Return error when reading zero byte from stdin

This commit changes the internal file system implementation for reading
data from stdin, it now returns an error when no bytes could be read. I
think it's worth failing in this case, the user instructed restic to
read some data from stdin, and no data was read at all. Maybe it was in
a pipe and some earlier stage failed.

See #2135 for a short discussion.

internal/archiver/archiver_test.go

@@ -160,7 +160,6 @@ func TestArchiverSaveFileReaderFS(t *testing.T) {
 	var tests = []struct {
 		Data string
 	}{
-		{Data: ""},
 		{Data: "foo"},
 		{Data: string(restictest.Random(23, 12*1024*1024+1287898))},
 	}
@@ -271,7 +270,6 @@ func TestArchiverSaveReaderFS(t *testing.T) {
 	var tests = []struct {
 		Data string
 	}{
-		{Data: ""},
 		{Data: "foo"},
 		{Data: string(restictest.Random(23, 12*1024*1024+1287898))},
 	}

internal/fs/fs_reader.go

@@ -1,6 +1,7 @@
 package fs
 
 import (
+	"fmt"
 	"io"
 	"os"
 	"path"
@@ -19,10 +20,13 @@ type Reader struct {
 	Name string
 	io.ReadCloser
 
+	// for FileInfo
 	Mode    os.FileMode
 	ModTime time.Time
 	Size    int64
 
+	AllowEmptyFile bool
+
 	open sync.Once
 }
 
@@ -40,7 +44,7 @@ func (fs *Reader) Open(name string) (f File, err error) {
 	switch name {
 	case fs.Name:
 		fs.open.Do(func() {
-			f = newReaderFile(fs.ReadCloser, fs.fi())
+			f = newReaderFile(fs.ReadCloser, fs.fi(), fs.AllowEmptyFile)
 		})
 
 		if f == nil {
@@ -78,7 +82,7 @@ func (fs *Reader) OpenFile(name string, flag int, perm os.FileMode) (f File, err
 	}
 
 	fs.open.Do(func() {
-		f = newReaderFile(fs.ReadCloser, fs.fi())
+		f = newReaderFile(fs.ReadCloser, fs.fi(), fs.AllowEmptyFile)
 	})
 
 	if f == nil {
@@ -158,9 +162,10 @@ func (fs *Reader) Dir(p string) string {
 	return path.Dir(p)
 }
 
-func newReaderFile(rd io.ReadCloser, fi os.FileInfo) readerFile {
-	return readerFile{
-		ReadCloser: rd,
+func newReaderFile(rd io.ReadCloser, fi os.FileInfo, allowEmptyFile bool) *readerFile {
+	return &readerFile{
+		ReadCloser:     rd,
+		AllowEmptyFile: allowEmptyFile,
 		fakeFile: fakeFile{
 			FileInfo: fi,
 			name:     fi.Name(),
@@ -170,19 +175,41 @@ func newReaderFile(rd io.ReadCloser, fi os.FileInfo) readerFile {
 
 type readerFile struct {
 	io.ReadCloser
+	AllowEmptyFile, bytesRead bool
+
 	fakeFile
 }
 
-func (r readerFile) Read(p []byte) (int, error) {
-	return r.ReadCloser.Read(p)
+// ErrFileEmpty is returned inside a *os.PathError by Read() for the file
+// opened from the fs provided by Reader when no data could be read and
+// AllowEmptyFile is not set.
+var ErrFileEmpty = errors.New("no data read")
+
+func (r *readerFile) Read(p []byte) (int, error) {
+	n, err := r.ReadCloser.Read(p)
+	if n > 0 {
+		r.bytesRead = true
+	}
+
+	// return an error if we did not read any data
+	if err == io.EOF && !r.AllowEmptyFile && !r.bytesRead {
+		fmt.Printf("reader: %d bytes read, err %v, bytesRead %v, allowEmpty %v\n", n, err, r.bytesRead, r.AllowEmptyFile)
+		return n, &os.PathError{
+			Path: r.fakeFile.name,
+			Op:   "read",
+			Err:  ErrFileEmpty,
+		}
+	}
+
+	return n, err
 }
 
-func (r readerFile) Close() error {
+func (r *readerFile) Close() error {
 	return r.ReadCloser.Close()
 }
 
 // ensure that readerFile implements File
-var _ File = readerFile{}
+var _ File = &readerFile{}
 
 // fakeFile implements all File methods, but only returns errors for anything
 // except Stat() and Name().

internal/fs/fs_reader_test.go

@@ -5,6 +5,7 @@ import (
 	"io/ioutil"
 	"os"
 	"sort"
+	"strings"
 	"testing"
 	"time"
 
@@ -317,3 +318,66 @@ func TestFSReader(t *testing.T) {
 		})
 	}
 }
+
+func TestFSReaderMinFileSize(t *testing.T) {
+	var tests = []struct {
+		name        string
+		data        string
+		allowEmpty  bool
+		readMustErr bool
+	}{
+		{
+			name: "regular",
+			data: "foobar",
+		},
+		{
+			name:        "empty",
+			data:        "",
+			allowEmpty:  false,
+			readMustErr: true,
+		},
+		{
+			name:        "empty2",
+			data:        "",
+			allowEmpty:  true,
+			readMustErr: false,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			fs := &Reader{
+				Name:           "testfile",
+				ReadCloser:     ioutil.NopCloser(strings.NewReader(test.data)),
+				Mode:           0644,
+				ModTime:        time.Now(),
+				AllowEmptyFile: test.allowEmpty,
+			}
+
+			f, err := fs.Open("testfile")
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			buf, err := ioutil.ReadAll(f)
+			if test.readMustErr {
+				if err == nil {
+					t.Fatal("expected error not found, got nil")
+				}
+			} else {
+				if err != nil {
+					t.Fatal(err)
+				}
+			}
+
+			if string(buf) != test.data {
+				t.Fatalf("wrong data returned, want %q, got %q", test.data, string(buf))
+			}
+
+			err = f.Close()
+			if err != nil {
+				t.Fatal(err)
+			}
+		})
+	}
+}