Fix calls for defer statements

Author: owen-mc

go/ql/lib/semmle/go/controlflow/ControlFlowGraphShared.qll

@@ -354,7 +354,15 @@ module GoCfg {
       )
     }
 
-    predicate preOrderExpr(Ast::Expr e) { none() }
+    predicate preOrderExpr(Ast::Expr e) {
+      // The call of a `defer` statement is not invoked at the statement
+      // itself; its callee and arguments are evaluated in place, but the call
+      // is only invoked later, at function exit (modelled by the `defer-invoke`
+      // node and `deferExitStep`). Marking it as pre-order means no in-order
+      // "invocation" node (and hence no inline exceptional-exit edge) is
+      // created at the `defer` statement.
+      e = any(Go::DeferStmt s).getCall()
+    }
 
     predicate propagatesValue(Ast::AstNode child, Ast::AstNode parent) {
       child = parent.(Go::ParenExpr).getExpr()

go/ql/lib/semmle/go/controlflow/IR.qll

@@ -218,6 +218,11 @@ module IR {
     EvalInstruction() {
       this.isIn(e)
       or
+      // The call of a `defer` statement is pre-order (it has no in-order
+      // "invocation" node at the statement), so its value is produced by the
+      // `defer-invoke` node that models the call at function exit.
+      this.isAdditional(e, "defer-invoke")
+      or
       // `NotExpr` and `LogicalBinaryExpr` are not in `postOrInOrder`, so they
       // don't have an `isIn` node. Only use the after-node when the
       // expression is not in a conditional context; otherwise the value is