net/http: Request Body error should not be ignored.

Fixes #7521.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/76320043

src/pkg/net/http/requestwrite_test.go

@@ -310,6 +310,46 @@ var reqWriteTests = []reqWriteTest{
 		WantError: errors.New("http: Request.ContentLength=5 with nil Body"),
 	},
 
+	// Request with a 0 ContentLength and a body with 1 byte content and an error.
+	{
+		Req: Request{
+			Method:        "POST",
+			URL:           mustParseURL("/"),
+			Host:          "example.com",
+			ProtoMajor:    1,
+			ProtoMinor:    1,
+			ContentLength: 0, // as if unset by user
+		},
+
+		Body: func() io.ReadCloser {
+			err := errors.New("Custom reader error")
+			errReader := &errorReader{err}
+			return ioutil.NopCloser(io.MultiReader(strings.NewReader("x"), errReader))
+		},
+
+		WantError: errors.New("Custom reader error"),
+	},
+
+	// Request with a 0 ContentLength and a body without content and an error.
+	{
+		Req: Request{
+			Method:        "POST",
+			URL:           mustParseURL("/"),
+			Host:          "example.com",
+			ProtoMajor:    1,
+			ProtoMinor:    1,
+			ContentLength: 0, // as if unset by user
+		},
+
+		Body: func() io.ReadCloser {
+			err := errors.New("Custom reader error")
+			errReader := &errorReader{err}
+			return ioutil.NopCloser(errReader)
+		},
+
+		WantError: errors.New("Custom reader error"),
+	},
+
 	// Verify that DumpRequest preserves the HTTP version number, doesn't add a Host,
 	// and doesn't add a User-Agent.
 	{

src/pkg/net/http/transfer.go

@@ -17,6 +17,14 @@ import (
 	"sync"
 )
 
+type errorReader struct {
+	err error
+}
+
+func (r *errorReader) Read(p []byte) (n int, err error) {
+	return 0, r.err
+}
+
 // transferWriter inspects the fields of a user-supplied Request or Response,
 // sanitizes them without changing the user object and provides methods for
 // writing the respective header, body and trailer in wire format.
@@ -53,8 +61,11 @@ func newTransferWriter(r interface{}) (t *transferWriter, err error) {
 			if t.ContentLength == 0 {
 				// Test to see if it's actually zero or just unset.
 				var buf [1]byte
-				n, _ := io.ReadFull(t.Body, buf[:])
-				if n == 1 {
+				n, rerr := io.ReadFull(t.Body, buf[:])
+				if rerr != nil && rerr != io.EOF {
+					t.ContentLength = -1
+					t.Body = &errorReader{rerr}
+				} else if n == 1 {
 					// Oh, guess there is data in this Body Reader after all.
 					// The ContentLength field just wasn't set.
 					// Stich the Body back together again, re-attaching our