avformat/hls: add http_multiple option

This improves network throughput of the hls demuxer by avoiding
the latency introduced by downloading segments one at a time.

The problem is particularly noticable over high-latency network
connections: for instance, if RTT is 250ms, there will a 250ms idle
period between when one segment response is read and the next one
starts.

The obvious solution to this is to use HTTP pipelining, where a
second request can be sent (on the persistent http/1.1 connection)
before the first response is fully read. Unfortunately the way the
http protocol is implemented in avformat makes implementing pipleining
very complex.

Instead, this commit simulates pipelining using two separate persistent
http connections. This has the advantage of working independently of
the http_persistent option, and can be used with http/1.0 servers as
well. The pair of connections is swapped every time a new segment starts
downloading, and a request for the next segment is sent on the secondary
connection right away. This means the second response will be ready and
waiting by the time the current response is fully read.

Signed-off-by: Aman Gupta <aman@tmm1.net>
Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>

doc/demuxers.texi

@@ -320,6 +320,10 @@ Default value is 1000.
 @item http_persistent
 Use persistent HTTP connections. Applicable only for HTTP streams.
 Enabled by default.
+
+@item http_multiple
+Use multiple HTTP connections for downloading HTTP segments.
+Enabled by default.
 @end table
 
 @section image2

libavformat/hls.c

@@ -96,6 +96,8 @@ struct playlist {
     uint8_t* read_buffer;
     AVIOContext *input;
     int input_read_done;
+    AVIOContext *input_next;
+    int input_next_requested;
     AVFormatContext *parent;
     int index;
     AVFormatContext *ctx;
@@ -209,6 +211,7 @@ typedef struct HLSContext {
     char *allowed_extensions;
     int max_reload;
     int http_persistent;
+    int http_multiple;
     AVIOContext *playlist_pb;
 } HLSContext;
 
@@ -261,6 +264,9 @@ static void free_playlist_list(HLSContext *c)
         if (pls->input)
             ff_format_io_close(c->ctx, &pls->input);
         pls->input_read_done = 0;
+        if (pls->input_next)
+            ff_format_io_close(c->ctx, &pls->input_next);
+        pls->input_next_requested = 0;
         if (pls->ctx) {
             pls->ctx->pb = NULL;
             avformat_close_input(&pls->ctx);
@@ -924,6 +930,14 @@ static struct segment *current_segment(struct playlist *pls)
     return pls->segments[pls->cur_seq_no - pls->start_seq_no];
 }
 
+static struct segment *next_segment(struct playlist *pls)
+{
+    int n = pls->cur_seq_no - pls->start_seq_no + 1;
+    if (n >= pls->n_segments)
+        return NULL;
+    return pls->segments[n];
+}
+
 enum ReadFromURLMode {
     READ_NORMAL,
     READ_COMPLETE,
@@ -1365,6 +1379,7 @@ static int read_data(void *opaque, uint8_t *buf, int buf_size)
     int ret;
     int just_opened = 0;
     int reload_count = 0;
+    struct segment *seg;
 
 restart:
     if (!v->needed)
@@ -1372,7 +1387,6 @@ restart:
 
     if (!v->input || (c->http_persistent && v->input_read_done)) {
         int64_t reload_interval;
-        struct segment *seg;
 
         /* Check that the playlist is still needed before opening a new
          * segment. */
@@ -1430,11 +1444,18 @@ reload:
         if (ret)
             return ret;
 
-        ret = open_input(c, v, seg, &v->input);
+        if (c->http_multiple && av_strstart(seg->url, "http", NULL) && v->input_next_requested) {
+            FFSWAP(AVIOContext *, v->input, v->input_next);
+            v->input_next_requested = 0;
+            ret = 0;
+        } else {
+            ret = open_input(c, v, seg, &v->input);
+        }
         if (ret < 0) {
             if (ff_check_interrupt(c->interrupt_callback))
                 return AVERROR_EXIT;
-            av_log(v->parent, AV_LOG_WARNING, "Failed to open segment of playlist %d\n",
+            av_log(v->parent, AV_LOG_WARNING, "Failed to open segment %d of playlist %d\n",
+                   v->cur_seq_no,
                    v->index);
             v->cur_seq_no += 1;
             goto reload;
@@ -1442,6 +1463,20 @@ reload:
         just_opened = 1;
     }
 
+    seg = next_segment(v);
+    if (c->http_multiple && !v->input_next_requested && seg) {
+        ret = open_input(c, v, seg, &v->input_next);
+        if (ret < 0) {
+            if (ff_check_interrupt(c->interrupt_callback))
+                return AVERROR_EXIT;
+            av_log(v->parent, AV_LOG_WARNING, "Failed to open segment %d of playlist %d\n",
+                   v->cur_seq_no + 1,
+                   v->index);
+        } else {
+            v->input_next_requested = 1;
+        }
+    }
+
     if (v->init_sec_buf_read_offset < v->init_sec_data_len) {
         /* Push init section out first before first actual segment */
         int copy_size = FFMIN(v->init_sec_data_len - v->init_sec_buf_read_offset, buf_size);
@@ -1964,6 +1999,9 @@ static int recheck_discard_flags(AVFormatContext *s, int first)
             if (pls->input)
                 ff_format_io_close(pls->parent, &pls->input);
             pls->input_read_done = 0;
+            if (pls->input_next)
+                ff_format_io_close(pls->parent, &pls->input_next);
+            pls->input_next_requested = 0;
             pls->needed = 0;
             changed = 1;
             av_log(s, AV_LOG_INFO, "No longer receiving playlist %d\n", i);
@@ -2199,6 +2237,9 @@ static int hls_read_seek(AVFormatContext *s, int stream_index,
         if (pls->input)
             ff_format_io_close(pls->parent, &pls->input);
         pls->input_read_done = 0;
+        if (pls->input_next)
+            ff_format_io_close(pls->parent, &pls->input_next);
+        pls->input_next_requested = 0;
         av_packet_unref(&pls->pkt);
         reset_packet(&pls->pkt);
         pls->pb.eof_reached = 0;
@@ -2255,6 +2296,8 @@ static const AVOption hls_options[] = {
         OFFSET(max_reload), AV_OPT_TYPE_INT, {.i64 = 1000}, 0, INT_MAX, FLAGS},
     {"http_persistent", "Use persistent HTTP connections",
         OFFSET(http_persistent), AV_OPT_TYPE_BOOL, {.i64 = 1}, 0, 1, FLAGS },
+    {"http_multiple", "Use multiple HTTP connections for fetching segments",
+        OFFSET(http_multiple), AV_OPT_TYPE_BOOL, {.i64 = 1}, 0, 1, FLAGS},
     {NULL}
 };