avformat/hls: Fix regression with ranged media segments

Commit 81306fd4bdf ("hls: eliminate ffurl_* usage", merged in d0fc5de3)
changed the hls demuxer to use AVIOContext instead of URLContext for its
HTTP requests.

HLS demuxer uses the "offset" option of the http demuxer, requesting
the initial file offset for the I/O (http URLProtocol uses the "Range:"
HTTP header to try to accommodate that).

However, the code in libavformat/aviobuf.c seems to be doing its own
accounting for the current file offset (AVIOContext.pos), with the
assumption that the initial offset is always zero.

HLS demuxer does an explicit seek after open_url to account for cases
where the "offset" was not effective (due to the URL being a local file
or the HTTP server not obeying it), which should be a no-op in case the
file offset is already at that position.

However, since aviobuf.c code thinks the starting offset is 0, this
doesn't work properly.

This breaks retrieval of ranged media segments.

To fix the regression, just drop the seek call from the HLS demuxer when
the HTTP(S) protocol is used.

libavformat/hls.c

@@ -590,7 +590,7 @@ static void update_options(char **dest, const char *name, void *src)
 }
 
 static int open_url(AVFormatContext *s, AVIOContext **pb, const char *url,
-                    AVDictionary *opts, AVDictionary *opts2)
+                    AVDictionary *opts, AVDictionary *opts2, int *is_http)
 {
     HLSContext *c = s->priv_data;
     AVDictionary *tmp = NULL;
@@ -631,6 +631,9 @@ static int open_url(AVFormatContext *s, AVIOContext **pb, const char *url,
 
     av_dict_free(&tmp);
 
+    if (is_http)
+        *is_http = av_strstart(proto_name, "http", NULL);
+
     return ret;
 }
 
@@ -1072,6 +1075,7 @@ static int open_input(HLSContext *c, struct playlist *pls, struct segment *seg)
 {
     AVDictionary *opts = NULL;
     int ret;
+    int is_http = 0;
 
     // broker prior HTTP options that should be consistent across requests
     av_dict_set(&opts, "user-agent", c->user_agent, 0);
@@ -1091,13 +1095,13 @@ static int open_input(HLSContext *c, struct playlist *pls, struct segment *seg)
            seg->url, seg->url_offset, pls->index);
 
     if (seg->key_type == KEY_NONE) {
-        ret = open_url(pls->parent, &pls->input, seg->url, c->avio_opts, opts);
+        ret = open_url(pls->parent, &pls->input, seg->url, c->avio_opts, opts, &is_http);
     } else if (seg->key_type == KEY_AES_128) {
         AVDictionary *opts2 = NULL;
         char iv[33], key[33], url[MAX_URL_SIZE];
         if (strcmp(seg->key, pls->key_url)) {
             AVIOContext *pb;
-            if (open_url(pls->parent, &pb, seg->key, c->avio_opts, opts) == 0) {
+            if (open_url(pls->parent, &pb, seg->key, c->avio_opts, opts, NULL) == 0) {
                 ret = avio_read(pb, pls->key, sizeof(pls->key));
                 if (ret != sizeof(pls->key)) {
                     av_log(NULL, AV_LOG_ERROR, "Unable to read key file %s\n",
@@ -1122,7 +1126,7 @@ static int open_input(HLSContext *c, struct playlist *pls, struct segment *seg)
         av_dict_set(&opts2, "key", key, 0);
         av_dict_set(&opts2, "iv", iv, 0);
 
-        ret = open_url(pls->parent, &pls->input, url, opts2, opts);
+        ret = open_url(pls->parent, &pls->input, url, opts2, opts, &is_http);
 
         av_dict_free(&opts2);
 
@@ -1140,8 +1144,15 @@ static int open_input(HLSContext *c, struct playlist *pls, struct segment *seg)
 
     /* Seek to the requested position. If this was a HTTP request, the offset
      * should already be where want it to, but this allows e.g. local testing
-     * without a HTTP server. */
-    if (ret == 0 && seg->key_type == KEY_NONE && seg->url_offset) {
+     * without a HTTP server.
+     *
+     * This is not done for HTTP at all as avio_seek() does internal bookkeeping
+     * of file offset which is out-of-sync with the actual offset when "offset"
+     * AVOption is used with http protocol, causing the seek to not be a no-op
+     * as would be expected. Wrong offset received from the server will not be
+     * noticed without the call, though.
+     */
+    if (ret == 0 && !is_http && seg->key_type == KEY_NONE && seg->url_offset) {
         int64_t seekret = avio_seek(pls->input, seg->url_offset, SEEK_SET);
         if (seekret < 0) {
             av_log(pls->parent, AV_LOG_ERROR, "Unable to seek to offset %"PRId64" of HLS segment '%s'\n", seg->url_offset, seg->url);