perf(sandbox): bound stderr carry and scan newlines incrementally

Cap incomplete-line carry, drop overflow until the next newline, and
avoid toString+split rebuilds on every chunk. Bound sanitize input length.

Closes #427

Author: ZhuchkaTriplesix

lib/core/extensions/sandbox/sandbox_sanitizer.dart

@@ -32,9 +32,15 @@ class SandboxSanitizer {
     caseSensitive: false,
   );
 
+  /// Soft bound for regex work on a single line (stderr pipe also caps).
+  static const maxInputChars = 256 * 1024;
+
   /// Sanitizes a single chunk / line of plugin output.
   static String sanitize(String input) {
     if (input.isEmpty) return input;
+    if (input.length > maxInputChars) {
+      input = input.substring(0, maxInputChars);
+    }
     var out = input;
     out = out.replaceAll(_privateKey, redactionToken);
     out = out.replaceAll(_jwt, redactionToken);

lib/core/extensions/sandbox/sandbox_stderr_pipe.dart

@@ -9,21 +9,40 @@ import 'package:querya_desktop/core/extensions/sandbox/sandbox_sanitizer.dart';
 import 'package:querya_desktop/core/extensions/sandbox/sandbox_security_audit.dart';
 
 /// Captures `process.stderr`, sanitizes it, and writes to a rotating log.
+///
+/// Incomplete lines are held in a capped carry buffer. Newline scanning walks
+/// only the new chunk (no full-buffer `split` rebuild each time). Oversized
+/// lines are truncated and the remainder is dropped until the next newline.
 class SandboxStderrPipe {
   SandboxStderrPipe({
     required this.pluginId,
     required this.log,
     this.audit,
     this.onSanitizedLine,
-  });
+    this.maxCarryChars = defaultMaxCarryChars,
+    this.maxSanitizeChars = defaultMaxSanitizeChars,
+  }) : assert(maxCarryChars > 0),
+       assert(maxSanitizeChars > 0);
+
+  /// Default cap for an incomplete stderr line held across chunks.
+  static const defaultMaxCarryChars = 256 * 1024;
+
+  /// Default max length passed into [SandboxSanitizer.sanitize].
+  static const defaultMaxSanitizeChars = 256 * 1024;
+
+  static const _truncatedSuffix = '…[truncated]';
 
   final String pluginId;
   final SandboxRotatingLog log;
   final SandboxSecurityAudit? audit;
   final void Function(String line)? onSanitizedLine;
+  final int maxCarryChars;
+  final int maxSanitizeChars;
 
   StreamSubscription<List<int>>? _subscription;
   final StringBuffer _carry = StringBuffer();
+  int _carryLength = 0;
+  var _dropUntilNewline = false;
   Future<void> _writeChain = Future<void>.value();
   var _closed = false;
 
@@ -35,6 +54,8 @@ class SandboxStderrPipe {
     SandboxSecurityAudit? audit,
     int maxBytes = 5 * 1024 * 1024,
     int maxFiles = 2,
+    int maxCarryChars = defaultMaxCarryChars,
+    int maxSanitizeChars = defaultMaxSanitizeChars,
     void Function(String line)? onSanitizedLine,
   }) async {
     final file = await SandboxLogPaths.pluginLogFile(handle.pluginId);
@@ -47,6 +68,8 @@ class SandboxStderrPipe {
       ),
       audit: audit,
       onSanitizedLine: onSanitizedLine,
+      maxCarryChars: maxCarryChars,
+      maxSanitizeChars: maxSanitizeChars,
     );
     pipe.listen(handle.process.stderr);
     return pipe;
@@ -80,36 +103,89 @@ class SandboxStderrPipe {
 
   void _onBytes(List<int> chunk) {
     if (chunk.isEmpty) return;
-    _carry.write(utf8.decode(chunk, allowMalformed: true));
-    _drainLines();
+    var text = utf8.decode(chunk, allowMalformed: true);
+    if (_dropUntilNewline) {
+      final nl = text.indexOf('\n');
+      if (nl < 0) return;
+      _dropUntilNewline = false;
+      text = text.substring(nl + 1);
+      if (text.isEmpty) return;
+    }
+    _drainIncoming(text);
   }
 
