server: keep request/access log out of the pySim output capture

The HTTP access log (log_message) and the --log-requests REQUEST/RESPONSE
lines wrote to the rebindable sys.stderr, so a concurrent card-free request
(the 2s /api/status poll) dropped its log line into the StringIO a card
command was capturing - corrupting file content read during a snapshot scan
(the access-log line appeared inside the file hex).

- Add _log_stderr(): write the request/access log to fd 2 directly (the same
  bypass the APDU tracer already uses), so it can never enter a capture.
  Route log_message, _log_req and _log_resp through it.
- Add _capture_app_output() context manager and deduplicate the six handlers
  (cardinfo/command/help/read/write/tree) that did the inline
  app.stdout/sys.stderr = out save/restore dance.

Bumps VERSION to 3.40.5 and the SW cache to simple-v491.

CHANGELOG.md

@@ -9,6 +9,26 @@ every change carries a descriptive commit message.
 
 ## [Unreleased]
 
+## [3.40.5] — 2026-11-08
+
+### Fixed
+
+- **Server request/access log leaked into captured file content.**  The HTTP
+  access log (`log_message`) and the `--log-requests` `REQUEST`/`RESPONSE`
+  lines wrote to the rebindable `sys.stderr`, so a concurrent card-free request
+  (the 2 s `/api/status` poll) dropped its log line into the `StringIO` a
+  card command was capturing — corrupting file content read during a snapshot
+  scan (the log line appeared in the snapshot, e.g.
+  `127.0.0.1 - - […] "GET /api/status HTTP/1.1" 200 -ffff…`).  The log now
+  writes to fd 2 via `_log_stderr`, exactly as the APDU tracer already does.
+
+### Changed
+
+- **Deduplicated the pySim output capture.**  The six handlers that captured
+  pySim command output by rebinding `sys.stdout`/`sys.stderr` to a `StringIO`
+  now share one `_capture_app_output` context manager (the same save/set/
+  restore semantics, in one place).
+
 ## [3.40.4] — 2026-11-04
 
 ### Added

frontend/index.html

@@ -1990,7 +1990,7 @@
 // ===== Version =====
 // Single source of truth for the PWA version: shown in the header and used
 // by the server version check in pysimConnect().
-const SIMPLE_VERSION = '3.40.4';
+const SIMPLE_VERSION = '3.40.5';
 document.getElementById('app-version').textContent = 'v' + SIMPLE_VERSION;
 
 // ===== view history (the browser Back/Forward buttons) =====

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'simple-v490';
+const CACHE = 'simple-v491';
 const URLS = [
   'index.html',
   'help.html',

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "pysim-simple-server"
-version = "3.40.4"
+version = "3.40.5"
 description = "HTTP REST server wrapping pysim for the SIMple PWA"
 license = { text = "GPL-2.0-or-later" }
 requires-python = ">=3.9"

pysim_simple_server/server.py

@@ -13,6 +13,7 @@ from urllib.parse import unquote_plus
 from datetime import datetime, timedelta
 from http.server import BaseHTTPRequestHandler
 from io import StringIO
+from contextlib import contextmanager
 from pySim.transport import ApduTracer, ProactiveHandler
 from pySim.cards import UiccCardBase
 from pysim_simple_server import httpota
@@ -37,7 +38,7 @@ from osmocom.tlv import BER_TLV_IE
 from osmocom.utils import rpad
 
 
-VERSION = '3.40.4'
+VERSION = '3.40.5'
 
 MAX_ENVELOPE_SEGMENTS = 5  # max SMS segments for outgoing C-APDU in ENVELOPE
 
@@ -267,6 +268,42 @@ def _cmd_exception(output):
     return 'command failed'
 
