Capture exit codes and stderr from background hooks

Extended graceful_terminate_background_hooks() to:
- Reap processes with os.waitpid() to get exit codes
- Write returncode to .returncode file for update_from_output()
- Return detailed result dict with status, returncode, and pid

Updated update_from_output() to:
- Read .returncode and .stderr.log files
- Determine status from returncode if no ArchiveResult JSONL record
- Include stderr in output_str for failed hooks
- Handle signal termination (negative returncodes like -9 for SIGKILL)
- Clean up .returncode files along with other hook output files

archivebox/core/models.py

@@ -2711,7 +2711,20 @@ class ArchiveResult(ModelWithOutputDir, ModelWithConfig, ModelWithNotes, ModelWi
 
         # Read and parse JSONL output from hook-specific stdout log
         stdout_file = plugin_dir / f'{hook_basename}.stdout.log'
+        stderr_file = plugin_dir / f'{hook_basename}.stderr.log'
+        returncode_file = plugin_dir / f'{hook_basename}.returncode'
+
         stdout = stdout_file.read_text() if stdout_file.exists() else ''
+        stderr = stderr_file.read_text() if stderr_file.exists() else ''
+
+        # Read returncode from file (written by graceful_terminate_background_hooks)
+        returncode = None
+        if returncode_file.exists():
+            try:
+                rc_text = returncode_file.read_text().strip()
+                returncode = int(rc_text) if rc_text else None
+            except (ValueError, OSError):
+                pass
 
         records = []
         for line in stdout.splitlines():
@@ -2746,9 +2759,30 @@ class ArchiveResult(ModelWithOutputDir, ModelWithConfig, ModelWithNotes, ModelWi
                 self._set_binary_from_cmd(hook_data['cmd'])
             # Note: cmd_version is derived from binary.version, not stored on Process
         else:
-            # No ArchiveResult record = failed
-            self.status = self.StatusChoices.FAILED
-            self.output_str = 'Hook did not output ArchiveResult record'
+            # No ArchiveResult JSONL record - determine status from returncode
+            if returncode is not None:
+                if returncode == 0:
+                    self.status = self.StatusChoices.SUCCEEDED
+                    self.output_str = 'Hook completed successfully (no JSONL output)'
+                elif returncode < 0:
+                    # Negative = killed by signal (e.g., -9 for SIGKILL, -15 for SIGTERM)
+                    sig_num = abs(returncode)
+                    sig_name = {9: 'SIGKILL', 15: 'SIGTERM'}.get(sig_num, f'signal {sig_num}')
+                    self.status = self.StatusChoices.FAILED
+                    self.output_str = f'Hook killed by {sig_name}'
+                    if stderr:
+                        self.output_str += f'\n\nstderr:\n{stderr[:2000]}'
+                else:
+                    self.status = self.StatusChoices.FAILED
+                    self.output_str = f'Hook failed with exit code {returncode}'
+                    if stderr:
+                        self.output_str += f'\n\nstderr:\n{stderr[:2000]}'
+            else:
+                # No returncode file and no JSONL = failed
+                self.status = self.StatusChoices.FAILED
+                self.output_str = 'Hook did not output ArchiveResult record'
+                if stderr:
+                    self.output_str += f'\n\nstderr:\n{stderr[:2000]}'
 
         # Walk filesystem and populate output_files, output_size, output_mimetypes
         # Exclude hook output files (hook-specific names like on_Snapshot__50_wget.stdout.log)
@@ -2758,6 +2792,7 @@ class ArchiveResult(ModelWithOutputDir, ModelWithConfig, ModelWithNotes, ModelWi
                 name.endswith('.stdout.log') or
                 name.endswith('.stderr.log') or
                 name.endswith('.pid') or
+                name.endswith('.returncode') or
                 (name.endswith('.sh') and name.startswith('on_'))
             )
 
@@ -2826,10 +2861,10 @@ class ArchiveResult(ModelWithOutputDir, ModelWithConfig, ModelWithNotes, ModelWi
         }
         process_hook_records(filtered_records, overrides=overrides)
 
-        # Cleanup PID files and empty logs (hook-specific names)
+        # Cleanup PID files, returncode files, and empty logs (hook-specific names)
         pid_file = plugin_dir / f'{hook_basename}.pid'
         pid_file.unlink(missing_ok=True)
-        stderr_file = plugin_dir / f'{hook_basename}.stderr.log'
+        returncode_file.unlink(missing_ok=True)
         if stdout_file.exists() and stdout_file.stat().st_size == 0:
             stdout_file.unlink()
         if stderr_file.exists() and stderr_file.stat().st_size == 0: