Fix bug relating to allowlist

Author: owen-mc

go/ql/src/RedundantCode/UnreachableStatement.ql

@@ -93,8 +93,28 @@ predicate allowlist(Stmt s) {
   exists(getPreviousStmt(s).(IfStmt).getCondition().getBoolValue())
 }
 
+/** Holds if `s` is part of an allowlisted prefix of a run of unreachable statements. */
+predicate isInAllowlistedUnreachablePrefix(Stmt s) {
+  allowlist(s) and
+  (
+    firstUnreachableStmt(s)
+    or
+    isInAllowlistedUnreachablePrefix(getPreviousStmt(s))
+  )
+}
+
+/** Holds if `s` is the first non-allowlisted statement in a run of unreachable statements. */
+predicate firstNonAllowlistedUnreachableStmt(Stmt s) {
+  not isReachable(s) and
+  not s instanceof EmptyStmt and
+  not allowlist(s) and
+  (
+    firstUnreachableStmt(s)
+    or
+    isInAllowlistedUnreachablePrefix(getPreviousStmt(s))
+  )
+}
+
 from Stmt s
-where
-  firstUnreachableStmt(s) and
-  not allowlist(s)
+where firstNonAllowlistedUnreachableStmt(s)
 select s, "This statement is unreachable."

go/ql/test/query-tests/RedundantCode/UnreachableStatement/UnreachableStatement.expected

@@ -10,3 +10,4 @@
 | main.go:147:2:147:17 | return statement | This statement is unreachable. |
 | main.go:153:2:153:22 | return statement | This statement is unreachable. |
 | main.go:159:2:159:43 | return statement | This statement is unreachable. |
+| main.go:165:2:165:14 | expression statement | This statement is unreachable. |

go/ql/test/query-tests/RedundantCode/UnreachableStatement/main.go

@@ -162,7 +162,7 @@ func test19() mystruct {
 func test20() {
 	select {}
 	panic("unreachable") // OK: allowlisted statement
-	unreachable()        // $ MISSING: Alert
+	unreachable()        // $ Alert
 }
 
 func main() {}