command: export more details about file seekability

If a file is unseekable (consider e.g. a http server without resume
functionality), but the stream cache is active, the player will enable
seeking anyway. Until know, client API user couldn't know that this
happens, and it has implications on how well seeking will work. So add a
property which exports whether this situation applies.

Fixes #1522.

DOCS/man/input.rst

@@ -1480,6 +1480,14 @@ Property list
 ``seekable``
     Return whether it's generally possible to seek in the current file.
 
+``partially-seekable``
+    Return ``yes`` if the current file is considered seekable, but only because
+    the cache is active. This means small relative seeks may be fine, but larger
+    seeks may fail anyway. Whether a seek will succeed or not is generally not
+    known in advance.
+
+    If this property returns true, ``seekable`` will also return true.
+
 ``playback-abort``
     Return whether playback is stopped or is to be stopped. (Useful in obscure
     situations like during ``on_load`` hook processing, when the user can

demux/demux.c

@@ -758,6 +758,7 @@ static void demux_copy(struct demuxer *dst, struct demuxer *src)
         dst->file_contents = src->file_contents;
         dst->playlist = src->playlist;
         dst->seekable = src->seekable;
+        dst->partially_seekable = src->partially_seekable;
         dst->filetype = src->filetype;
         dst->ts_resets_possible = src->ts_resets_possible;
         dst->rel_seeks = src->rel_seeks;
@@ -905,6 +906,7 @@ static struct demuxer *open_given_type(struct mpv_global *global,
         if (!in->d_thread->seekable && stream->uncached_stream) {
             mp_verbose(log, "Enabling seeking because stream cache is active.\n");
             in->d_thread->seekable = true;
+            in->d_thread->partially_seekable = true;
         }
         demux_init_cache(demuxer);
         demux_changed(in->d_thread, DEMUX_EVENT_ALL);

demux/demux.h

@@ -185,7 +185,8 @@ typedef struct demuxer {
     int64_t filepos;  // input stream current pos.
     char *filename;  // same as stream->url
     enum demuxer_type type;
-    int seekable; // flag
+    bool seekable;
+    bool partially_seekable; // implies seekable=true
     double start_time;
     // File format allows PTS resets (even if the current file is without)
     bool ts_resets_possible;

player/command.c

@@ -1438,6 +1438,15 @@ static int mp_property_seekable(void *ctx, struct m_property *prop,
     return m_property_flag_ro(action, arg, mpctx->demuxer->seekable);
 }
 
+static int mp_property_partially_seekable(void *ctx, struct m_property *prop,
+                                          int action, void *arg)
+{
+    MPContext *mpctx = ctx;
+    if (!mpctx->demuxer)
+        return M_PROPERTY_UNAVAILABLE;
+    return m_property_flag_ro(action, arg, mpctx->demuxer->partially_seekable);
+}
+
 /// Volume (RW)
 static int mp_property_volume(void *ctx, struct m_property *prop,
                               int action, void *arg)
@@ -3294,6 +3303,7 @@ static const struct m_property mp_properties[] = {
     {"hr-seek", mp_property_generic_option},
     {"clock", mp_property_clock},
     {"seekable", mp_property_seekable},
+    {"partially-seekable", mp_property_partially_seekable},
     {"idle", mp_property_idle},
 
     {"chapter-list", mp_property_list_chapters},