send-email: add an auto option for transfer encoding

For most patches, using a transfer encoding of 8bit provides good
compatibility with most servers and makes it as easy as possible to view
patches.  However, there are some patches for which 8bit is not a valid
encoding: RFC 5322 specifies that a message must not have lines
exceeding 998 octets.

Add a transfer encoding value, auto, which indicates that a patch should
use 8bit where allowed and quoted-printable otherwise.  Choose
quoted-printable instead of base64, since base64-encoded plain text is
treated as suspicious by some spam filters.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-send-email.txt

@@ -137,15 +137,18 @@ Note that no attempts whatsoever are made to validate the encoding.
 	Specify encoding of compose message. Default is the value of the
 	'sendemail.composeencoding'; if that is unspecified, UTF-8 is assumed.
 
---transfer-encoding=(7bit|8bit|quoted-printable|base64)::
+--transfer-encoding=(7bit|8bit|quoted-printable|base64|auto)::
 	Specify the transfer encoding to be used to send the message over SMTP.
 	7bit will fail upon encountering a non-ASCII message.  quoted-printable
 	can be useful when the repository contains files that contain carriage
 	returns, but makes the raw patch email file (as saved from a MUA) much
 	harder to inspect manually.  base64 is even more fool proof, but also
-	even more opaque.  Default is the value of the `sendemail.transferEncoding`
-	configuration value; if that is unspecified, git will use 8bit and not
-	add a Content-Transfer-Encoding header.
+	even more opaque.  auto will use 8bit when possible, and quoted-printable
+	otherwise.
++
+Default is the value of the `sendemail.transferEncoding` configuration
+value; if that is unspecified, git will use 8bit and not add a
+Content-Transfer-Encoding header.
 
 --xmailer::
 --no-xmailer::

git-send-email.perl

@@ -1739,9 +1739,8 @@ sub process_file {
 	}
 	if (defined $target_xfer_encoding) {
 		$xfer_encoding = '8bit' if not defined $xfer_encoding;
-		$message = apply_transfer_encoding(
+		($message, $xfer_encoding) = apply_transfer_encoding(
 			$message, $xfer_encoding, $target_xfer_encoding);
-		$xfer_encoding = $target_xfer_encoding;
 	}
 	if (defined $xfer_encoding) {
 		push @xh, "Content-Transfer-Encoding: $xfer_encoding";
@@ -1852,13 +1851,16 @@ sub apply_transfer_encoding {
 	$message = MIME::Base64::decode($message)
 		if ($from eq 'base64');
 
+	$to = ($message =~ /.{999,}/) ? 'quoted-printable' : '8bit'
+		if $to eq 'auto';
+
 	die __("cannot send message as 7bit")
 		if ($to eq '7bit' and $message =~ /[^[:ascii:]]/);
-	return $message
+	return ($message, $to)
 		if ($to eq '7bit' or $to eq '8bit');
-	return MIME::QuotedPrint::encode($message, "\n", 0)
+	return (MIME::QuotedPrint::encode($message, "\n", 0), $to)
 		if ($to eq 'quoted-printable');
-	return MIME::Base64::encode($message, "\n")
+	return (MIME::Base64::encode($message, "\n"), $to)
 		if ($to eq 'base64');
 	die __("invalid transfer encoding");
 }

t/t9001-send-email.sh

@@ -456,6 +456,29 @@ test_expect_success $PREREQ 'allow long lines with --no-validate' '
 		2>errors
 '
 
+test_expect_success $PREREQ 'short lines with auto encoding are 8bit' '
+	clean_fake_sendmail &&
+	git send-email \
+		--from="A <author@example.com>" \
+		--to=nobody@example.com \
+		--smtp-server="$(pwd)/fake.sendmail" \
+		--transfer-encoding=auto \
+		$patches &&
+	grep "Content-Transfer-Encoding: 8bit" msgtxt1
+'
+
+test_expect_success $PREREQ 'long lines with auto encoding are quoted-printable' '
+	clean_fake_sendmail &&
+	git send-email \
+		--from="Example <nobody@example.com>" \
+		--to=nobody@example.com \
+		--smtp-server="$(pwd)/fake.sendmail" \
+		--transfer-encoding=auto \
+		--no-validate \
+		longline.patch &&
+	grep "Content-Transfer-Encoding: quoted-printable" msgtxt1
+'
+
 test_expect_success $PREREQ 'Invalid In-Reply-To' '
 	clean_fake_sendmail &&
 	git send-email \