Fix #80595: Resetting POSTFIELDS to empty array breaks request

This is mainly to work around curl/curl#6455,
but not building the mime structure for empty hashtables is a general
performance optimization, so we do not restrict it to affected cURL
versions (7.56.0 to 7.75.0).

The minor change to bug79033.phpt is unexpected, but should not matter
in practice.

Closes GH-6606.

Author: cmb69

NEWS

@@ -7,6 +7,9 @@ PHP                                                                        NEWS
   . Fixed bug #80384 (filter buffers entire read until file closed). (Adam
     Seitz, cmb)
 
+- Curl:
+  . Fixed bug #80595 (Resetting POSTFIELDS to empty array breaks request). (cmb)
+
 - Date:
   . Fixed bug #80376 (last day of the month causes runway cpu usage. (Derick)
 

ext/curl/interface.c

@@ -2969,7 +2969,14 @@ static int _php_curl_setopt(php_curl *ch, zend_long option, zval *zvalue) /* {{{
 
 		case CURLOPT_POSTFIELDS:
 			if (Z_TYPE_P(zvalue) == IS_ARRAY || Z_TYPE_P(zvalue) == IS_OBJECT) {
-				return build_mime_structure_from_hash(ch, zvalue);
+				if (zend_hash_num_elements(HASH_OF(zvalue)) == 0) {
+					/* no need to build the mime structure for empty hashtables;
+					   also works around https://github.com/curl/curl/issues/6455 */
+					curl_easy_setopt(ch->cp, CURLOPT_POSTFIELDS, "");
+					error = curl_easy_setopt(ch->cp, CURLOPT_POSTFIELDSIZE, 0);
+				} else {
+					return build_mime_structure_from_hash(ch, zvalue);
+				}
 			} else {
 #if LIBCURL_VERSION_NUM >= 0x071101
 				zend_string *tmp_str;

ext/curl/tests/bug79033.phpt

@@ -21,9 +21,10 @@ var_dump(curl_getinfo($ch)["request_header"]);
 string(%d) "array(0) {
 }
 "
-string(90) "POST /get.inc?test=post HTTP/1.1
+string(%d) "POST /get.inc?test=post HTTP/1.1
 Host: localhost:%d
 Accept: */*
 Content-Length: 0
+Content-Type: application/x-www-form-urlencoded
 
 "

ext/curl/tests/bug80595.phpt

@@ -0,0 +1,30 @@
+--TEST--
+Bug #80595 (Resetting POSTFIELDS to empty array breaks request)
+--SKIPIF--
+<?php include 'skipif.inc'; ?>
+--FILE--
+<?php
+include 'server.inc';
+$host = curl_cli_server_start();
+$ch = curl_init();
+curl_setopt_array($ch, [
+    CURLOPT_RETURNTRANSFER => true,
+    CURLOPT_POST           => true,
+    CURLOPT_URL            => "{$host}/get.inc?test=post",
+]);
+
+curl_setopt($ch, CURLOPT_POSTFIELDS, ['foo' => 'bar']);
+var_dump(curl_exec($ch));
+
+curl_setopt($ch, CURLOPT_POSTFIELDS, []);
+var_dump(curl_exec($ch));
+?>
+--EXPECT--
+string(43) "array(1) {
+  ["foo"]=>
+  string(3) "bar"
+}
+"
+string(13) "array(0) {
+}
+"