Subject: Re: [PATCH 1/2] http: reset POSTFIELDSIZE when clearing curl handle

From: Patrick Steinhardt <[email-masked]>
Date: Wed, 3 Apr 2024 08:30:54 +0200

On Tue, Apr 02, 2024 at 04:05:17PM -0400, Jeff King wrote:
> In get_active_slot(), we return a CURL handle that may have been used
> before (reusing them is good because it lets curl reuse the same
> connection across many requests). We set a few curl options back to
> defaults that may have been modified by previous requests.
> 
> We reset POSTFIELDS to NULL, but do not reset POSTFIELDSIZE (which
> defaults to "-1"). This usually doesn't matter because most POSTs will
> set both fields together anyway. But there is one exception: when
> handling a large request in remote-curl's post_rpc(), we don't set
> _either_, and instead set a READFUNCTION to stream data into libcurl.
> 
> This can interact weirdly with a stale POSTFIELDSIZE setting, because
> curl will assume it should read only some set number of bytes from our
> READFUNCTION. However, it has worked in practice because we also
> manually set a "Transfer-Encoding: chunked" header, which libcurl uses
> as a clue to set the POSTFIELDSIZE to -1 itself.
> 
> So everything works, but we're better off resetting the size manually
> for a few reasons:
> 
>   - there was a regression in curl 8.7.0 where the chunked header
>     detection didn't kick in, causing any large HTTP requests made by
>     Git to fail. This has since been fixed (but not yet released). In
>     the issue, curl folks recommended setting it explicitly to -1:
> 
>       https://github.com/curl/curl/issues/13229#issuecomment-2029826058
> 
>     and it indeed works around the regression. So even though it won't
>     be strictly necessary after the fix there, this will help folks who
>     end up using the affected libcurl versions.
> 
>   - it's consistent with what a new curl handle would look like. Since
>     get_active_slot() may or may not return a used handle, this reduces
>     the possibility of heisenbugs that only appear with certain request
>     patterns.
> 
> Note that the recommendation in the curl issue is to actually drop the
> manual Transfer-Encoding header. Modern libcurl will add the header
> itself when streaming from a READFUNCTION. However, that code wasn't
> added until 802aa5ae2 (HTTP: use chunked Transfer-Encoding for HTTP_POST
> if size unknown, 2019-07-22), which is in curl 7.66.0. We claim to
> support back to 7.19.5, so those older versions still need the manual
> header.
> 
> Signed-off-by: Jeff King <[email-masked]>
> ---
>  http.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/http.c b/http.c
> index e73b136e58..3d80bd6116 100644
> --- a/http.c
> +++ b/http.c
> @@ -1452,6 +1452,7 @@ struct active_request_slot *get_active_slot(void)
>  	curl_easy_setopt(slot->curl, CURLOPT_READFUNCTION, NULL);
>  	curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, NULL);
>  	curl_easy_setopt(slot->curl, CURLOPT_POSTFIELDS, NULL);
> +	curl_easy_setopt(slot->curl, CURLOPT_POSTFIELDSIZE, -1L);
>  	curl_easy_setopt(slot->curl, CURLOPT_UPLOAD, 0);
>  	curl_easy_setopt(slot->curl, CURLOPT_HTTPGET, 1);
>  	curl_easy_setopt(slot->curl, CURLOPT_FAILONERROR, 1);

Can't we refactor this code to instead use `curl_easy_reset()`? That
function already resets most of the data we want to reset and would also
end up setting `POSFIELDSIZE = -1` via `Curl_init_userdefined()`. So
wouldn't the following be a more sensible fix?

diff --git a/http.c b/http.c
index e73b136e58..e5f5bc23db 100644
--- a/http.c
+++ b/http.c
@@ -1442,20 +1442,14 @@ struct active_request_slot *get_active_slot(void)
 	slot->finished = NULL;
 	slot->callback_data = NULL;
 	slot->callback_func = NULL;
+	curl_easy_reset(slot->curl);
 	curl_easy_setopt(slot->curl, CURLOPT_COOKIEFILE, curl_cookie_file);
 	if (curl_save_cookies)
 		curl_easy_setopt(slot->curl, CURLOPT_COOKIEJAR, curl_cookie_file);
 	curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, pragma_header);
 	curl_easy_setopt(slot->curl, CURLOPT_RESOLVE, host_resolutions);
 	curl_easy_setopt(slot->curl, CURLOPT_ERRORBUFFER, curl_errorstr);
-	curl_easy_setopt(slot->curl, CURLOPT_CUSTOMREQUEST, NULL);
-	curl_easy_setopt(slot->curl, CURLOPT_READFUNCTION, NULL);
-	curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, NULL);
-	curl_easy_setopt(slot->curl, CURLOPT_POSTFIELDS, NULL);
-	curl_easy_setopt(slot->curl, CURLOPT_UPLOAD, 0);
-	curl_easy_setopt(slot->curl, CURLOPT_HTTPGET, 1);
 	curl_easy_setopt(slot->curl, CURLOPT_FAILONERROR, 1);
-	curl_easy_setopt(slot->curl, CURLOPT_RANGE, NULL);
 
 	/*
 	 * Default following to off unless "ALWAYS" is configured; this gives

Patrick