send-email: validate charset name in 8bit encoding prompt

When a non-ASCII character is detected in the body or subject of the email
the user is prompted with,

        Which 8bit encoding should I declare [UTF-8]? foo

After this the input string is validated by the regex, based on the fact
that the charset string will be minimum 4 characters [1]. If the string is
more than 4 letters the email is sent, if not then a second prompt to
confirm is asked to the user,

        Are you sure you want to use <foo> [y/N]? y

This relies on a length based regex heuristic check to validate the user
input, and can allow clearly invalid charset names to pass if the input is
greater than 4 characters.

Add a semantic validation of the charset name using the
Encode::find_encoding() module of perl. If the encoding is not recognized,
warn the user and ask for confirmation before proceeding. After this
validation the lenght based validation becomes redundant and also breaks
flow, so change the regex of valid input to any non blank string.

Introduce a dedicated helper for confirmation handling that can be reused
both by ask() and the custom 8bit prompt flow. Make the encoding warning
logic specific to the 8bit prompt, this reduces the load on ask(), and
improves maintainability.

Additionally, the wording of the first prompt can confuse the user if not
read properly or under any default assumptions for a yes/no prompt. Change
the wording to make it explicitly clear to the user that the prompt needs a
string input, UTF-8 being the default.

The intended flow is,

        Declare which 8bit encoding to use [default: UTF-8]? foobar
        warning: 'foobar' does not appear to be a valid charset name.
        Are you sure you want to use <foobar> [y/N]?

[1]- git@852a15d

Signed-off-by: Shreyansh Paliwal <shreyanshpaliwalcmsmn@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: shreyp135

git-send-email.perl

@@ -23,6 +23,7 @@
 use Git::LoadCPAN::Error qw(:try);
 use Git;
 use Git::I18N;
+use Encode qw(find_encoding);
 
 Getopt::Long::Configure qw/ pass_through /;
 
@@ -984,6 +985,18 @@ sub get_patch_subject {
 	}
 }
 
+sub confirm_ask {
+	my ($resp) = @_;
+	my $term = term();
+	return 0
+		unless defined $term->IN and defined fileno($term->IN) and
+		       defined $term->OUT and defined fileno($term->OUT);
+	my $yesno = $term->readline(
+		# TRANSLATORS: please keep [y/N] as is.
+		sprintf(__("Are you sure you want to use <%s> [y/N]? "), $resp));
+	return defined $yesno && $yesno =~ /y/i;
+}
+
 sub ask {
 	my ($prompt, %arg) = @_;
 	my $valid_re = $arg{valid_re};
@@ -1008,10 +1021,7 @@ sub ask {
 			return $resp;
 		}
 		if ($confirm_only) {
-			my $yesno = $term->readline(
-				# TRANSLATORS: please keep [y/N] as is.
-				sprintf(__("Are you sure you want to use <%s> [y/N]? "), $resp));
-			if (defined $yesno && $yesno =~ /y/i) {
+			if (confirm_ask($resp)) {
 				return $resp;
 			}
 		}
@@ -1044,9 +1054,21 @@ sub file_declares_8bit_cte {
 	foreach my $f (sort keys %broken_encoding) {
 		print "    $f\n";
 	}
-	$auto_8bit_encoding = ask(__("Which 8bit encoding should I declare [UTF-8]? "),
-				  valid_re => qr/.{4}/, confirm_only => 1,
-				  default => "UTF-8");
+	while(1) {
+		my $encoding = ask(__("Declare which 8bit encoding to use [default: UTF-8]? "),
+		valid_re => qr/^\S+$/,
+		default  => "UTF-8");
+		next unless defined $encoding;
+		if (find_encoding($encoding)) {
+			$auto_8bit_encoding = $encoding;
+			last;
+		}
+		printf STDERR __("warning: '%s' does not appear to be a valid charset name.\n"), $encoding;
+		if (confirm_ask($encoding)) {
+			$auto_8bit_encoding = $encoding;
+			last;
+		}
+	}
 }
 
 if (!$force) {

t/t9001-send-email.sh

@@ -1691,7 +1691,7 @@ test_expect_success $PREREQ 'asks about and fixes 8bit encodings' '
 			email-using-8bit >stdout &&
 	grep "do not declare a Content-Transfer-Encoding" stdout &&
 	grep email-using-8bit stdout &&
-	grep "Which 8bit encoding" stdout &&
+	grep "Declare which 8bit encoding to use" stdout &&
 	grep -E "Content|MIME" msgtxt1 >actual &&
 	test_cmp content-type-decl actual
 '