cmd/go: move automatic testing.Init call into generated test code

In CL 173722, we moved the flag registration in the testing package into
an Init function. In order to avoid needing changes to user code, we
called Init automatically as part of testing.MainStart.

However, that isn't early enough if flag.Parse is called before the
tests run, as part of package initialization.

Fix this by injecting a bit of code to call testing.Init into test
packages. This runs before any other initialization code in the user's
test package, so testing.Init will be called before any user code can
call flag.Parse.

Fixes #31859

Change-Id: Ib42cd8d3819150c49a3cecf7eef2472319d0c7e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/176098
Run-TryBot: Caleb Spare <cespare@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

Author: cespare

src/cmd/go/go_test.go

@@ -3176,12 +3176,6 @@ func TestGoTestFooTestWorks(t *testing.T) {
 	tg.run("test", "testdata/standalone_test.go")
 }
 
-func TestGoTestTestMainSeesTestingFlags(t *testing.T) {
-	tg := testgo(t)
-	defer tg.cleanup()
-	tg.run("test", "testdata/standalone_testmain_flag_test.go")
-}
-
 // Issue 22388
 func TestGoTestMainWithWrongSignature(t *testing.T) {
 	tg := testgo(t)

src/cmd/go/internal/list/list.go

@@ -459,7 +459,7 @@ func runList(cmd *base.Command, args []string) {
 				}
 				if pmain != nil {
 					pkgs = append(pkgs, pmain)
-					data := *pmain.Internal.TestmainGo
+					data := pmain.Internal.TestmainGo
 					h := cache.NewHash("testmain")
 					h.Write([]byte("testmain\n"))
 					h.Write(data)

src/cmd/go/internal/load/pkg.go

@@ -177,7 +177,8 @@ type PackageInternal struct {
 	OmitDebug         bool                 // tell linker not to write debug information
 	GobinSubdir       bool                 // install target would be subdir of GOBIN
 	BuildInfo         string               // add this info to package main
-	TestmainGo        *[]byte              // content for _testmain.go
+	TestinginitGo     []byte               // content for _testinginit.go
+	TestmainGo        []byte               // content for _testmain.go
 
 	Asmflags   []string // -asmflags for this package
 	Gcflags    []string // -gcflags for this package

src/cmd/go/internal/load/test.go

@@ -102,6 +102,7 @@ func TestPackagesAndErrors(p *Package, cover *TestCover) (pmain, ptest, pxtest *
 	var stk ImportStack
 	stk.Push(p.ImportPath + " (test)")
 	rawTestImports := str.StringList(p.TestImports)
+	var ptestImportsTesting, pxtestImportsTesting bool
 	for i, path := range p.TestImports {
 		p1 := loadImport(pre, path, p.Dir, p, &stk, p.Internal.Build.TestImportPos[path], ResolveImport)
 		if str.Contains(p1.Deps, p.ImportPath) || p1.ImportPath == p.ImportPath {
@@ -116,6 +117,9 @@ func TestPackagesAndErrors(p *Package, cover *TestCover) (pmain, ptest, pxtest *
 		}
 		p.TestImports[i] = p1.ImportPath
 		imports = append(imports, p1)
+		if path == "testing" {
+			ptestImportsTesting = true
+		}
 	}
 	stk.Pop()
 	stk.Push(p.ImportPath + "_test")
@@ -129,6 +133,9 @@ func TestPackagesAndErrors(p *Package, cover *TestCover) (pmain, ptest, pxtest *
 			ximports = append(ximports, p1)
 		}
 		p.XTestImports[i] = p1.ImportPath
+		if path == "testing" {
+			pxtestImportsTesting = true
+		}
 	}
 	stk.Pop()
 
@@ -138,6 +145,9 @@ func TestPackagesAndErrors(p *Package, cover *TestCover) (pmain, ptest, pxtest *
 		*ptest = *p
 		ptest.Error = ptestErr
 		ptest.ForTest = p.ImportPath
+		if ptestImportsTesting {
+			ptest.Internal.TestinginitGo = formatTestinginit(p)
+		}
 		ptest.GoFiles = nil
 		ptest.GoFiles = append(ptest.GoFiles, p.GoFiles...)
 		ptest.GoFiles = append(ptest.GoFiles, p.TestGoFiles...)
@@ -201,6 +211,9 @@ func TestPackagesAndErrors(p *Package, cover *TestCover) (pmain, ptest, pxtest *
 				Gccgoflags: p.Internal.Gccgoflags,
 			},
 		}
+		if pxtestImportsTesting {
+			pxtest.Internal.TestinginitGo = formatTestinginit(pxtest)
+		}
 		if pxtestNeedsPtest {
 			pxtest.Internal.Imports = append(pxtest.Internal.Imports, ptest)
 		}
@@ -323,9 +336,7 @@ func TestPackagesAndErrors(p *Package, cover *TestCover) (pmain, ptest, pxtest *
 	if err != nil && pmain.Error == nil {
 		pmain.Error = &PackageError{Err: err.Error()}
 	}
-	if data != nil {
-		pmain.Internal.TestmainGo = &data
-	}
+	pmain.Internal.TestmainGo = data
 
 	return pmain, ptest, pxtest
 }
@@ -473,6 +484,15 @@ func loadTestFuncs(ptest *Package) (*testFuncs, error) {
 	return t, err
 }
 
+// formatTestinginit returns the content of the _testinginit.go file for p.
+func formatTestinginit(p *Package) []byte {
+	var buf bytes.Buffer
+	if err := testinginitTmpl.Execute(&buf, p); err != nil {
+		panic("testinginit template execution failed") // shouldn't be possible
+	}
+	return buf.Bytes()
+}
+
 // formatTestmain returns the content of the _testmain.go file for t.
 func formatTestmain(t *testFuncs) ([]byte, error) {
 	var buf bytes.Buffer
@@ -602,6 +622,24 @@ func checkTestFunc(fn *ast.FuncDecl, arg string) error {
 	return nil
 }
 
