net/textproto: initialize commonHeader in canonicalMIMEHeaderKey

Call initCommonHeader in canonicalMIMEHeaderKey to ensure that
commonHeader is initialized before use. Remove all other calls to
initCommonHeader, since commonHeader is only used in
canonicalMIMEHeaderKey.

This prevents a race condition: read of commonHeader before
commonHeader has been initialized.

Add regression test that triggers the race condition which can be
detected by the race detector.

Fixes #46363

Change-Id: I00c8c52c6f4c78c0305978c876142c1b388174af
Reviewed-on: https://go-review.googlesource.com/c/go/+/397575
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/net/textproto/reader.go

@@ -28,7 +28,6 @@ type Reader struct {
 // should be reading from an io.LimitReader or similar Reader to bound
 // the size of responses.
 func NewReader(r *bufio.Reader) *Reader {
-	commonHeaderOnce.Do(initCommonHeader)
 	return &Reader{R: r}
 }
 
@@ -579,8 +578,6 @@ func (r *Reader) upcomingHeaderNewlines() (n int) {
 // If s contains a space or invalid header field bytes, it is
 // returned without modifications.
 func CanonicalMIMEHeaderKey(s string) string {
-	commonHeaderOnce.Do(initCommonHeader)
-
 	// Quick check for canonical encoding.
 	upper := true
 	for i := 0; i < len(s); i++ {
@@ -642,6 +639,7 @@ func canonicalMIMEHeaderKey(a []byte) string {
 		a[i] = c
 		upper = c == '-' // for next time
 	}
+	commonHeaderOnce.Do(initCommonHeader)
 	// The compiler recognizes m[string(byteSlice)] as a special
 	// case, so a copy of a's bytes into a new string does not
 	// happen in this map lookup:

src/net/textproto/reader_test.go

@@ -8,8 +8,10 @@ import (
 	"bufio"
 	"bytes"
 	"io"
+	"net"
 	"reflect"
 	"strings"
+	"sync"
 	"testing"
 )
 
@@ -324,6 +326,33 @@ func TestCommonHeaders(t *testing.T) {
 	}
 }
 
+func TestIssue46363(t *testing.T) {
+	// Regression test for data race reported in issue 46363:
+	// ReadMIMEHeader reads commonHeader before commonHeader has been initialized.
+	// Run this test with the race detector enabled to catch the reported data race.
+
+	// Reset commonHeaderOnce, so that commonHeader will have to be initialized
+	commonHeaderOnce = sync.Once{}
+	commonHeader = nil
+
+	// Test for data race by calling ReadMIMEHeader and CanonicalMIMEHeaderKey concurrently
+
+	// Send MIME header over net.Conn
+	r, w := net.Pipe()
+	go func() {
+		// ReadMIMEHeader calls canonicalMIMEHeaderKey, which reads from commonHeader
+		NewConn(r).ReadMIMEHeader()
+	}()
+	w.Write([]byte("A: 1\r\nB: 2\r\nC: 3\r\n\r\n"))
+
+	// CanonicalMIMEHeaderKey calls commonHeaderOnce.Do(initCommonHeader) which initializes commonHeader
+	CanonicalMIMEHeaderKey("a")
+
+	if commonHeader == nil {
+		t.Fatal("CanonicalMIMEHeaderKey should initialize commonHeader")
+	}
+}
+
 var clientHeaders = strings.Replace(`Host: golang.org
 Connection: keep-alive
 Cache-Control: max-age=0