Ensure empty files do not have a mimetype

pkg/file/mime_type.go

@@ -14,11 +14,31 @@ func MIMEType(reader io.Reader) string {
 	if reader == nil {
 		return ""
 	}
+
+	s := sizer{reader: reader}
+
 	var mTypeStr string
-	mType, err := mimetype.DetectReader(reader)
+	mType, err := mimetype.DetectReader(&s)
 	if err == nil {
 		// extract the string mimetype and ignore aux information (e.g. 'text/plain; charset=utf-8' -> 'text/plain')
 		mTypeStr = strings.Split(mType.String(), ";")[0]
 	}
+
+	// we may have a reader that is not nil but the observed contents was empty
+	if s.size == 0 {
+		return ""
+	}
+
 	return mTypeStr
 }
+
+type sizer struct {
+	reader io.Reader
+	size   int64
+}
+
+func (s *sizer) Read(p []byte) (int, error) {
+	n, err := s.reader.Read(p)
+	s.size += int64(n)
+	return n, err
+}

pkg/file/mime_type_test.go

@@ -2,41 +2,50 @@ package file
 
 import (
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"io"
 	"os"
+	"strings"
 	"testing"
 )
 
 func Test_MIMEType(t *testing.T) {
 
+	fileReader := func(path string) io.Reader {
+		f, err := os.Open(path)
+		require.NoError(t, err)
+		return f
+	}
+
 	tests := []struct {
-		fixture  string
+		name     string
+		fixture  io.Reader
 		expected string
 	}{
 		{
-			// darwin binary
-			fixture:  "test-fixtures/mime/mach-binary",
+			name:     "binary",
+			fixture:  fileReader("test-fixtures/mime/mach-binary"),
 			expected: "application/x-mach-binary",
 		},
 		{
-			// script
-			fixture:  "test-fixtures/mime/capture.sh",
+			name:     "script",
+			fixture:  fileReader("test-fixtures/mime/capture.sh"),
 			expected: "text/plain",
 		},
 		{
-			// no contents
-			fixture:  "",
+			name:     "no contents",
+			fixture:  strings.NewReader(""),
+			expected: "",
+		},
+		{
+			name:     "no reader",
+			fixture:  nil,
 			expected: "",
 		},
 	}
 	for _, test := range tests {
-		t.Run(test.fixture, func(t *testing.T) {
-			var f *os.File
-			var err error
-			if test.fixture != "" {
-				f, err = os.Open(test.fixture)
-				assert.NoError(t, err)
-			}
-			assert.Equal(t, test.expected, MIMEType(f))
+		t.Run(test.name, func(t *testing.T) {
+			assert.Equal(t, test.expected, MIMEType(test.fixture))
 		})
 	}
 }