rtmp: Don't blindly skip the 4 trailer bytes from the FLV packets

If not enough bytes are available, keep track of them and skip
them on next call.

In practice, if these trailer bytes are written in a separate
call, there is no other data written in this call, making it
fall into the "FLV packet too small" case currently - working,
but not as intended.

This patch makes the code more robust, handling all cases
except for having the FLV packet header split over multiple
write calls.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtmpproto.c

@@ -71,6 +71,7 @@ typedef struct RTMPContext {
     uint32_t      client_report_size;         ///< number of bytes after which client should report to server
     uint32_t      bytes_read;                 ///< number of bytes read from server
     uint32_t      last_bytes_read;            ///< number of bytes read last reported to server
+    int           skip_bytes;                 ///< number of bytes to skip from the input FLV stream in the next write call
 } RTMPContext;
 
 #define PLAYER_KEY_OPEN_PART_LEN 30   ///< length of partial key used for first client digest signing
@@ -925,7 +926,16 @@ static int rtmp_write(URLContext *s, const uint8_t *buf, int size)
     uint32_t ts;
     const uint8_t *buf_temp = buf;
 
-    if (!rt->flv_off && size < 11) {
+    if (rt->skip_bytes) {
+        int skip = FFMIN(rt->skip_bytes, size);
+        buf_temp       += skip;
+        size_temp      -= skip;
+        rt->skip_bytes -= skip;
+        if (size_temp <= 0)
+            return size;
+    }
+
+    if (!rt->flv_off && size_temp < 11) {
         av_log(s, AV_LOG_DEBUG, "FLV packet too small %d\n", size);
         return 0;
     }
@@ -974,8 +984,14 @@ static int rtmp_write(URLContext *s, const uint8_t *buf, int size)
         }
 
         if (rt->flv_off == rt->flv_size) {
-            bytestream_get_be32(&buf_temp);
-            size_temp -= 4;
+            if (size_temp < 4) {
+                rt->skip_bytes = 4 - size_temp;
+                buf_temp += size_temp;
+                size_temp = 0;
+            } else {
+                bytestream_get_be32(&buf_temp);
+                size_temp -= 4;
+            }
             ff_rtmp_packet_write(rt->stream, &rt->out_pkt, rt->chunk_size, rt->prev_pkt[1]);
             ff_rtmp_packet_destroy(&rt->out_pkt);
             rt->flv_size = 0;