curl_error: return an empty string if no error occurred

[jay]

CURLOPT_ERRORBUFFER doc says "Do not rely on the contents of the
buffer unless an error code was returned." 1

Prior to this change the error buffer was returned even if no error had
occurred, and that buffer may contain incorrect information in such a
case. 2

Closes #xxxx

---

Also I noticed a comment "CURLE_PARTIAL_FILE is returned by HEAD requests". I don't see that happening unless HEAD was sent as a CURLOPT_CUSTOMREQUEST... The right way is CURLOPT_NOBODY, also see https://daniel.haxx.se/blog/2015/09/11/unnecessary-use-of-curl-x/

php-src/ext/curl/interface.c

Lines 3114 to 3120 in 54ef8d1

	error = curl_easy_perform(ch->cp);
	SAVE_CURL_ERROR(ch, error);
	/* CURLE_PARTIAL_FILE is returned by HEAD requests */
	if (error != CURLE_OK && error != CURLE_PARTIAL_FILE) {
	smart_str_free(&ch->handlers->write->buf);
	RETURN_FALSE;
	}

/cc @bagder @Schnitzel

[bagder]

Looks like a good fix.

That CURLE_PARTIAL_FILE thing looks like a separate bug. curl does not return that error for HEAD requests, only if you trick it with weirdo option combinations and then the user asked for it.

[nikic]

This should use RETURN_EMPTY_STRING().

[nikic]

Merged as 5025eb0 into 7.2+ together with the suggested tweak. Thanks!

Would you like to also submit a PR to drop the CURLE_PARTIAL_FILE check? At least the very least it should be fine for master, even if it breaks some code using incorrect options.