net/http: don't ignore errors in Request.Write

LGTM=josharian, adg
R=golang-codereviews, josharian, adg
CC=golang-codereviews
https://golang.org/cl/119110043

src/pkg/net/http/request.go

@@ -390,10 +390,16 @@ func (req *Request) write(w io.Writer, usingProxy bool, extraHeaders Header) err
 		w = bw
 	}
 
-	fmt.Fprintf(w, "%s %s HTTP/1.1\r\n", valueOrDefault(req.Method, "GET"), ruri)
+	_, err := fmt.Fprintf(w, "%s %s HTTP/1.1\r\n", valueOrDefault(req.Method, "GET"), ruri)
+	if err != nil {
+		return err
+	}
 
 	// Header lines
-	fmt.Fprintf(w, "Host: %s\r\n", host)
+	_, err = fmt.Fprintf(w, "Host: %s\r\n", host)
+	if err != nil {
+		return err
+	}
 
 	// Use the defaultUserAgent unless the Header contains one, which
 	// may be blank to not send the header.
@@ -404,7 +410,10 @@ func (req *Request) write(w io.Writer, usingProxy bool, extraHeaders Header) err
 		}
 	}
 	if userAgent != "" {
-		fmt.Fprintf(w, "User-Agent: %s\r\n", userAgent)
+		_, err = fmt.Fprintf(w, "User-Agent: %s\r\n", userAgent)
+		if err != nil {
+			return err
+		}
 	}
 
 	// Process Body,ContentLength,Close,Trailer
@@ -429,7 +438,10 @@ func (req *Request) write(w io.Writer, usingProxy bool, extraHeaders Header) err
 		}
 	}
 
-	io.WriteString(w, "\r\n")
+	_, err = io.WriteString(w, "\r\n")
+	if err != nil {
+		return err
+	}
 
 	// Write body and trailer
 	err = tw.WriteBody(w)

src/pkg/net/http/requestwrite_test.go

@@ -563,3 +563,61 @@ func mustParseURL(s string) *url.URL {
 	}
 	return u
 }
+
+type writerFunc func([]byte) (int, error)
+
+func (f writerFunc) Write(p []byte) (int, error) { return f(p) }
+
+// TestRequestWriteError tests the Write err != nil checks in (*Request).write.
+func TestRequestWriteError(t *testing.T) {
+	failAfter, writeCount := 0, 0
+	errFail := errors.New("fake write failure")
+
+	// w is the buffered io.Writer to write the request to.  It
+	// fails exactly once on its Nth Write call, as controlled by
+	// failAfter. It also tracks the number of calls in
+	// writeCount.
+	w := struct {
+		io.ByteWriter // to avoid being wrapped by a bufio.Writer
+		io.Writer
+	}{
+		nil,
+		writerFunc(func(p []byte) (n int, err error) {
+			writeCount++
+			if failAfter == 0 {
+				err = errFail
+			}
+			failAfter--
+			return len(p), err
+		}),
+	}
+
+	req, _ := NewRequest("GET", "http://example.com/", nil)
+	const writeCalls = 4 // number of Write calls in current implementation
+	sawGood := false
+	for n := 0; n <= writeCalls+2; n++ {
+		failAfter = n
+		writeCount = 0
+		err := req.Write(w)
+		var wantErr error
+		if n < writeCalls {
+			wantErr = errFail
+		}
+		if err != wantErr {
+			t.Errorf("for fail-after %d Writes, err = %v; want %v", n, err, wantErr)
+			continue
+		}
+		if err == nil {
+			sawGood = true
+			if writeCount != writeCalls {
+				t.Fatalf("writeCalls constant is outdated in test")
+			}
+		}
+		if writeCount > writeCalls || writeCount > n+1 {
+			t.Errorf("for fail-after %d, saw unexpectedly high (%d) write calls", n, writeCount)
+		}
+	}
+	if !sawGood {
+		t.Fatalf("writeCalls constant is outdated in test")
+	}
+}