[PATCH v2] send-email: validate charset name in 8bit encoding prompt

[Shreyansh Paliwal <shreyanshpaliwalcmsmn@gmail.com>]

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.

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]- https://github.com/git/git/commit/852a15d748034eec87adbee73a72689c8936fb8b

Signed-off-by: Shreyansh Paliwal <shreyanshpaliwalcmsmn@gmail.com>
---
Changes in v2:
 - Added braces in if-else block.

 git-send-email.perl   | 17 ++++++++++++++---
 t/t9001-send-email.sh |  2 +-
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/git-send-email.perl b/git-send-email.perl
index cd4b316ddc..15387ac377 100755
--- a/git-send-email.perl
+++ b/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 /;

@@ -987,6 +988,7 @@ sub get_patch_subject {
 sub ask {
 	my ($prompt, %arg) = @_;
 	my $valid_re = $arg{valid_re};
+	my $warn_invalid = $arg{warn_invalid};
 	my $default = $arg{default};
 	my $confirm_only = $arg{confirm_only};
 	my $resp;
@@ -1005,7 +1007,15 @@ sub ask {
 			return $default;
 		}
 		if (!defined $valid_re or $resp =~ /$valid_re/) {
-			return $resp;
+			if ($warn_invalid) {
+				if (find_encoding($resp)) {
+					return $resp;
+				} else {
+					printf STDERR __("warning: '%s' does not appear to be a valid charset name.\n"), $resp;
+				}
+			} else {
+				return $resp;
+			}
 		}
 		if ($confirm_only) {
 			my $yesno = $term->readline(
@@ -1044,8 +1054,9 @@ 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,
+	$auto_8bit_encoding = ask(__("Declare which 8bit encoding to use [default: UTF-8]? "),
+				  valid_re => qr/^\S+$/, confirm_only => 1,
+				  warn_invalid => 1,
 				  default => "UTF-8");
 }

diff --git a/t/t9001-send-email.sh b/t/t9001-send-email.sh
index e56e0c8d77..24f6c76aee 100755
--- a/t/t9001-send-email.sh
+++ b/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
 '

Range-diff against v1:
1:  70fa4d2899 ! 1:  954c1dae9f send-email: validate charset name in 8bit encoding prompt
    @@ git-send-email.perl: sub ask {
      		if (!defined $valid_re or $resp =~ /$valid_re/) {
     -			return $resp;
     +			if ($warn_invalid) {
    -+				if (find_encoding($resp))
    ++				if (find_encoding($resp)) {
     +					return $resp;
    -+				else
    ++				} else {
     +					printf STDERR __("warning: '%s' does not appear to be a valid charset name.\n"), $resp;
    -+			} else
    ++				}
    ++			} else {
     +				return $resp;
    ++			}
      		}
      		if ($confirm_only) {
      			my $yesno = $term->readline(
--
2.53.0