MB-35297: Fix "RangeScan::setStateIdle invalid state:State::Cancelled"

The continue "self-cancelling" code is relying on the cancel being
detected in a key/value callback. If the scan has no data, no callback
and the code attempts to move from cancel to idle (invalid).

Add a check for cancel before calling into KVStore::scan

Change-Id: I4ecfd42cd57770883b18b63035b2a5dc500e5696
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/174585
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@couchbase.com>

engines/ep/src/range_scans/range_scan.cc

@@ -74,6 +74,15 @@ cb::rangescan::Id RangeScan::createScan(const CookieIface& cookie,
 
 cb::engine_errc RangeScan::continueScan(KVStoreIface& kvstore) {
     EP_LOG_DEBUG("RangeScan {} continue for {}", uuid, getVBucketId());
+
+    // Only attempt scan when !cancelled
+    if (isCancelled()) {
+        // ensure the client/cookie sees cancelled
+        handleStatus(cb::engine_errc::range_scan_cancelled);
+        // but return success so the scan now gets cleaned-up
+        return cb::engine_errc::success;
+    }
+
     auto status = kvstore.scan(*scanCtx);
 
     switch (status) {

engines/ep/tests/module_tests/range_scan_test.cc

@@ -425,6 +425,20 @@ TEST_P(RangeScanTest, create_cancel) {
     EXPECT_TRUE(handler->scannedItems.empty());
 }
 
+TEST_P(RangeScanTest, create_cancel_no_data) {
+    // this scan will generate no callbacks internally, but must still safely
+    // cancel
+    auto uuid = createScan(scanCollection, {"0"}, {"0\xFF"});
+    auto vb = store->getVBucket(vbid);
+    EXPECT_EQ(cb::engine_errc::success, vb->cancelRangeScan(uuid, true));
+    runNextTask(*task_executor->getLpTaskQ()[AUXIO_TASK_IDX],
+                "RangeScanContinueTask");
+
+    // Nothing read
+    EXPECT_TRUE(handler->scannedKeys.empty());
+    EXPECT_TRUE(handler->scannedItems.empty());
+}
+
 // Check that if a scan has been continue (but is waiting to run), it can be
 // cancelled. When the task runs the scan cancels.
 TEST_P(RangeScanTest, create_continue_is_cancelled) {