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/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)
+			}
+		})
+	}
+}