ftp: reconnect on seek

ABOR command usually takes long (FTP server implementation dependent).
It also produces different response codes from different servers.

It is save for ffmpeg to reconnect both connection during seek for two reasons:
1. Alreay mentioned heavy manner of ABOR command.
2. Server may disconnected due to no transfer (seek after a long pause in ffmpeg client)

libavformat/ftp.c

@@ -221,6 +221,14 @@ static int ftp_send_command(FTPContext *s, const char *command,
     return ftp_status(s, response, response_codes);
 }
 
+static void ftp_close_both_connections(FTPContext *s)
+{
+    ffurl_closep(&s->conn_control);
+    ffurl_closep(&s->conn_data);
+    s->position = 0;
+    s->state = DISCONNECTED;
+}
+
 static int ftp_auth(FTPContext *s)
 {
     const char *user = NULL, *pass = NULL;
@@ -398,29 +406,6 @@ static int ftp_type(FTPContext *s)
     return 0;
 }
 
-static int ftp_abort(FTPContext *s)
-{
-    int err;
-    const char *command = "ABOR\r\n";
-    const int abor_codes[] = {225, 226, 0};
-
-    if ((err = ftp_flush_control_input(s)) < 0)
-        return err;
-
-    if (s->state == DOWNLOADING) {
-        s->conn_control_block_flag = 0;
-        if ((err = ffurl_write(s->conn_control, command, strlen(command))) < 0)
-            return err;
-    }
-    ffurl_closep(&s->conn_data);
-    s->state = DISCONNECTED;
-
-    if (!ftp_status(s, NULL, abor_codes))
-        return AVERROR(EIO);
-
-    return 0;
-}
-
 static int ftp_restart(FTPContext *s, int64_t pos)
 {
     char command[CONTROL_BUFFER_SIZE];
@@ -502,9 +487,21 @@ static int ftp_connect_data_connection(URLContext *h)
         av_dict_free(&opts);
         if (err < 0)
             return err;
+
+        if (s->position)
+            if ((err = ftp_restart(s, s->position)) < 0)
+                return err;
     }
     s->state = READY;
-    s->position = 0;
+    return 0;
+}
+
+static int ftp_abort(URLContext *h)
+{
+    int err;
+    ftp_close_both_connections(h->priv_data);
+    if ((err = ftp_connect_control_connection(h)) < 0)
+        return err;
     return 0;
 }
 
@@ -518,6 +515,7 @@ static int ftp_open(URLContext *h, const char *url, int flags)
 
     s->state = DISCONNECTED;
     s->filesize = -1;
+    s->position = 0;
     s->conn_control_interrupt_cb.opaque = s;
     s->conn_control_interrupt_cb.callback = ftp_conn_control_block_control;
 
@@ -543,10 +541,8 @@ static int ftp_open(URLContext *h, const char *url, int flags)
     if (s->write_seekable != 1 && flags & AVIO_FLAG_WRITE)
         h->is_streamed = 1;
 
-    if ((err = ftp_connect_data_connection(h)) < 0)
-        goto fail;
-
     return 0;
+
   fail:
     av_log(h, AV_LOG_ERROR, "FTP open failed\n");
     ffurl_closep(&s->conn_control);
@@ -583,23 +579,15 @@ static int64_t ftp_seek(URLContext *h, int64_t pos, int whence)
     if  (h->is_streamed)
         return AVERROR(EIO);
 
-    if (new_pos < 0 || (s->filesize >= 0 && new_pos > s->filesize))
-        return AVERROR(EINVAL);
+    new_pos = FFMAX(0, new_pos);
+    if (s->filesize >= 0)
+        new_pos = FFMIN(s->filesize, new_pos);
 
     if (new_pos != s->position) {
-        /* close existing data connection */
-        if (s->state != READY) {
-            if (s->conn_data) {
-                /* abort existing transfer */
-                if ((err = ftp_abort(s)) < 0)
-                    return err;
-            }
-            /* open new data connection */
-            if ((err = ftp_connect_data_connection(h)) < 0)
-                return err;
-        }
-
-        if ((err = ftp_restart(s, new_pos)) < 0)
+        /* XXX: Full abort is a save solution here.
+           Some optimalizations are possible, but may lead to crazy states of FTP server.
+           The worst scenario would be when FTP server closed both connection due to no transfer. */
+        if ((err = ftp_abort(h)) < 0)
             return err;
 
         s->position = new_pos;
@@ -614,15 +602,20 @@ static int ftp_read(URLContext *h, unsigned char *buf, int size)
 
     av_dlog(h, "ftp protocol read %d bytes\n", size);
   retry:
+    if (s->state == DISCONNECTED) {
+        if ((err = ftp_connect_data_connection(h)) < 0)
+            return err;
+    }
     if (s->state == READY) {
-        ftp_retrieve(s);
+        if ((err = ftp_retrieve(s)) < 0)
+            return err;
     }
     if (s->conn_data && s->state == DOWNLOADING) {
         read = ffurl_read(s->conn_data, buf, size);
         if (read >= 0) {
             s->position += read;
             if (s->position >= s->filesize) {
-                if (ftp_abort(s) < 0)
+                if (ftp_abort(h) < 0)
                     return AVERROR(EIO);
             }
         }
@@ -631,16 +624,12 @@ static int ftp_read(URLContext *h, unsigned char *buf, int size)
             /* TODO: Consider retry before reconnect */
             int64_t pos = s->position;
             av_log(h, AV_LOG_INFO, "Reconnect to FTP server.\n");
-            ffurl_closep(&s->conn_control);
-            ffurl_closep(&s->conn_data);
-            s->position = 0;
-            s->state = DISCONNECTED;
-            if ((err = ftp_connect_control_connection(h)) < 0) {
-                av_log(h, AV_LOG_ERROR, "Reconnect failed\n");
+            if ((err = ftp_abort(h)) < 0) {
+                av_log(h, AV_LOG_ERROR, "Reconnect failed.\n");
                 return err;
             }
             if ((err = ftp_seek(h, pos, SEEK_SET)) < 0) {
-                av_dlog(h, "Seek failed after reconnect\n");
+                av_log(h, AV_LOG_ERROR, "Position cannot be restored.\n");
                 return err;
             }
             if (!retry_done) {
@@ -657,13 +646,19 @@ static int ftp_read(URLContext *h, unsigned char *buf, int size)
 
 static int ftp_write(URLContext *h, const unsigned char *buf, int size)
 {
+    int err;
     FTPContext *s = h->priv_data;
     int written;
 
     av_dlog(h, "ftp protocol write %d bytes\n", size);
 
+    if (s->state == DISCONNECTED) {
+        if ((err = ftp_connect_data_connection(h)) < 0)
+            return err;
+    }
     if (s->state == READY) {
-        ftp_store(s);
+        if ((err = ftp_store(s)) < 0)
+            return err;
     }
     if (s->conn_data && s->state == UPLOADING) {
         written = ffurl_write(s->conn_data, buf, size);
@@ -680,12 +675,9 @@ static int ftp_write(URLContext *h, const unsigned char *buf, int size)
 
 static int ftp_close(URLContext *h)
 {
-    FTPContext *s = h->priv_data;
-
     av_dlog(h, "ftp protocol close\n");
 
-    ffurl_closep(&s->conn_control);
-    ffurl_closep(&s->conn_data);
+    ftp_close_both_connections(h->priv_data);
 
     return 0;
 }