ParseHeader: print hexdump on error

Should help debugging https://github.com/rfjakob/gocryptfs/issues/363

internal/contentenc/file_header.go

@@ -7,6 +7,7 @@ package contentenc
 import (
 	"bytes"
 	"encoding/binary"
+	"encoding/hex"
 	"fmt"
 	"log"
 
@@ -52,11 +53,13 @@ func ParseHeader(buf []byte) (*FileHeader, error) {
 	var h FileHeader
 	h.Version = binary.BigEndian.Uint16(buf[0:headerVersionLen])
 	if h.Version != CurrentVersion {
-		return nil, fmt.Errorf("ParseHeader: invalid version, want=%d have=%d", CurrentVersion, h.Version)
+		return nil, fmt.Errorf("ParseHeader: invalid version, want=%d have=%d. Hexdump: %s",
+			CurrentVersion, h.Version, hex.EncodeToString(buf))
 	}
 	h.ID = buf[headerVersionLen:]
 	if bytes.Equal(h.ID, allZeroFileID) {
-		return nil, fmt.Errorf("ParseHeader: file id is all-zero")
+		return nil, fmt.Errorf("ParseHeader: file id is all-zero. Hexdump: %s",
+			hex.EncodeToString(buf))
 	}
 	return &h, nil
 }

tests/matrix/matrix_test.go

@@ -98,13 +98,13 @@ func testWriteN(t *testing.T, fn string, n int) string {
 	}
 
 	d := make([]byte, n)
-	written, err := file.Write(d)
-	if err != nil || written != len(d) {
-		t.Errorf("err=\"%s\", written=%d", err, written)
+	_, err := file.Write(d)
+	if err != nil {
+		t.Fatal(err)
 	}
 	err = file.Close()
 	if err != nil {
-		t.Error(err)
+		t.Fatal(err)
 	}
 
 	test_helpers.VerifySize(t, test_helpers.DefaultPlainDir+"/"+fn, n)