Fix convert plugin attempting to process a non-media file (#5261)

## Description

My library is managed using Beets for organization and
[git-annex](https://git-annex.branchable.com/) as storage backend.
Therefore when using this system, while my library files always exists
on my filesystem, some files may be empty (without content). In this
case, when I'm running the `convert` plugin, I don't wants it to process
files which are empty (same apply for any Beets plugin). Hence, I added
a check that the file is readable as a `MediaFile` before doing any
process.

Before this fix, trying to encode an empty file would have lead to an
error while leaving `convert` doing its side-effects **and** `convert`
would also copy empty files to destination for files that doesn't need
to be re-encoded.

In my case, this is empty files, but the problem can be anything else
(depending on the storage backend) and/or corrupted files. Conclusion, I
think **checking that the file is readable is always recommended before
proceeding to heavy operation** like this.

beetsplug/convert.py

@@ -22,6 +22,7 @@ import tempfile
 import threading
 from string import Template
 
+import mediafile
 from confuse import ConfigTypeError, Optional
 
 from beets import art, config, plugins, ui, util
@@ -351,6 +352,15 @@ class ConvertPlugin(BeetsPlugin):
             item = yield (item, original, converted)
             dest = item.destination(basedir=dest_dir, path_formats=path_formats)
 
+            # Ensure that desired item is readable before processing it. Needed
+            # to avoid any side-effect of the conversion (linking, keep_new,
+            # refresh) if we already know that it will fail.
+            try:
+                mediafile.MediaFile(util.syspath(item.path))
+            except mediafile.UnreadableFileError as exc:
+                self._log.error("Could not open file to convert: {0}", exc)
+                continue
+
             # When keeping the new file in the library, we first move the
             # current (pristine) file to the destination. We'll then copy it
             # back to its old path or transcode it to a new path.

docs/changelog.rst

@@ -519,6 +519,8 @@ Bug fixes:
   `requests` timeout.
 * Fix cover art resizing logic to support multiple steps of resizing
   :bug:`5151`
+* :doc:`/plugins/convert`: Fix attempt to convert and perform side-effects if
+  library file is not readable.
 
 For plugin developers: