flvdec: Validate index entries added from metadata while reading

By validating the index entries while reading, we don't need to
seek at startup to validate the entries. If the error in the
index entries is not pointing to (our definition of) the start
of packets, and there is an index entry pointing at some of the
first packets after the metadata, the invalid index can be discarded
almost immediately.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/flvdec.c

@@ -39,12 +39,20 @@
 #define KEYFRAMES_TIMESTAMP_TAG  "times"
 #define KEYFRAMES_BYTEOFFSET_TAG "filepositions"
 
+#define VALIDATE_INDEX_TS_THRESH 2500
+
 typedef struct {
     int wrong_dts; ///< wrong dts due to negative cts
     uint8_t *new_extradata[2];
     int      new_extradata_size[2];
     int      last_sample_rate;
     int      last_channels;
+    struct {
+        int64_t dts;
+        int64_t pos;
+    } validate_index[2];
+    int validate_next;
+    int validate_count;
 } FLVContext;
 
 static int flv_probe(AVProbeData *p)
@@ -137,6 +145,7 @@ static int amf_get_string(AVIOContext *ioc, char *buffer, int buffsize) {
 }
 
 static int parse_keyframes_index(AVFormatContext *s, AVIOContext *ioc, AVStream *vstream, int64_t max_pos) {
+    FLVContext *flv = s->priv_data;
     unsigned int arraylen = 0, timeslen = 0, fileposlen = 0, i;
     double num_val;
     char str_val[256];
@@ -206,11 +215,17 @@ static int parse_keyframes_index(AVFormatContext *s, AVIOContext *ioc, AVStream
         }
     }
 
-    if (!ret && timeslen == fileposlen)
-        for (i = 0; i < fileposlen; i++)
+    if (!ret && timeslen == fileposlen) {
+        for (i = 0; i < fileposlen; i++) {
             av_add_index_entry(vstream, filepositions[i], times[i]*1000,
                                0, 0, AVINDEX_KEYFRAME);
-    else
+            if (i < 2) {
+                flv->validate_index[i].pos = filepositions[i];
+                flv->validate_index[i].dts = times[i] * 1000;
+                flv->validate_count = i + 1;
+            }
+        }
+    } else
         av_log(s, AV_LOG_WARNING, "Invalid keyframes object, skipping.\n");
 
 finish:
@@ -434,6 +449,22 @@ static int flv_queue_extradata(FLVContext *flv, AVIOContext *pb, int stream,
     return 0;
 }
 
+static void clear_index_entries(AVFormatContext *s, int64_t pos)
+{
+    int i, j, out;
+    av_log(s, AV_LOG_WARNING, "Found invalid index entries, clearing the index.\n");
+    for (i = 0; i < s->nb_streams; i++) {
+        AVStream *st = s->streams[i];
+        /* Remove all index entries that point to >= pos */
+        out = 0;
+        for (j = 0; j < st->nb_index_entries; j++) {
+            if (st->index_entries[j].pos < pos)
+                st->index_entries[out++] = st->index_entries[j];
+        }
+        st->nb_index_entries = out;
+    }
+}
+
 static int flv_read_packet(AVFormatContext *s, AVPacket *pkt)
 {
     FLVContext *flv = s->priv_data;
@@ -455,6 +486,22 @@ static int flv_read_packet(AVFormatContext *s, AVPacket *pkt)
     avio_skip(s->pb, 3); /* stream id, always 0 */
     flags = 0;
 
+    if (flv->validate_next < flv->validate_count) {
+        int64_t validate_pos = flv->validate_index[flv->validate_next].pos;
+        if (pos == validate_pos) {
+            if (FFABS(dts - flv->validate_index[flv->validate_next].dts) <=
+                VALIDATE_INDEX_TS_THRESH) {
+                flv->validate_next++;
+            } else {
+                clear_index_entries(s, validate_pos);
+                flv->validate_count = 0;
+            }
+        } else if (pos > validate_pos) {
+            clear_index_entries(s, validate_pos);
+            flv->validate_count = 0;
+        }
+    }
+
     if(size == 0)
         continue;
 
@@ -633,6 +680,8 @@ leave:
 static int flv_read_seek(AVFormatContext *s, int stream_index,
     int64_t ts, int flags)
 {
+    FLVContext *flv = s->priv_data;
+    flv->validate_count = 0;
     return avio_seek_time(s->pb, stream_index, ts, flags);
 }