Democracy.fast_track not allowed with zero voting period (#11666)

* Democracy.fast_track not allowed with zero voting period

* revert static parameter alter line

* ensure voting period greater zero

* update doc for fast_track

* unit test: instant fast track to the next block referendum is backed

* fix typos in comments

substrate/frame/democracy/src/tests/fast_tracking.rs

@@ -67,6 +67,10 @@ fn instant_referendum_works() {
 			Error::<Test>::InstantNotAllowed
 		);
 		INSTANT_ALLOWED.with(|v| *v.borrow_mut() = true);
+		assert_noop!(
+			Democracy::fast_track(Origin::signed(6), h, 0, 0),
+			Error::<Test>::VotingPeriodLow
+		);
 		assert_ok!(Democracy::fast_track(Origin::signed(6), h, 1, 0));
 		assert_eq!(
 			Democracy::referendum_status(0),
@@ -81,6 +85,60 @@ fn instant_referendum_works() {
 	});
 }
 
+#[test]
+fn instant_next_block_referendum_backed() {
+	new_test_ext().execute_with(|| {
+		// arrange
+		let start_block_number = 10;
+		let majority_origin_id = 3;
+		let instant_origin_id = 6;
+		let voting_period = 1;
+		let proposal_hash = set_balance_proposal_hash_and_note(2);
+		let delay = 2; // has no effect on test
+
+		// init
+		System::set_block_number(start_block_number);
+		InstantAllowed::set(true);
+
+		// propose with majority origin
+		assert_ok!(Democracy::external_propose_majority(
+			Origin::signed(majority_origin_id),
+			proposal_hash
+		));
+
+		// fast track with instant origin and voting period pointing to the next block
+		assert_ok!(Democracy::fast_track(
+			Origin::signed(instant_origin_id),
+			proposal_hash,
+			voting_period,
+			delay
+		));
+
+		// fetch the status of the only referendum at index 0
+		assert_eq!(
+			Democracy::referendum_status(0),
+			Ok(ReferendumStatus {
+				end: start_block_number + voting_period,
+				proposal_hash,
+				threshold: VoteThreshold::SimpleMajority,
+				delay,
+				tally: Tally { ayes: 0, nays: 0, turnout: 0 },
+			})
+		);
+
+		// referendum expected to be baked with the start of the next block
+		next_block();
+
+		// assert no active referendums
+		assert_noop!(Democracy::referendum_status(0), Error::<Test>::ReferendumInvalid);
+		// the only referendum in the storage is finished and not approved
+		assert_eq!(
+			ReferendumInfoOf::<Test>::get(0).unwrap(),
+			ReferendumInfo::Finished { approved: false, end: start_block_number + voting_period }
+		);
+	});
+}
+
 #[test]
 fn fast_track_referendum_fails_when_no_simple_majority() {
 	new_test_ext().execute_with(|| {