 
+def _log_stderr(msg):
+    """Write a server log line to the real stderr fd, bypassing ``sys.stderr``.
+
+    The HTTP handlers temporarily rebind the global ``sys.stderr`` to a
+    ``StringIO`` to capture pySim command output.  Concurrent card-free
+    requests (the 2 s /api/status poll) log via ``log_message`` / ``_log_req``
+    / ``_log_resp``; writing to ``sys.stderr`` would drop those lines into the
+    active capture buffer and corrupt the file content (the snapshot "access
+    log line in contents" bug).  Writing to fd 2 keeps the request log out of
+    the capture, exactly as the APDU tracer already does below."""
+    os.write(2, msg.encode())
+
+
+@contextmanager
+def _capture_app_output(app):
+    """Temporarily capture the pySim app's output into a ``StringIO``.
+
+    cmd2 writes command output to ``app.stdout``; stray writes to the global
+    ``sys.stderr`` (``print(file=sys.stderr)``, ``sys.stderr.write``, logging,
+    tracebacks) are captured too by rebinding it.  Card access is serialized by
+    ``_CARD_LOCK``, so only one capture is active at a time, and the server's
+    own request/access logging bypasses it via ``_log_stderr``.  Yields the
+    buffer and restores both streams in ``finally``.
+    """
+    out = StringIO()
+    old_stdout = app.stdout
+    old_stderr = sys.stderr
+    app.stdout = out
+    sys.stderr = out
+    try:
+        yield out
+    finally:
+        app.stdout = old_stdout
+        sys.stderr = old_stderr
+
+
 def _parse_help_text(text):
     result = {'usage': '', 'description': '', 'args': []}
     lines = text.split('\n')
@@ -9472,13 +9509,13 @@ class PysimHandler(BaseHTTPRequestHandler):
     def _log_req(self, body=None):
         if self.server.log_requests:
             if body is not None:
-                sys.stderr.write("REQUEST %s %s: %s\n" % (self.command, self.path, json.dumps(body)))
+                _log_stderr("REQUEST %s %s: %s\n" % (self.command, self.path, json.dumps(body)))
             else:
-                sys.stderr.write("REQUEST %s %s\n" % (self.command, self.path))
+                _log_stderr("REQUEST %s %s\n" % (self.command, self.path))
 
     def _log_resp(self, data):
         if self.server.log_requests:
-            sys.stderr.write("RESPONSE %s: %s\n" % (self.path, json.dumps(data, ensure_ascii=False)))
+            _log_stderr("RESPONSE %s: %s\n" % (self.path, json.dumps(data, ensure_ascii=False)))
 