-  void _drainLines() {
-    final text = _carry.toString();
-    final parts = text.split('\n');
-    _carry.clear();
-    if (!text.endsWith('\n')) {
-      _carry.write(parts.removeLast());
-    } else if (parts.isNotEmpty && parts.last.isEmpty) {
-      parts.removeLast();
+  /// Scan [incoming] for newlines; only the incomplete tail stays in [_carry].
+  void _drainIncoming(String incoming) {
+    var start = 0;
+    while (true) {
+      final nl = incoming.indexOf('\n', start);
+      if (nl < 0) {
+        _appendCarry(incoming.substring(start));
+        return;
+      }
+      final segment = incoming.substring(start, nl);
+      final line = _carryLength == 0
+          ? segment
+          : (_carry..write(segment)).toString();
+      if (_carryLength != 0) {
+        _carry.clear();
+        _carryLength = 0;
+      }
+      _enqueueLine(line);
+      start = nl + 1;
+    }
+  }
+
+  void _appendCarry(String rest) {
+    if (rest.isEmpty) return;
+    if (_carryLength + rest.length <= maxCarryChars) {
+      _carry.write(rest);
+      _carryLength += rest.length;
+      return;
+    }
+
+    final room = maxCarryChars - _carryLength;
+    if (room > 0) {
+      _carry.write(rest.substring(0, room));
+      _carryLength += room;
     }
+    final flushed = '${_carry.toString()}$_truncatedSuffix';
+    _carry.clear();
+    _carryLength = 0;
+    _dropUntilNewline = true;
+    _enqueueLine(flushed);
 
-    for (final raw in parts) {
-      _writeChain = _writeChain.then((_) => _writeSanitized(raw));
+    if (room < rest.length) {
+      final nl = rest.indexOf('\n', room);
+      if (nl >= 0) {
+        _dropUntilNewline = false;
+        final after = rest.substring(nl + 1);
+        if (after.isNotEmpty) {
+          _drainIncoming(after);
+        }
+      }
     }
   }
 
+  void _enqueueLine(String raw) {
+    _writeChain = _writeChain.then((_) => _writeSanitized(raw));
+  }
+
   Future<void> _flushCarry() async {
-    if (_carry.isEmpty) return;
+    if (_carryLength == 0) return;
     final raw = _carry.toString();
     _carry.clear();
+    _carryLength = 0;
     await _writeSanitized(raw);
   }
 
   Future<void> _writeSanitized(String raw) async {
     try {
-      final sanitized = SandboxSanitizer.sanitize(raw);
-      if (sanitized != raw && audit != null) {
+      final bounded = raw.length > maxSanitizeChars
+          ? raw.substring(0, maxSanitizeChars)
+          : raw;
+      final sanitized = SandboxSanitizer.sanitize(bounded);
+      if (sanitized != bounded && audit != null) {
         await audit!.record(
           type: SandboxSecurityEventType.secretLeakBlocked,
           pluginId: pluginId,

test/core/extensions/sandbox/sandbox_sanitization_pipe_test.dart

@@ -270,5 +270,82 @@ QyNTUxOQAAACBA1m7X8J9H6P8Q9J8H6P8Q9J8H6P8Q9J8H6P8Q9J8H6Q==
 
       await handle.dispose();
     });
+
+    test('caps carry and drops remainder until newline', () async {
+      final process = _FakeProcess();
+      final scratch = await SandboxScratchDirectory.create(
+        pluginId: 'pipe.cap',
+        baseDirectory: temp,
+        token: '3',
+      );
+      final handle = SandboxProcessHandle(
+        pluginId: 'pipe.cap',
+        process: process,
+        scratch: scratch,
+        launchCommand: const SandboxLaunchCommand(
+          executable: '/bin/true',
+          arguments: [],
+          platform: 'linux',
+          usesOsSandbox: false,
+        ),
+      );
+
+      final lines = <String>[];
+      final pipe = await SandboxStderrPipe.attach(
+        handle,
+        maxCarryChars: 8,
+        onSanitizedLine: lines.add,
+      );
+
+      // No newline: force truncate at 8 chars, then more without newline.
+      process.emitStderr('abcdefghij');
+      process.emitStderr('ignored');
+      process.emitStderr('\nafter\n');
+      await Future<void>.delayed(const Duration(milliseconds: 50));
+      await pipe.close();
+
+      expect(lines, hasLength(2));
+      expect(lines[0], startsWith('abcdefgh'));
+      expect(lines[0], contains('[truncated]'));
+      expect(lines[0], isNot(contains('ij')));
+      expect(lines[1], 'after');
+
+      await handle.dispose();
+    });
+
+    test('drains split lines without rebuilding full carry each chunk', () async {
+      final process = _FakeProcess();
+      final scratch = await SandboxScratchDirectory.create(
+        pluginId: 'pipe.split',
+        baseDirectory: temp,
+        token: '4',
+      );
+      final handle = SandboxProcessHandle(
+        pluginId: 'pipe.split',
+        process: process,
+        scratch: scratch,
+        launchCommand: const SandboxLaunchCommand(
+          executable: '/bin/true',
+          arguments: [],
+          platform: 'linux',
+          usesOsSandbox: false,
+        ),
+      );
+
+      final lines = <String>[];
+      final pipe = await SandboxStderrPipe.attach(
+        handle,
+        onSanitizedLine: lines.add,
+      );
+
+      process.emitStderr('hel');
+      process.emitStderr('lo\nwor');
+      process.emitStderr('ld\n');
+      await Future<void>.delayed(const Duration(milliseconds: 50));
+      await pipe.close();
+
+      expect(lines, ['hello', 'world']);
+      await handle.dispose();
+    });
   });
 }