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write base64 message parts with 76 data bytes on a line instead of 78

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As required by RFC 2045 (MIME). The 78 byte lines work in practice, except that
SpamAssassin has rules that give messages with 78-byte lines spam points.

Mentioned by kjetilho on irc.
This commit is contained in:
Mechiel Lukkien
2025-04-03 10:22:15 +02:00
parent 00c8db98e6
commit 69d2699961
9 changed files with 16 additions and 13 deletions
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7
moxio/base64writer.go
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@ -13,7 +13,7 @@ func (f closerFunc) Close() error {
}
// Base64Writer turns a writer for data into one that writes base64 content on
// \r\n separated lines of max 78+2 characters length.
// \r\n separated lines of max 76+2 characters length.
func Base64Writer(w io.Writer) io.WriteCloser {
lw := &lineWrapper{w: w}
bw := base64.NewEncoder(base64.StdEncoding, lw)
@ -39,7 +39,8 @@ type lineWrapper struct {
func (lw *lineWrapper) Write(buf []byte) (int, error) {
wrote := 0
for len(buf) > 0 {
n := min(78-lw.n, len(buf))
// base64 has max 76 data bytes on per line. ../rfc/2045:1372
n := min(76-lw.n, len(buf))
nn, err := lw.w.Write(buf[:n])
if nn > 0 {
wrote += nn
@ -49,7 +50,7 @@ func (lw *lineWrapper) Write(buf []byte) (int, error) {
return wrote, err
}
lw.n += nn
if lw.n == 78 {
if lw.n == 76 {
_, err := lw.w.Write([]byte("\r\n"))
if err != nil {
return wrote, err