net/http: improve FileServer error logging

Errors returned during copying the file content in ServeContent,
ServeFile and FileServer were ignored to avoid excessive, meaningless
logging. This commit introduces error filtering and logging to ensure
that the errors occurring during reading the content are logged.

Updates golang#27128

src/net/http/export_test.go

@@ -29,6 +29,7 @@ var (
 	ExportErrRequestCanceledConn      = errRequestCanceledConn
 	ExportErrServerClosedIdle         = errServerClosedIdle
 	ExportServeFile                   = serveFile
+	ExportCopyNIgnoreWriteError       = copyNIgnoreWriteError
 	ExportScanETag                    = scanETag
 	ExportHttp2ConfigureServer        = http2ConfigureServer
 	Export_shouldCopyHeaderOnRedirect = shouldCopyHeaderOnRedirect

src/net/http/fs.go

@@ -295,8 +295,38 @@ func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time,
 	w.WriteHeader(code)
 
 	if r.Method != "HEAD" {
-		io.CopyN(w, sendContent, sendSize)
+		if err := copyNIgnoreWriteError(w, sendContent, sendSize); err != nil {
+			n := name
+			if n == "" {
+				n = r.URL.Path
+			}
+			logf(r, "http: read error in ServeContent: %s: %v", n, err)
+		}
+	}
+}
+
+// errorSaverWriter wraps an io.Writer and saves the error returned
+// by the last invocation of Write in err
+type errorSaverWriter struct {
+	w   io.Writer
+	err error
+}
+
+func (w *errorSaverWriter) Write(b []byte) (int, error) {
+	var n int
+	n, w.err = w.w.Write(b)
+	return n, w.err
+}
+
+// copyNIgnoreWriteError copies n bytes (or until an error) from src to dst.
+// It returns only the errors encountered during reading from src.
+func copyNIgnoreWriteError(dst io.Writer, src io.Reader, n int64) error {
+	ew := &errorSaverWriter{w: dst}
+	_, err := io.CopyN(ew, src, n)
+	if err == ew.err {
+		return nil
 	}
+	return err
 }
 
 // scanETag determines if a syntactically valid ETag is present at s. If so,

src/net/http/fs_test.go

@@ -1285,6 +1285,28 @@ func (d fileServerCleanPathDir) Open(path string) (File, error) {
 
 type panicOnSeek struct{ io.ReadSeeker }
 
+func Test_copyNIgnoreWriteError(t *testing.T) {
+	e := errors.New("io error")
+	tests := []struct {
+		name    string
+		dst     io.Writer
+		src     io.Reader
+		n       int64
+		wantErr error
+	}{
+		{"short read", ioutil.Discard, new(bytes.Buffer), 1, io.EOF},
+		{"read err", ioutil.Discard, errorReader{e}, 1, e},
+		{"no err", ioutil.Discard, strings.NewReader("content"), 3, nil},
+		{"write err", &net.IPConn{}, strings.NewReader("content"), 3, nil},
+	}
+	for _, test := range tests {
+		err := ExportCopyNIgnoreWriteError(test.dst, test.src, test.n)
+		if err != test.wantErr {
+			t.Errorf("%s copyNIgnoreWriteError got %v, want %v", test.name, err, test.wantErr)
+		}
+	}
+}
+
 func Test_scanETag(t *testing.T) {
 	tests := []struct {
 		in         string