Read ffmpeg progress from a file, not a pipe, to avoid deadlock

Driving -progress over pipe:1 made ffmpeg block on the progress write
whenever the GUI stopped draining the pipe (e.g. the parent terminal or
the app's event loop wedging), deadlocking every running encode.

Write -progress to a temp file instead: a file write never blocks, so the
encoder runs to completion regardless of GUI state. The executor calls
optional start_/stop_progress_polling hooks; ffmpeg polls the file on a
500ms GLib timer and removes it when done. Verified an encode completes
with no reader at all (previously a hard deadlock).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/devedeng/executor.py

@@ -109,6 +109,12 @@ class executor(GObject.GObject):
         self.launch_process(self.command_var)
         if self.use_pulse_mode != self.pulse_mode:
             self.set_pulse_mode(self.use_pulse_mode)
+        # let subclasses start any timer-based progress polling (e.g. ffmpeg
+        # reads its -progress file on a GLib timer). No-op by default.
+        try:
+            self.start_progress_polling()
+        except AttributeError:
+            pass
 
     def remove_ansi(self, line):
 
@@ -308,6 +314,11 @@ class executor(GObject.GObject):
             retval = -1
 
         self.set_pulse_mode(False)
+        # stop any subclass progress polling timer
+        try:
+            self.stop_progress_polling()
+        except AttributeError:
+            pass
 
         # call, if it exists, the post-function
         try:

src/devedeng/ffmpeg.py

@@ -19,6 +19,7 @@
 import subprocess
 import os
 import re
+from gi.repository import GLib
 import devedeng.configuration_data
 import devedeng.executor
 import devedeng.mux_dvd_menu
@@ -182,11 +183,19 @@ class ffmpeg(devedeng.executor.executor):
         self.command_var.append("-n")
         self.command_var.append("10")
         self.command_var.append("ffmpeg")
-        # emit structured, steadily-paced progress on stdout (pipe:1) instead of
-        # the carriage-return stats line, which barely updates over a pipe; this
-        # is what keeps the progress bar moving (see process_stdout)
+        # Write structured, steadily-paced progress to a FILE (never a pipe):
+        # a file write can't block the encoder, so a wedged or crashed GUI can
+        # never deadlock ffmpeg on backpressure. The app polls this file on a
+        # timer to update the progress bar (see start_progress_polling).
+        self.progress_file = output_file + ".progress"
+        try:
+            # start clean so we never read a previous run's position
+            if os.path.exists(self.progress_file):
+                os.remove(self.progress_file)
+        except OSError:
+            pass
         self.command_var.append("-progress")
-        self.command_var.append("pipe:1")
+        self.command_var.append(self.progress_file)
         self.command_var.append("-stats_period")
         self.command_var.append("0.5")
         self.command_var.append("-nostats")
@@ -725,37 +734,74 @@ class ffmpeg(devedeng.executor.executor):
         return (final_path)
 
     def process_stdout(self, data):
-        # ffmpeg -progress emits newline-delimited key=value pairs on stdout,
-        # e.g. out_time=00:01:23.456 / out_time_ms=83456000 / progress=continue.
-        # This updates steadily over a pipe, unlike the carriage-return stats.
-        if self.progress_bar is None:
+        # progress is read from the -progress file (see start_progress_polling),
+        # not stdout, so nothing to do here
+        return
+
+    def start_progress_polling(self):
+        """ Poll ffmpeg's -progress file on a GLib timer to drive the progress
+            bar. Reading a file never blocks the encoder, so a wedged or crashed
+            GUI can't deadlock ffmpeg on pipe backpressure. """
+        self._progress_timer = None
+        if getattr(self, "progress_file", None) is None:
             return
-        for line in data:
+        self._progress_timer = GLib.timeout_add(500, self._poll_progress_file)
+
+    def stop_progress_polling(self):
+        timer = getattr(self, "_progress_timer", None)
+        if timer is not None:
+            GLib.source_remove(timer)
+            self._progress_timer = None
+        # tidy up the temp progress file
+        pf = getattr(self, "progress_file", None)
+        if pf is not None:
+            try:
+                os.remove(pf)
+            except OSError:
+                pass
+
+    def _poll_progress_file(self):
+        # returning True keeps the GLib timer alive
+        if self.progress_bar is None:
+            return True
+        pf = getattr(self, "progress_file", None)
+        if (pf is None) or (not os.path.exists(pf)):
+            return True
+        try:
+            with open(pf, "r") as f:
+                content = f.read()
+        except OSError:
+            return True
+        # ffmpeg appends progress blocks; the last out_time/progress is current
+        seconds = None
+        ended = False
+        for line in content.splitlines():
             line = line.strip()
             if line.startswith("out_time_ms=") or line.startswith("out_time_us="):
                 try:
-                    micros = float(line.split("=", 1)[1])
+                    seconds = float(line.split("=", 1)[1]) / 1000000.0
                 except (ValueError, IndexError):
-                    continue
-                seconds = micros / 1000000.0
-                self._set_progress_fraction(seconds)
+                    pass
             elif line.startswith("out_time="):
-                value = line.split("=", 1)[1]
-                parts = value.split(":")
-                seconds = 0.0
+                parts = line.split("=", 1)[1].split(":")
+                acc = 0.0
                 ok = True
                 for e in parts:
                     try:
-                        seconds = seconds * 60.0 + float(e)
+                        acc = acc * 60.0 + float(e)
                     except ValueError:
                         ok = False
                         break
                 if ok:
-                    self._set_progress_fraction(seconds)
+                    seconds = acc
             elif line == "progress=end":
-                self.progress_bar[1].set_fraction(1.0)
-                self.progress_bar[1].set_text("100.0%")
-        return
+                ended = True
+        if seconds is not None:
+            self._set_progress_fraction(seconds)
+        if ended:
+            self.progress_bar[1].set_fraction(1.0)
+            self.progress_bar[1].set_text("100.0%")
+        return True
 
     def _set_progress_fraction(self, seconds):
         if (self.progress_bar is None) or (not self.final_length):