Fix #53891: iconv_mime_encode() fails to Q-encode UTF-8 string

The minimum length of an encoded-word is actually the pure encoding
overhead plus the length of the `output-charset` plus the minimum unit
of encoded text, which is 4 for B-encoding and (for simplicity) 3 for
Q-encoding.  We also cater to the possibility that we need further
encoded words, which would be split by the `line-break-chars` followed
by a space character.  Obviously, the former `out_charset_len + 12` is
too simplistic and wrong in the given case (where the magic number
would be 13).

These simplifications are somewhat wasteful, but iconv_mime_encode()
with Q-encoding is wasteful anyway (see bug 66828[1]), and the proper
solution to convert the whole input to the desired output charset
upfront, and applying the encoding afterwards appears too much a change
for the stable releases.

[1] <https://bugs.php.net/66828>

NEWS

@@ -16,6 +16,9 @@ PHP                                                                        NEWS
   . Fixed bug #76285 (DOMDocument::formatOutput attribute sometimes ignored).
     (Andrew Nester, Laruence, Anatol)
 
+- iconv:
+  . Fixed bug #53891 (iconv_mime_encode() fails to Q-encode UTF-8 string). (cmb)
+
 - libxml:
   . Fixed bug #76777 ("public id" parameter of libxml_set_external_entity_loader
     callback undefined). (Ville Hukkamäki)

ext/iconv/iconv.c

@@ -1224,8 +1224,9 @@ static php_iconv_err_t _php_iconv_mime_encode(smart_str *pretval, const char *fn
 	do {
 		size_t prev_in_left;
 		size_t out_size;
+		size_t encoded_word_min_len = sizeof("=??X??=")-1 + out_charset_len + (enc_scheme == PHP_ICONV_ENC_SCHEME_BASE64 ? 4 : 3);
 
-		if (char_cnt < (out_charset_len + 12)) {
+		if (char_cnt < encoded_word_min_len + lfchars_len + 1) {
 			/* lfchars must be encoded in ASCII here*/
 			smart_str_appendl(pretval, lfchars, lfchars_len);
 			smart_str_appendc(pretval, ' ');

ext/iconv/tests/bug53891.phpt

@@ -0,0 +1,22 @@
+--TEST--
+Bug #53891 (iconv_mime_encode() fails to Q-encode UTF-8 string)
+--SKIPIF--
+<?php
+if (!extension_loaded('iconv')) die('skip iconv extension not available');
+?>
+--FILE--
+<?php
+$preferences = array(
+    'scheme' => 'Q',
+    'input-charset'  => 'utf-8',
+    'output-charset' => 'utf-8',
+    'line-length' => 74,
+    'line-break-chars' => "\r\n",
+);
+var_dump(iconv_mime_encode('subject', "d obeybiubrsfqllpdtpge…", $preferences));
+?>
+===DONE===
+--EXPECT--
+string(81) "subject: =?utf-8?Q?d=20obeybiubrsfqllp?==?utf-8?Q?dtpge?=
+ =?utf-8?Q?=E2=80=A6?="
+===DONE===