always emit header when forced

Signed-off-by: James Hillyerd <james@hillyerd.com>

Author: jhillyerd

encode.go

@@ -119,13 +119,24 @@ func (p *Part) setupMIMEHeaders() transferEncoding {
 	p.Header.Del(hnContentEncoding)
 
 	cte := te7Bit
-	forcedCTE := false
-	if len(p.Content) > 0 {
-		// Check for explicit override first.
-		if f := p.resolveForcedCTE(); f != teRaw {
-			cte = f
-			forcedCTE = true
-		} else if strings.Index(strings.ToLower(p.ContentType), "message/") == 0 {
+	if f := p.resolveForcedCTE(); f != teRaw {
+		// An explicit override applies regardless of content length, so the header is always
+		// emitted (including 7bit), even when the body is empty.
+		cte = f
+		switch f {
+		case te7Bit:
+			p.Header.Set(hnContentEncoding, cte7Bit)
+		case te8Bit:
+			p.Header.Set(hnContentEncoding, cte8Bit)
+		case teBase64:
+			p.Header.Set(hnContentEncoding, cteBase64)
+		case teQuoted:
+			p.Header.Set(hnContentEncoding, cteQuotedPrintable)
+		}
+	} else if len(p.Content) > 0 {
+		// No override: select automatically. RFC 2045: 7bit is assumed if the CTE header is not
+		// present, so for auto-detected 7bit (and for empty content) no header is emitted.
+		if strings.Index(strings.ToLower(p.ContentType), "message/") == 0 {
 			// RFC 1341: `message` types must have no encoding other than "7bit", "8bit", or
 			// "binary". The message header fields are always US-ASCII in any case, and data within
 			// the body can still be encoded, in which case the Content-Transfer-Encoding header
@@ -140,13 +151,7 @@ func (p *Part) setupMIMEHeaders() transferEncoding {
 				}
 			}
 		}
-
-		// RFC 2045: 7bit is assumed if CTE header not present.
 		switch cte {
-		case te7Bit:
-			if forcedCTE {
-				p.Header.Set(hnContentEncoding, cte7Bit)
-			}
 		case te8Bit:
 			p.Header.Set(hnContentEncoding, cte8Bit)
 		case teBase64:

encode_test.go

@@ -626,6 +626,80 @@ func TestEncodePartForcedCTEWithContentReader(t *testing.T) {
 	assert.Nil(t, p.ContentReader, "ContentReader should have been consumed")
 }
 
+// TestEncodePartForcedCTEEmptyContent verifies that a forced Content-Transfer-Encoding is still
+// applied to the header when the body is empty -- either nil/zero-length Content or a ContentReader
+// that yields zero bytes. Previously the override was gated on len(Content) > 0 and was silently
+// dropped, contradicting the documented "forces" contract.
+func TestEncodePartForcedCTEEmptyContent(t *testing.T) {
+	for _, tc := range []struct {
+		name string
+		cte  string
+		part func() *enmime.Part
+	}{
+		{
+			name: "7bit nil content",
+			cte:  "7bit",
+			part: func() *enmime.Part {
+				p := enmime.NewPart("application/pgp-encrypted")
+				p.ContentTransferEncoding = "7bit"
+				return p
+			},
+		},
+		{
+			name: "base64 nil content",
+			cte:  "base64",
+			part: func() *enmime.Part {
+				p := enmime.NewPart("application/octet-stream")
+				p.ContentTransferEncoding = "base64"
+				return p
+			},
+		},
+		{
+			name: "7bit zero-byte reader",
+			cte:  "7bit",
+			part: func() *enmime.Part {
+				p := enmime.NewPart("application/pgp-encrypted")
+				p.ContentTransferEncoding = "7bit"
+				p.ContentReader = bytes.NewReader(nil)
+				return p
+			},
+		},
+		{
+			name: "base64 zero-byte reader",
+			cte:  "base64",
+			part: func() *enmime.Part {
+				p := enmime.NewPart("application/octet-stream")
+				p.ContentTransferEncoding = "base64"
+				p.ContentReader = bytes.NewReader(nil)
+				return p
+			},
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			p := tc.part()
+			b := &bytes.Buffer{}
+			if err := p.Encode(b); err != nil {
+				t.Fatal(err)
+			}
+			assert.Equal(t, tc.cte, p.Header.Get("Content-Transfer-Encoding"))
+		})
+	}
+}
+
+// TestEncodePartAutoCTEEmptyContent verifies that, when no CTE override is set, an empty body still
+// results in no Content-Transfer-Encoding header (RFC 2045: 7bit is assumed when the header is
+// absent). This guards against the forced-override refactor accidentally emitting a header for
+// empty auto-detected parts.
+func TestEncodePartAutoCTEEmptyContent(t *testing.T) {
+	p := enmime.NewPart("text/plain")
+	// No ContentTransferEncoding set; Content is empty.
+	b := &bytes.Buffer{}
+	if err := p.Encode(b); err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, "", p.Header.Get("Content-Transfer-Encoding"))
+}
+
 // TestEncodePartForcedCTEPrecedenceOverEncoderOption verifies that ContentTransferEncoding on the part
 // takes precedence over the encoder-level ForceQuotedPrintableCte option. Without the override, the
 // non-ASCII text content combined with ForceQuotedPrintableCte(true) would be QP-encoded; the forced