runtime/coverage: improve unit tests

Add a testpoint to cover support routines used to help
implement "go test -cover".

Change-Id: Ic28bf884a4e0d2c0a6d8fd04fc29c0c949227f21
Reviewed-on: https://go-review.googlesource.com/c/go/+/432315
Reviewed-by: Bryan Mills <bcmills@google.com>

src/runtime/coverage/testsupport.go

@@ -13,6 +13,7 @@ import (
 	"internal/coverage/decodecounter"
 	"internal/coverage/decodemeta"
 	"internal/coverage/pods"
+	"io"
 	"os"
 )
 
@@ -21,6 +22,12 @@ import (
 // intended to be used other than internally by the Go command's
 // generated code.
 func processCoverTestDir(dir string, cfile string, cm string, cpkg string) error {
+	return processCoverTestDirInternal(dir, cfile, cm, cpkg, os.Stdout)
+}
+
+// processCoverTestDirInternal is an io.Writer version of processCoverTestDir,
+// exposed for unit testing.
+func processCoverTestDirInternal(dir string, cfile string, cm string, cpkg string, w io.Writer) error {
 	cmode := coverage.ParseCounterMode(cm)
 	if cmode == coverage.CtrModeInvalid {
 		return fmt.Errorf("invalid counter mode %q", cm)
@@ -80,7 +87,7 @@ func processCoverTestDir(dir string, cfile string, cm string, cpkg string) error
 	}
 
 	// Emit percent.
-	if err := ts.cf.EmitPercent(os.Stdout, cpkg, true); err != nil {
+	if err := ts.cf.EmitPercent(w, cpkg, true); err != nil {
 		return err
 	}
 

src/runtime/coverage/ts_test.go

@@ -0,0 +1,58 @@
+// Copyright 2022 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 coverage
+
+import (
+	"internal/goexperiment"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	_ "unsafe"
+)
+
+//go:linkname testing_testGoCoverDir testing.testGoCoverDir
+func testing_testGoCoverDir() string
+
+// TestTestSupport does a basic verification of the functionality in
+// runtime/coverage.processCoverTestDir (doing this here as opposed to
+// relying on other test paths will provide a better signal when
+// running "go test -cover" for this package).
+func TestTestSupport(t *testing.T) {
+	if !goexperiment.CoverageRedesign {
+		return
+	}
+	if testing.CoverMode() == "" {
+		return
+	}
+	t.Logf("testing.testGoCoverDir() returns %s mode=%s\n",
+		testing_testGoCoverDir(), testing.CoverMode())
+
+	textfile := filepath.Join(t.TempDir(), "file.txt")
+	var sb strings.Builder
+	err := processCoverTestDirInternal(testing_testGoCoverDir(), textfile,
+		testing.CoverMode(), "", &sb)
+	if err != nil {
+		t.Fatalf("bad: %v", err)
+	}
+
+	// Check for existence of text file.
+	if inf, err := os.Open(textfile); err != nil {
+		t.Fatalf("problems opening text file %s: %v", textfile, err)
+	} else {
+		inf.Close()
+	}
+
+	// Check for percent output with expected tokens.
+	strout := sb.String()
+	want1 := "runtime/coverage"
+	want2 := "of statements"
+	if !strings.Contains(strout, want1) ||
+		!strings.Contains(strout, want2) {
+		t.Logf("output from run: %s\n", strout)
+		t.Fatalf("percent output missing key tokens: %q and %q",
+			want1, want2)
+	}
+}

src/testing/newcover.go

@@ -39,3 +39,10 @@ func coverReport2() {
 		os.Exit(2)
 	}
 }
+
+// testGoCoverDir returns the value passed to the -test.gocoverdir
+// flag by the Go command, if goexperiment.CoverageRedesign is
+// in effect.
+func testGoCoverDir() string {
+	return *gocoverdir
+}