ffmpeg demuxer concat helper uses absolute paths in ffconcat file.

The ffmpeg demuxer concat helper uses absolute paths in the ffconcat file, rather than relative
paths, because ffmpeg interprets relative paths with respect to the location of the ffconcat file,
rather than with respect to CWD. Fixes #93 reported by @Cocalus.

CHANGELOG.md

@@ -5,6 +5,10 @@
 
 - ffmpeg muxing support supports the use of the `DASHMPD_PERSIST_FILES` environment variable to retain
   the temporary files created during muxing.
+
+- The ffmpeg demuxer concat helper uses absolute paths in the ffconcat file, rather than relative
+  paths, because ffmpeg interprets relative paths with respect to the location of the ffconcat file,
+  rather than with respect to CWD. Fixes #93 reported by @Cocalus.
 
 
 ## [0.18.0] - 2025-01-12

src/ffmpeg.rs

@@ -1178,13 +1178,14 @@ pub(crate) fn concat_output_files_ffmpeg_demuxer(
     // https://ffmpeg.org/ffmpeg-formats.html#concat
     writeln!(&demuxlist, "ffconcat version 1.0")
         .map_err(|e| DashMpdError::Io(e, String::from("writing to demuxer cmd file")))?;
-    let mut inputs = Vec::<PathBuf>::new();
-    inputs.push(tmppath.into());
-    writeln!(&demuxlist, "file '{}'", tmppath)
+    let canonical = fs::canonicalize(tmppath)
+        .map_err(|e| DashMpdError::Io(e, String::from("canonicalizing temporary filename")))?;
+    writeln!(&demuxlist, "file '{}'", canonical.display())
         .map_err(|e| DashMpdError::Io(e, String::from("writing to demuxer cmd file")))?;
     for p in &paths[1..] {
-        inputs.push(p.to_path_buf());
-        writeln!(&demuxlist, "file '{}'", p.to_path_buf().display())
+        let canonical = fs::canonicalize(p)
+            .map_err(|e| DashMpdError::Io(e, String::from("canonicalizing temporary filename")))?;
+        writeln!(&demuxlist, "file '{}'", canonical.display())
             .map_err(|e| DashMpdError::Io(e, String::from("writing to demuxer cmd file")))?;
     }
     let demuxlistpath = &demuxlist