avformat/libcurl: no error is ever retried when opening with offset > 0 (HLS byte-range segments, resumed opens)

[ronag]

Summary

http_error_is_recoverable() and curl_error_is_recoverable() both start with a guard meant for "mid-stream on a non-seekable resource":

    if (!c->seekable && c->request_start > 0 && !c->reconnect_streamed)
        return 0;

Before the first reply has been probed, c->seekable is still 0 (meaning "unknown") and c->request_start == offset. So every open with offset > 0 is classified as "non-seekable, already streaming", and nothing is retried: not 429/503 from the default reconnect_on_http_error list, and not recoverable curl errors. The main victims are HLS EXT-X-BYTERANGE segments, where hls.c passes offset/end_offset, and applications resuming a download with offset.

Location

• FFmpeg/libavformat/libcurl.c

  Lines 194 to 199 in fe63f8f

  	static int http_error_is_recoverable(CurlContext *c, long http_status)
  	{
  	if (!c->reconnect_on_http_error || !http_status)
  	return 0;
  	if (!c->seekable && c->request_start > 0 && !c->reconnect_streamed)
  	return 0;

• FFmpeg/libavformat/libcurl.c

  Lines 214 to 217 in fe63f8f

  	static int curl_error_is_recoverable(CurlContext *c, CURLcode code)
  	{
  	if (!c->seekable && c->request_start > 0 && !c->reconnect_streamed)
  	return 0;

Reproduction

Server answers the first request with 503 + Retry-After: 1, then 206 (harness from #71):

$ t libcurl:http://127.0.0.1:18080/once503ra-b offset=1000
open: -1482175992 (Server returned 5XX Server Error reply)

The server sees a single request (Range: bytes=1000-), so no retry was attempted. The same resource with offset=0 does enter the retry path (which then fails for the reason in #71).

Suggested fix

Only apply the guard once the resource has actually been probed: if (c->probed && !c->seekable && c->request_start > 0 && !c->reconnect_streamed). Alternatively, key it on "bytes already delivered to the caller" rather than on request_start.

[haasn]

Fixed upstream, need to resolve this when rebasing our deviation of libcurl.c onto the upstream version