2 files changed, 119 insertions, 0 deletions
diff --git a/pkg/rfc822/writer.go b/pkg/rfc822/writer.go
new file mode 100644
index 0000000..07751ea
--- /dev/null
+++ b/pkg/rfc822/writer.go
@@ -0,0 +1,72 @@
+// Package rfc822 provides a writer that ensures the intrinsics of RFC 822.
+//
+// Rationale
+//
+// Cyrus IMAP really cares about the hard specifics of RFC 822, namely not allowing single \r and \n.
+//
+// See also: https://www.cyrusimap.org/imap/reference/faqs/interop-barenewlines.html
+// and: https://github.com/Necoro/feed2imap-go/issues/46
+//
+// NB: This package currently only cares about the newlines.
+package rfc822
+
+import "io"
+
+type rfc822Writer struct {
+	w io.Writer
+}
+
+var lf = []byte{'\n'}
+var cr = []byte{'\r'}
+
+func (f rfc822Writer) Write(p []byte) (n int, err error) {
+	crFound := false
+	start := 0
+
+	write := func(str []byte) {
+		var j int
+		j, err = f.w.Write(str)
+		n = n + j
+	}
+
+	for idx, b := range p {
+		if crFound && b != '\n' {
+			// insert '\n'
+			if write(p[start:idx]); err != nil {
+				return
+			}
+			if write(lf); err != nil {
+				return
+			}
+
+			start = idx
+		} else if !crFound && b == '\n' {
+			// insert '\r'
+			if write(p[start:idx]); err != nil {
+				return
+			}
+			if write(cr); err != nil {
+				return
+			}
+
+			start = idx
+		}
+		crFound = b == '\r'
+	}
+
+	// write the remainder
+	if write(p[start:]); err != nil {
+		return
+	}
+
+	if crFound { // dangling \r
+		write(lf)
+	}
+
+	return
+}
+
+// Writer creates a new RFC 822 conform writer.
+func Writer(w io.Writer) io.Writer {
+	return rfc822Writer{w}
+}
diff --git a/pkg/rfc822/writer_test.go b/pkg/rfc822/writer_test.go
new file mode 100644
index 0000000..7beae8d
--- /dev/null
+++ b/pkg/rfc822/writer_test.go
@@ -0,0 +1,47 @@
+package rfc822
+
+import (
+	"bytes"
+	"io"
+	"testing"
+)
+
+func TestRfc822Writer_Write(t *testing.T) {
+	tests := []struct {
+		before string
+		after  string
+	}{
+		{"", ""},
+		{"foo", "foo"},
+		{"foo\r", "foo\r\n"},
+		{"foo\n", "foo\r\n"},
+		{"foo\r\n", "foo\r\n"},
+		{"\r", "\r\n"},
+		{"\n", "\r\n"},
+		{"\r\n", "\r\n"},
+		{"foo\rbar", "foo\r\nbar"},
+		{"foo\nbar", "foo\r\nbar"},
+		{"foo\r\nbar", "foo\r\nbar"},
+		{"\r\r", "\r\n\r\n"},
+		{"\n\n", "\r\n\r\n"},
+		{"\r\r\n", "\r\n\r\n"},
+		{"\n\r", "\r\n\r\n"},
+		{"\rbar", "\r\nbar"},
+		{"\nbar", "\r\nbar"},
+		{"\r\nbar", "\r\nbar"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.before, func(t *testing.T) {
+			b := bytes.Buffer{}
+			w := Writer(&b)
+			if _, err := io.WriteString(w, tt.before); err != nil {
+				t.Errorf("Error: %v", err)
+				return
+			}
+			res := b.String()
+			if tt.after != res {
+				t.Errorf("Expected: %q, got: %q", tt.after, res)
+			}
+		})
+	}
+}