[FFmpeg-devel,FFmpeg-trac] #7592(avformat:new): FFmpeg download data twice

From 71cadd60a2f71d2d39bd7999e080c206291b55da Mon Sep 17 00:00:00 2001
From: Adrian Trzcinski <atrzcinski@vewd.com>
Date: Tue, 4 Dec 2018 19:00:22 +0100
Subject: [PATCH] avformat/avio: fill_buffer drops less data

Signed-off-by: Adrian Trzcinski <atrzcinski@vewd.com>
---
 libavformat/avio.h    |  1 +
 libavformat/aviobuf.c | 21 ++++++++++++++++-----
 libavformat/utils.c   |  1 +
 3 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/libavformat/avio.h b/libavformat/avio.h
index 75912ce6be..c44e1c7e9c 100644
--- a/libavformat/avio.h
+++ b/libavformat/avio.h
@@ -225,6 +225,7 @@  typedef struct AVIOContext {
      */
     unsigned char *buffer;  /**< Start of the buffer. */
     int buffer_size;        /**< Maximum buffer size */
+    int time_units_in_buffer; /**< Number of time units in buffer */
     unsigned char *buf_ptr; /**< Current position in the buffer */
     unsigned char *buf_end; /**< End of the data, may be less than
                                  buffer+buffer_size if the read function returned
diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
index 5a33f82950..afccc61491 100644
--- a/libavformat/aviobuf.c
+++ b/libavformat/aviobuf.c
@@ -92,6 +92,7 @@  int ffio_init_context(AVIOContext *s,
     s->buffer      = buffer;
     s->orig_buffer_size =
     s->buffer_size = buffer_size;
+    s->time_units_in_buffer = 0;
     s->buf_ptr     = buffer;
     s->buf_ptr_max = buffer;
     s->opaque      = opaque;
@@ -550,9 +551,19 @@  static void fill_buffer(AVIOContext *s)
 {
     int max_buffer_size = s->max_packet_size ?
                           s->max_packet_size : IO_BUFFER_SIZE;
-    uint8_t *dst        = s->buf_end - s->buffer + max_buffer_size < s->buffer_size ?
-                          s->buf_end : s->buffer;
-    int len             = s->buffer_size - (dst - s->buffer);
+    int need_erase = !(s->buf_end - s->buffer + max_buffer_size < s->buffer_size);
+    uint8_t *dst = need_erase ? s->buffer : s->buf_end;
+    int len;
+
+    if (need_erase && s->time_units_in_buffer > 1) {
+        const int bytes_in_time_unit = (s->buf_end - s->buffer) / s->time_units_in_buffer;
+        const int offset = bytes_in_time_unit * (s->time_units_in_buffer - 1);
+        memcpy(s->buffer, s->buffer + offset, bytes_in_time_unit);
+        dst = s->buf_end = s->buffer + bytes_in_time_unit;
+        s->buf_ptr = s->buf_ptr - s->buffer < offset ? s->buffer : s->buf_ptr - offset;
+    }
+
+    len = s->buffer_size - (dst - s->buffer);
 
     /* can't fill the buffer without read_packet, just set EOF if appropriate */
     if (!s->read_packet && s->buf_ptr >= s->buf_end)
@@ -562,7 +573,7 @@  static void fill_buffer(AVIOContext *s)
     if (s->eof_reached)
         return;
 
-    if (s->update_checksum && dst == s->buffer) {
+    if (s->update_checksum && need_erase) {
         if (s->buf_end > s->checksum_ptr)
             s->checksum = s->update_checksum(s->checksum, s->checksum_ptr,
                                              s->buf_end - s->checksum_ptr);
@@ -571,7 +582,7 @@  static void fill_buffer(AVIOContext *s)
 
     /* make buffer smaller in case it ended up large after probing */
     if (s->read_packet && s->orig_buffer_size && s->buffer_size > s->orig_buffer_size) {
-        if (dst == s->buffer && s->buf_ptr != dst) {
+        if (need_erase  && s->buf_ptr != dst) {
             int ret = ffio_set_buf_size(s, s->orig_buffer_size);
             if (ret < 0)
                 av_log(s, AV_LOG_WARNING, "Failed to decrease buffer size\n");
diff --git a/libavformat/utils.c b/libavformat/utils.c
index 93e588ee1e..62deaa710b 100644
--- a/libavformat/utils.c
+++ b/libavformat/utils.c
@@ -2138,6 +2138,7 @@  void ff_configure_buffers_for_index(AVFormatContext *s, int64_t time_tolerance)
         av_log(s, AV_LOG_VERBOSE, "Reconfiguring buffers to size %"PRId64"\n", pos_delta);
         ffio_set_buf_size(s->pb, pos_delta);
         s->pb->short_seek_threshold = FFMAX(s->pb->short_seek_threshold, pos_delta/2);
+        s->pb->time_units_in_buffer = 2;
     }
 
     if (skip < (1<<23)) {
-- 
2.19.2