +1083,12 @@ func TestEvalAddSSTable(t *testing.T) {
 			sst:          kvs{pointKV("oe", 11, "foo"), pointKV("oih", 12, "foo"), rangeKV("ods", "ogvh", 10, ""), rangeKV("ogvh", "ohl", 10, ""), rangeKV("ogvh", "ohl", 9, "")},
 			ignoreExpect: true,
 		},
+		"DisallowConflict maintains ext iter ahead of sst iter": {
+			noConflict:   true,
+			data:         kvs{pointKV("c", 6, "foo"), rangeKV("c", "e", 5, "")},
+			sst:          kvs{rangeKV("a", "b", 10, ""), pointKV("d", 9, "foo")},
+			ignoreExpect: true,
+		},
 	}
 	testutils.RunTrueAndFalse(t, "IngestAsWrites", func(t *testing.T, ingestAsWrites bool) {
 		testutils.RunValues(t, "RewriteConcurrency", []interface{}{0, 8}, func(t *testing.T, c interface{}) {

pkg/storage/sst.go

@@ -1137,6 +1137,10 @@ func CheckSSTConflicts(
 					if sstChangedKeys && !extChangedKeys {
 						sstIter.SeekGE(MVCCKey{Key: extIter.UnsafeKey().Key})
 						sstOK, sstErr = sstIter.Valid()
+						if sstOK && extIter.UnsafeKey().Key.Compare(sstIter.UnsafeKey().Key) < 0 {
+							extIter.SeekGE(MVCCKey{Key: sstIter.UnsafeKey().Key})
+							extOK, extErr = extIter.Valid()
+						}
 					}
 					// Re-seek the ext iterator if the ext iterator changed keys and:
 					// 1) the SST iterator did not change keys, and we need to bring the ext