     def do_OPTIONS(self):
         self._log_req()
@@ -9777,19 +9814,12 @@ class PysimHandler(BaseHTTPRequestHandler):
                 self._send_json({'error': _err('app_not_init', lang)}, 503)
                 self._log_resp({'error': _err('app_not_init', lang)})
                 return
-            out = StringIO()
+            with _capture_app_output(app) as out:
-            old_stdout = app.stdout
-            old_stderr = sys.stderr
-            app.stdout = out
-            sys.stderr = out
                 try:
                     app.onecmd_plus_hooks('cardinfo')
                     output = _strip_ansi(out.getvalue())
                 except Exception as e:
                     output = str(e) + '\n' + traceback.format_exc()
-            finally:
-                app.stdout = old_stdout
-                sys.stderr = old_stderr
             resp = {'output': output}
             self._send_json(resp)
             self._log_resp(resp)
@@ -9931,19 +9961,12 @@ class PysimHandler(BaseHTTPRequestHandler):
                 # transport so a manual Equip recovers too.
                 _ensure_transport(self.server)
             t0 = time.time()
-            out = StringIO()
+            with _capture_app_output(app) as out:
-            old_stdout = app.stdout
-            old_stderr = sys.stderr
-            app.stdout = out
-            sys.stderr = out
                 try:
                     stop = app.onecmd_plus_hooks(cmd)
                     output = _strip_ansi(out.getvalue())
                 except Exception as e:
                     output = str(e) + '\n' + traceback.format_exc()
-            finally:
-                app.stdout = old_stdout
-                sys.stderr = old_stderr
             elapsed = int((time.time() - t0) * 1000)
             status = 'OK' if not output or 'not a recognized command' not in output else 'ERROR'
             if is_equip:
@@ -10199,17 +10222,9 @@ class PysimHandler(BaseHTTPRequestHandler):
             body = self._read_body()
             self._log_req(body)
             cmd = body.get('cmd', '')
-            out = StringIO()
+            with _capture_app_output(app) as out:
-            old_stdout = app.stdout
-            old_stderr = sys.stderr
-            app.stdout = out
-            sys.stderr = out
-            try:
                 app.onecmd_plus_hooks('help ' + cmd)
                 raw = out.getvalue()
-            finally:
-                app.stdout = old_stdout
-                sys.stderr = old_stderr
             clean = _strip_ansi(raw)
             parsed = _parse_help_text(clean)
             self._send_json(parsed)
@@ -10307,17 +10322,9 @@ class PysimHandler(BaseHTTPRequestHandler):
                         cmd = 'read_records_decoded' if is_record else 'read_binary_decoded'
                     else:
                         cmd = 'read_records' if is_record else 'read_binary'
-                    out = StringIO()
+                    with _capture_app_output(app) as out:
-                    old_stdout = app.stdout
-                    old_stderr = sys.stderr
-                    app.stdout = out
-                    sys.stderr = out
-                    try:
                         app.onecmd_plus_hooks(cmd)
                         output = _strip_ansi(out.getvalue())
-                    finally:
-                        app.stdout = old_stdout
-                        sys.stderr = old_stderr
                 finally:
                     apdu_times = _end_apdu_time_collection()
                 exc = _cmd_exception(output)
@@ -10403,17 +10410,9 @@ class PysimHandler(BaseHTTPRequestHandler):
                     cmd = 'update_record 1 %s' % data
                 else:
                     cmd = 'update_binary %s' % data
-                out = StringIO()
+                with _capture_app_output(app) as out:
-                old_stdout = app.stdout
-                old_stderr = sys.stderr
-                app.stdout = out
-                sys.stderr = out
-                try:
                     app.onecmd_plus_hooks(cmd)
                     output = out.getvalue()
-                finally:
-                    app.stdout = old_stdout
-                    sys.stderr = old_stderr
                 exc = _cmd_exception(output)
                 if exc:
                     resp = {'success': False, 'error': exc}
@@ -10472,17 +10471,9 @@ class PysimHandler(BaseHTTPRequestHandler):
                 sel = fid if fid else name
                 _, cleanup = _select_with_parent(lchan, sel, parent_sel, app, parent_path, allow_probe)
                 cur = lchan.selected_file
-                out = StringIO()
+                with _capture_app_output(app) as out:
-                old_stdout = app.stdout
-                old_stderr = sys.stderr
-                app.stdout = out
-                sys.stderr = out
-                try:
                     app.onecmd_plus_hooks('tree')
                     output = _strip_ansi(out.getvalue())
-                finally:
-                    app.stdout = old_stdout
-                    sys.stderr = old_stderr
                 children = _parse_tree_output(output)
                 # EF.PBR (TS 31.102 4.4.2.1): the phonebook files the DF's PBR
                 # references are not in pySim's model, so they are appended to
@@ -12178,4 +12169,4 @@ class PysimHandler(BaseHTTPRequestHandler):
             self._log_resp({'error': _err('not_found', lang)})
 
     def log_message(self, format, *args):
-        sys.stderr.write('%s - - [%s] %s\n' % (self.client_address[0], self.log_date_time_string(), format % args))
+        _log_stderr('%s - - [%s] %s\n' % (self.client_address[0], self.log_date_time_string(), format % args))

