cmd/vet: lostcancel: suppress the check in the main.main function

When main.main returns, the process exits, so there's no need to cancel contexts.

This CL was originally reviewed as
  https://go-review.googlesource.com/c/go/+/106915
and then retried in
  https://go-review.googlesource.com/c/go/+/148758
but then reverted due to an embarrassing sequence
of careless moves.

Change-Id: Icdee0650996a442023e030697f10d2c31fd5fdff
Reviewed-on: https://go-review.googlesource.com/c/148877
Run-TryBot: Alan Donovan <adonovan@google.com>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

go/analysis/passes/lostcancel/lostcancel.go

@@ -132,11 +132,17 @@ func runFunc(pass *analysis.Pass, node ast.Node) {
 	var sig *types.Signature
 	switch node := node.(type) {
 	case *ast.FuncDecl:
-		g = cfgs.FuncDecl(node)
 		sig, _ = pass.TypesInfo.Defs[node.Name].Type().(*types.Signature)
+		if node.Name.Name == "main" && sig.Recv() == nil && pass.Pkg.Name() == "main" {
+			// Returning from main.main terminates the process,
+			// so there's no need to cancel contexts.
+			return
+		}
+		g = cfgs.FuncDecl(node)
+
 	case *ast.FuncLit:
-		g = cfgs.FuncLit(node)
 		sig, _ = pass.TypesInfo.Types[node.Type].Type.(*types.Signature)
+		g = cfgs.FuncLit(node)
 	}
 	if sig == nil {
 		return // missing type information

go/analysis/passes/lostcancel/lostcancel_test.go

@@ -13,5 +13,5 @@ import (
 
 func Test(t *testing.T) {
 	testdata := analysistest.TestData()
-	analysistest.Run(t, testdata, lostcancel.Analyzer, "a")
+	analysistest.Run(t, testdata, lostcancel.Analyzer, "a", "b")
 }

go/analysis/passes/lostcancel/testdata/src/b/b.go

@@ -0,0 +1,26 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "context"
+
+// Return from main is handled specially.
+// Since the program exits, there's no need to call cancel.
+func main() {
+	_, cancel := context.WithCancel(nil)
+	if maybe {
+		cancel()
+	}
+}
+
+func notMain() {
+	_, cancel := context.WithCancel(nil) // want "cancel function.*not used"
+
+	if maybe {
+		cancel()
+	}
+} // want "return statement.*reached without using the cancel"
+
+var maybe bool