From 37de8de1c592619e155f93640eab70f2efa0dddc Mon Sep 17 00:00:00 2001
From: Mechiel Lukkien <mechiel@ueber.net>
Date: Fri, 16 Feb 2024 20:14:31 +0100
Subject: [PATCH] fix incorrect error about bare cr/lf when sending a message
 over smtp

we weren't properly tracking the cr's and lf's when being strict about message
lines when sending data.

we are reading buffered data from a Reader. if that chunk happens to start with
a newline, we weren't looking at the previously written data, which could be a
cr. instead, in that case, we would always claim the cr/lf wasn't correct.

the new test case triggered the behaviour before having the fix.

should solve issue #129 by x8x, thanks for the report!
---
 smtp/data.go      |  6 ++++--
 smtp/data_test.go | 24 ++++++++++++++++++++++++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/smtp/data.go b/smtp/data.go
index c3f293b..7a59d75 100644
--- a/smtp/data.go
+++ b/smtp/data.go
@@ -40,8 +40,10 @@ func DataWrite(w io.Writer, r io.Reader) error {
 					c := p[n]
 					if c == '\n' {
 						if firstcr < 0 {
-							// Bare newline.
-							return ErrCRLF
+							if n > 0 || last != '\r' {
+								// Bare newline.
+								return ErrCRLF
+							}
 						} else if firstcr != n-1 {
 							// Bare carriage return.
 							return ErrCRLF
diff --git a/smtp/data_test.go b/smtp/data_test.go
index ac0496c..f438b08 100644
--- a/smtp/data_test.go
+++ b/smtp/data_test.go
@@ -129,3 +129,27 @@ func TestDataReader(t *testing.T) {
 		t.Fatalf("got err %v, expected io.ErrUnexpectedEOF", err)
 	}
 }
+
+func TestDataWriteLineBoundaries(t *testing.T) {
+	const valid = "Subject: test\r\n\r\nbody\r\n"
+	if err := DataWrite(io.Discard, &oneReader{[]byte(valid)}); err != nil {
+		t.Fatalf("data write: %v", err)
+	}
+}
+
+// oneReader returns data one byte at a time.
+type oneReader struct {
+	buf []byte
+}
+
+func (r *oneReader) Read(buf []byte) (int, error) {
+	if len(r.buf) == 0 {
+		return 0, io.EOF
+	}
+	if len(buf) == 0 {
+		return 0, nil
+	}
+	buf[0] = r.buf[0]
+	r.buf = r.buf[1:]
+	return 1, nil
+}