avformat/aviobuf: Add ffio_ensure_seekback()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/avio_internal.h

@@ -71,6 +71,15 @@ uint64_t ffio_read_varlen(AVIOContext *bc);
 /** @warning must be called before any I/O */
 int ffio_set_buf_size(AVIOContext *s, int buf_size);
 
+/**
+ * Ensures that the requested seekback buffer size will be available
+ *
+ * Will ensure that when reading sequentially up to buf_size, seeking
+ * within the current pos and pos+buf_size is possible.
+ * Once the stream position moves outside this window this gurantee is lost.
+ */
+int ffio_ensure_seekback(AVIOContext *s, int buf_size);
+
 int ffio_limit(AVIOContext *s, int size);
 
 void ffio_init_checksum(AVIOContext *s,

libavformat/aviobuf.c

@@ -725,6 +725,31 @@ int ffio_fdopen(AVIOContext **s, URLContext *h)
     return 0;
 }
 
+int ffio_ensure_seekback(AVIOContext *s, int buf_size)
+{
+    uint8_t *buffer;
+    int max_buffer_size = s->max_packet_size ?
+                          s->max_packet_size : IO_BUFFER_SIZE;
+
+    buf_size += s->buf_ptr - s->buffer + max_buffer_size;
+
+    if (buf_size < s->buffer_size || s->seekable)
+        return 0;
+    av_assert0(!s->write_flag);
+
+    buffer = av_malloc(buf_size);
+    if (!buffer)
+        return AVERROR(ENOMEM);
+
+    memcpy(buffer, s->buffer, s->buffer_size);
+    av_free(s->buffer);
+    s->buf_ptr = buffer + (s->buf_ptr - s->buffer);
+    s->buf_end = buffer + (s->buf_end - s->buffer);
+    s->buffer = buffer;
+    s->buffer_size = buf_size;
+    return 0;
+}
+
 int ffio_set_buf_size(AVIOContext *s, int buf_size)
 {
     uint8_t *buffer;