2 files changed, 91 insertions, 2 deletions

diff --git a/src/testing/internal/testdeps/deps.go b/src/testing/internal/testdeps/deps.go
index 042f69614e..8c0b3fded1 100644
--- a/src/testing/internal/testdeps/deps.go
+++ b/src/testing/internal/testdeps/deps.go
@@ -11,9 +11,13 @@
 package testdeps
 
 import (
+	"bufio"
+	"internal/testlog"
 	"io"
 	"regexp"
 	"runtime/pprof"
+	"strings"
+	"sync"
 )
 
 // TestDeps is an implementation of the testing.testDeps interface,
@@ -56,3 +60,61 @@ var ImportPath string
 func (TestDeps) ImportPath() string {
 	return ImportPath
 }
+
+// testLog implements testlog.Interface, logging actions by package os.
+type testLog struct {
+	mu sync.Mutex
+	w  *bufio.Writer
+}
+
+func (l *testLog) Getenv(key string) {
+	l.add("getenv", key)
+}
+
+func (l *testLog) Open(name string) {
+	l.add("open", name)
+}
+
+func (l *testLog) Stat(name string) {
+	l.add("stat", name)
+}
+
+func (l *testLog) Chdir(name string) {
+	l.add("chdir", name)
+}
+
+// add adds the (op, name) pair to the test log.
+func (l *testLog) add(op, name string) {
+	if strings.Contains(name, "\n") || name == "" {
+		return
+	}
+
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	if l.w == nil {
+		return
+	}
+	l.w.WriteString(op)
+	l.w.WriteByte(' ')
+	l.w.WriteString(name)
+	l.w.WriteByte('\n')
+}
+
+var log testLog
+
+func (TestDeps) StartTestLog(w io.Writer) {
+	log.mu.Lock()
+	log.w = bufio.NewWriter(w)
+	log.w.WriteString("# test log\n") // known to cmd/go/internal/test/test.go
+	log.mu.Unlock()
+
+	testlog.SetLogger(&log)
+}
+
+func (TestDeps) StopTestLog() error {
+	log.mu.Lock()
+	defer log.mu.Unlock()
+	err := log.w.Flush()
+	log.w = nil
+	return err
+}
diff --git a/src/testing/testing.go b/src/testing/testing.go
index cddd475fd7..402780ad64 100644
--- a/src/testing/testing.go
+++ b/src/testing/testing.go
@@ -268,10 +268,12 @@ var (
 	timeout              = flag.Duration("test.timeout", 0, "panic test binary after duration `d` (default 0, timeout disabled)")
 	cpuListStr           = flag.String("test.cpu", "", "comma-separated `list` of cpu counts to run each test with")
 	parallel             = flag.Int("test.parallel", runtime.GOMAXPROCS(0), "run at most `n` tests in parallel")
+	testlog              = flag.String("test.testlogfile", "", "write test action log to `file` (for use only by cmd/go)")
 
 	haveExamples bool // are there examples?
 
-	cpuList []int
+	cpuList     []int
+	testlogFile *os.File
 
 	numFailed uint32 // number of test failures
 )
@@ -889,6 +891,8 @@ func (f matchStringOnly) StopCPUProfile()                             {}
 func (f matchStringOnly) WriteHeapProfile(w io.Writer) error          { return errMain }
 func (f matchStringOnly) WriteProfileTo(string, io.Writer, int) error { return errMain }
 func (f matchStringOnly) ImportPath() string                          { return "" }
+func (f matchStringOnly) StartTestLog(io.Writer)                      {}
+func (f matchStringOnly) StopTestLog() error                          { return errMain }
 
 // Main is an internal function, part of the implementation of the "go test" command.
 // It was exported because it is cross-package and predates "internal" packages.
@@ -916,12 +920,14 @@ type M struct {
 // The canonical implementation of this interface is
 // testing/internal/testdeps's TestDeps.
 type testDeps interface {
+	ImportPath() string
 	MatchString(pat, str string) (bool, error)
 	StartCPUProfile(io.Writer) error
 	StopCPUProfile()
+	StartTestLog(io.Writer)
+	StopTestLog() error
 	WriteHeapProfile(io.Writer) error
 	WriteProfileTo(string, io.Writer, int) error
-	ImportPath() string
 }
 
 // MainStart is meant for use by tests generated by 'go test'.
@@ -1100,6 +1106,17 @@ func (m *M) before() {
 		fmt.Fprintf(os.Stderr, "testing: cannot use -test.coverprofile because test binary was not built with coverage enabled\n")
 		os.Exit(2)
 	}
+	if *testlog != "" {
+		// Note: Not using toOutputDir.
+		// This file is for use by cmd/go, not users.
+		f, err := os.Create(*testlog)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "testing: %s\n", err)
+			os.Exit(2)
+		}
+		m.deps.StartTestLog(f)
+		testlogFile = f
+	}
 }
 
 // after runs after all testing.
@@ -1110,6 +1127,16 @@ func (m *M) after() {
 }
 
 func (m *M) writeProfiles() {
+	if *testlog != "" {
+		if err := m.deps.StopTestLog(); err != nil {
+			fmt.Fprintf(os.Stderr, "testing: can't write %s: %s\n", *testlog, err)
+			os.Exit(2)
+		}
+		if err := testlogFile.Close(); err != nil {
+			fmt.Fprintf(os.Stderr, "testing: can't write %s: %s\n", *testlog, err)
+			os.Exit(2)
+		}
+	}
 	if *cpuProfile != "" {
 		m.deps.StopCPUProfile() // flushes profile to disk
 	}