[core] permit env w/ blank value (fix regression)

(thx Lars Bingchong)

empty env var must be set to blank string and not left unset
(regression in lighttpd 1.4.56 - lighttpd 1.4.64)

x-ref:
  https://stackoverflow.com/a/52913064/1338888

src/http_header.c

@@ -353,7 +353,6 @@ void http_header_env_set(request_st * const r, const char *k, uint32_t klen, con
 void http_header_env_append(request_st * const r, const char *k, uint32_t klen, const char *v, uint32_t vlen) {
     /*if (0 == vlen) return;*//* skip check; permit env var w/ blank value */
     buffer * const vb = array_get_buf_ptr(&r->env, k, klen);
-    if (0 == vlen) return;
     http_header_token_append(vb, v, vlen);
 }
 

tests/docroot/cgi.pl

@@ -2,7 +2,8 @@
 
 # env
 if ($ENV{"QUERY_STRING"} =~ /^env=(\w+)/) {
-    print "Status: 200\r\n\r\n$ENV{$1}";
+    my $v = defined($ENV{$1}) ? $ENV{$1} : "[$1 not found]";
+    print "Status: 200\r\n\r\n$v";
     exit 0;
 }
 

tests/lighttpd.conf

@@ -63,6 +63,7 @@ setenv.add-environment = (
 )
 setenv.set-environment = (
 	"NEWENV" => "newenv",
+	"BLANK_VALUE" => "",
 )
 setenv.add-request-header = (
 	"FOO" => "foo",

tests/request.t

@@ -8,7 +8,7 @@ BEGIN {
 
 use strict;
 use IO::Socket;
-use Test::More tests => 176;
+use Test::More tests => 178;
 use LightyTest;
 
 my $tf = LightyTest->new();
@@ -1399,6 +1399,24 @@ EOF
 $t->{RESPONSE} = [ { 'HTTP-Protocol' => 'HTTP/1.1', 'HTTP-Status' => 200, '+Content-Length' => '' } ];
 ok($tf->handle_http($t) == 0, 'cgi-env: HTTP_HOST');
 
+$t->{REQUEST}  = ( <<EOF
+GET /cgi.pl?env=ABSENT HTTP/1.1
+Host: www.example.org
+Connection: close
+EOF
+ );
+$t->{RESPONSE} = [ { 'HTTP-Protocol' => 'HTTP/1.1', 'HTTP-Status' => 200, 'HTTP-Content' => '[ABSENT not found]' } ];
+ok($tf->handle_http($t) == 0, 'cgi-env: ABSENT');
+
+$t->{REQUEST}  = ( <<EOF
+GET /cgi.pl?env=BLANK_VALUE HTTP/1.1
+Host: www.example.org
+Connection: close
+EOF
+ );
+$t->{RESPONSE} = [ { 'HTTP-Protocol' => 'HTTP/1.1', 'HTTP-Status' => 200, 'HTTP-Content' => '' } ];
+ok($tf->handle_http($t) == 0, 'cgi-env: BLANK_VALUE');
+
 # broken header crash
 $t->{REQUEST}  = ( <<EOF
 GET /cgi.pl?crlfcrash HTTP/1.0