+var testinginitTmpl = lazytemplate.New("init", `
+package {{.Name}}
+
+{{/* Avoid a name collision with a name "testing" in user code. */}}
+import testing_xxxxxxxxxxxx "testing"
+
+{{/*
+Call testing.Init before any other user initialization code runs.
+(This file is passed to the compiler first.)
+This provides the illusion of the old behavior where testing flags
+were registered as part of the testing package's initialization.
+*/}}
+var _ = func() bool {
+	testing_xxxxxxxxxxxx.Init()
+	return true
+}()
+`)
+
 var testmainTmpl = lazytemplate.New("main", `
 package main
 

src/cmd/go/internal/test/test.go

@@ -835,7 +835,7 @@ func builderTest(b *work.Builder, p *load.Package) (buildAction, runAction, prin
 	if !cfg.BuildN {
 		// writeTestmain writes _testmain.go,
 		// using the test description gathered in t.
-		if err := ioutil.WriteFile(testDir+"_testmain.go", *pmain.Internal.TestmainGo, 0666); err != nil {
+		if err := ioutil.WriteFile(testDir+"_testmain.go", pmain.Internal.TestmainGo, 0666); err != nil {
 			return nil, nil, nil, err
 		}
 	}

src/cmd/go/internal/work/exec.go

@@ -523,6 +523,15 @@ func (b *Builder) build(a *Action) (err error) {
 		}
 	}
 
+	// Write out the _testinginit.go file for any test packages that import "testing".
+	if a.Package.Internal.TestinginitGo != nil {
+		initfile := objdir + "_testinginit.go"
+		if err := b.writeFile(initfile, a.Package.Internal.TestinginitGo); err != nil {
+			return err
+		}
+		gofiles = append([]string{initfile}, gofiles...)
+	}
+
 	// Run cgo.
 	if a.Package.UsesCgo() || a.Package.UsesSwig() {
 		// In a package using cgo, cgo compiles the C, C++ and assembly files with gcc.

src/cmd/go/testdata/flag_test.go

@@ -1,16 +1,19 @@
+// Copyright 2019 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 flag_test
 
 import (
 	"flag"
-	"log"
 	"testing"
 )
 
 var v = flag.Int("v", 0, "v flag")
 
-// Run this as go test pkg -v=7
+// Run this as go test pkg -args -v=7
 func TestVFlagIsSet(t *testing.T) {
 	if *v != 7 {
-		log.Fatal("v flag not set")
+		t.Fatal("v flag not set")
 	}
 }

src/cmd/go/testdata/script/test_init.txt

@@ -0,0 +1,98 @@
+# Tests for automatic testing.Init calls when using 'go test'.
+
+env GO111MODULE=on
+
+# A TestMain should be able to access testing flags if it calls flag.Parse
+# without needing to use testing.Init.
+go test testmain_flag_test.go
+
+# Test code can use the name 'testing' without colliding with generated
+# testinginit code.
+go test testing_collision_test.go
+
+# Tests running under 'go test' should observe that testing.Init is called
+# before any user package initialization code runs.
+go test ./testinitflag
+
+-- testmain_flag_test.go --
+package testmain_flag_test
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	flag.Parse()
+	found := false
+	flag.VisitAll(func(f *flag.Flag) {
+		if f.Name == "test.count" {
+			found = true
+		}
+	})
+	if !found {
+		fmt.Println("testing flags not registered")
+		os.Exit(1)
+	}
+	os.Exit(m.Run())
+}
+
+func TestX(t *testing.T) {}
+
+-- testing_collision_test.go --
+package testing_collision_test
+
+import testing2 "testing"
+
+var testing = 3
+
+func TestX(t *testing2.T) {}
+
+-- go.mod --
+module m
+
+-- testinitflag/init.go --
+package testinitflag
+
+import "flag"
+
+func TestFlagsInitialized() bool {
+	found := false
+	flag.VisitAll(func(f *flag.Flag) {
+		if f.Name == "test.count" {
+			found = true
+		}
+	})
+	return found
+}
+
+-- testinitflag/init_test.go --
+package testinitflag
+
+import "testing"
+
+var testingInitAtInitialization = TestFlagsInitialized()
+
+func TestInit(t *testing.T) {
+	if !testingInitAtInitialization {
+		t.Fatal("testing.Init not called before package initialization")
+	}
+}
+
+-- testinitflag/external_test.go --
+package testinitflag_test
+
+import (
+	"testing"
+	"m/testinitflag"
+)
+
+var testingInitAtInitialization = testinitflag.TestFlagsInitialized()
+
+func TestInitExternal(t *testing.T) {
+	if !testingInitAtInitialization {
+		t.Fatal("testing.Init not called before package initialization")
+	}
+}

src/cmd/go/testdata/standalone_testmain_flag_test.go

@@ -1078,6 +1078,11 @@ type testDeps interface {
 // It is not meant to be called directly and is not subject to the Go 1 compatibility document.
 // It may change signature from release to release.
 func MainStart(deps testDeps, tests []InternalTest, benchmarks []InternalBenchmark, examples []InternalExample) *M {
+	// In most cases, Init has already been called by the testinginit code
+	// that 'go test' injects into test packages.
+	// Call it again here to handle cases such as:
+	// - test packages that don't import "testing" (such as example-only packages)
+	// - direct use of MainStart (though that isn't well-supported)
 	Init()
 	return &M{
 		deps:       deps,