tests/test_output_capture.py

@@ -0,0 +1,91 @@
+"""The request/access log must never enter a pySim output capture.
+
+Handlers capture pySim output by temporarily rebinding the global ``sys.stderr``
+to a StringIO (``_capture_app_output``).  Concurrent card-free requests (the 2 s
+/api/status poll) log via ``log_message`` / ``_log_req`` / ``_log_resp``; if
+those wrote to ``sys.stderr`` their lines would land in the active capture
+buffer and corrupt the file content (the snapshot "access log line in contents"
+bug).  The log now writes to fd 2 directly, and the capture is a context
+manager that restores both streams.
+"""
+
+import io
+import pathlib
+import sys
+import types
+import unittest
+from unittest import mock
+
+from pysim_simple_server import server as srv
+from pysim_simple_server.server import PysimHandler, _capture_app_output, _log_stderr
+
+
+class LogStderrTests(unittest.TestCase):
+    def test_log_stderr_writes_to_fd2_not_the_rebindable_stream(self):
+        writes = []
+        err = io.StringIO()
+        with mock.patch.object(srv.os, 'write', side_effect=lambda fd, data: writes.append((fd, data))), \
+                mock.patch('sys.stderr', new=err):
+            _log_stderr('hello\n')
+        self.assertEqual(writes, [(2, b'hello\n')])
+        self.assertEqual(err.getvalue(), '')
+
+    def test_log_message_goes_to_fd2(self):
+        h = object.__new__(PysimHandler)
+        h.client_address = ('127.0.0.1', 1234)
+        h.log_date_time_string = lambda: '05/Nov/2026 14:09:10'
+        writes = []
+        err = io.StringIO()
+        with mock.patch.object(srv.os, 'write', side_effect=lambda fd, data: writes.append((fd, data))), \
+                mock.patch('sys.stderr', new=err):
+            h.log_message('"%s" %s %s', 'GET /api/status HTTP/1.1', '200', '-')
+        self.assertEqual(
+            writes,
+            [(2, b'127.0.0.1 - - [05/Nov/2026 14:09:10] '
+                  b'"GET /api/status HTTP/1.1" 200 -\n')])
+        self.assertEqual(err.getvalue(), '',
+                         'the access log must not land in a redirected sys.stderr')
+
+
+class CaptureOutputTests(unittest.TestCase):
+    def test_captures_both_streams_and_restores(self):
+        orig_stdout = io.StringIO()
+        app = types.SimpleNamespace(stdout=orig_stdout)
+        real_stderr = sys.stderr
+        with _capture_app_output(app) as out:
+            app.stdout.write('AABB\n')
+            sys.stderr.write('SW: 9000\n')
+        self.assertIn('AABB\n', out.getvalue())
+        self.assertIn('SW: 9000\n', out.getvalue())
+        self.assertIs(app.stdout, orig_stdout)
+        self.assertIs(sys.stderr, real_stderr)
+
+    def test_restores_on_exception(self):
+        orig_stdout = io.StringIO()
+        app = types.SimpleNamespace(stdout=orig_stdout)
+        real_stderr = sys.stderr
+        with self.assertRaises(RuntimeError):
+            with _capture_app_output(app) as out:
+                app.stdout.write('partial\n')
+                raise RuntimeError('boom')
+        self.assertIs(app.stdout, orig_stdout)
+        self.assertIs(sys.stderr, real_stderr)
+
+
+class SourcePinTests(unittest.TestCase):
+    def test_log_message_uses_fd2(self):
+        src = pathlib.Path(srv.__file__).read_text()
+        seg = src[src.index('def log_message('):]
+        self.assertIn('_log_stderr(', seg)
+        self.assertNotIn('sys.stderr.write', seg)
+
+    def test_the_handlers_use_the_capture_context_manager(self):
+        # The six handlers that capture pySim output share one context manager;
+        # a re-introduced inline ``sys.stderr = out`` block would be a
+        # thread-safety regression.
+        src = pathlib.Path(srv.__file__).read_text()
+        self.assertGreaterEqual(src.count('with _capture_app_output(app) as out:'), 6)
+
+
+if __name__ == '__main__':
+    unittest.main()