avformat/file: seek back to initial position for fd protocol

So user's fd can be passed to libavformat multiple times in sequence
without changing the position.

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

libavformat/file.c

@@ -98,6 +98,7 @@ typedef struct FileContext {
 #if HAVE_DIRENT_H
     DIR *dir;
 #endif
+    int64_t initial_pos;
 } FileContext;
 
 static const AVOption file_options[] = {
@@ -218,7 +219,12 @@ static int fd_dup(URLContext *h, int oldfd)
 static int file_close(URLContext *h)
 {
     FileContext *c = h->priv_data;
-    int ret = close(c->fd);
+    int ret;
+
+    if (c->initial_pos >= 0 && !h->is_streamed)
+        lseek(c->fd, c->initial_pos, SEEK_SET);
+
+    ret = close(c->fd);
     return (ret == -1) ? AVERROR(errno) : 0;
 }
 
@@ -286,6 +292,7 @@ static int file_open(URLContext *h, const char *filename, int flags)
 
     av_strstart(filename, "file:", &filename);
 
+    c->initial_pos = -1;
     if (flags & AVIO_FLAG_WRITE && flags & AVIO_FLAG_READ) {
         access = O_CREAT | O_RDWR;
         if (c->trunc)
@@ -494,6 +501,11 @@ static int fd_open(URLContext *h, const char *filename, int flags)
     if (c->fd == -1)
         return AVERROR(errno);
 
+    if (h->is_streamed)
+        c->initial_pos = -1;
+    else
+        c->initial_pos = lseek(c->fd, 0, SEEK_CUR);
+
     return 0;
 }