fix: a store that cannot brick itself on its own history

Matthias Nott

yesterday e6eb093cc63ba020799844905439f1cabfddcd3c

 lib/services/message_store.dart
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@@ -88,19 +88,32 @@
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     }
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   }
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+  /// Session id, without decoding the message it belongs to.
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+  ///
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+  /// The index needs ONE short field per line. jsonDecode gave it by parsing
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+  /// the whole record — including an inline base64 attachment that can be tens
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+  /// of megabytes — so building the index cost the size of the entire history.
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+  /// With enough of it, startup exceeded the iOS watchdog and the app could
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+  /// never open again: 9.5 seconds of CPU, killed at 0x8BADF00D, on a store
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+  /// that had grown a few megabytes at a time for weeks.
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+  ///
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+  /// A line is written by us and always carries "sessionId":"…", so reading it
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+  /// is a scan, not a parse.
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+  static final RegExp _sessionIdPattern = RegExp(r'"sessionId"\s*:\s*"([^"]+)"');
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+
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+  static String? _sessionIdOf(String line) =>
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+      _sessionIdPattern.firstMatch(line)?.group(1);
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+
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   static Future<void> _rebuildIndex(File logFile) async {
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     _index.clear();
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     final lines = logFile.readAsLinesSync();
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     for (var i = 0; i < lines.length; i++) {
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       final line = lines[i].trim();
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       if (line.isEmpty) continue;
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-      try {
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-        final map = jsonDecode(line) as Map<String, dynamic>;
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-        final sessionId = map['sessionId'] as String?;
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-        if (sessionId != null) {
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-          _index.putIfAbsent(sessionId, () => []).add(i);
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-        }
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-      } catch (_) {}
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+      final sessionId = _sessionIdOf(line);
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+      if (sessionId != null) {
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+        _index.putIfAbsent(sessionId, () => []).add(i);
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+      }
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     }
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   }
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@@ -189,6 +202,9 @@
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   // ------------------------------------------------------------- compact --
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+  /// Above this, compact regardless of how few lines there are.
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+  static const int _compactAboveBytes = 8 * 1024 * 1024;
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+
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   /// Rewrite the log keeping at most [keepPerSession] messages per session.
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   /// Called once on startup after initialize(). NOT called during normal use.
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   static Future<void> compact({int keepPerSession = 200}) async {
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@@ -198,7 +214,12 @@
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       if (!logFile.existsSync()) return;
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       final allLines = logFile.readAsLinesSync();
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-      if (allLines.length < 500) return; // nothing worth compacting
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+      // Compact on SIZE as well as line count. A store is bounded at 200
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+      // messages per session, which bounds nothing when a message carries an
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+      // inline attachment: 200 screenshots is tens of megabytes, and the count
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+      // never trips the threshold. Bytes are what startup has to read.
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+      final bytes = logFile.lengthSync();
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+      if (allLines.length < 500 && bytes < _compactAboveBytes) return;
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       // Build a set of line numbers to keep: last keepPerSession per session.
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       final keepLines = <int>{};
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@@ -224,14 +245,13 @@
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         final line = allLines[i].trim();
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         if (line.isEmpty) continue;
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         buffer.write('$line\n');
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-        // Extract sessionId for new index.
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-        try {
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-          final map = jsonDecode(line) as Map<String, dynamic>;
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-          final sid = map['sessionId'] as String?;
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-          if (sid != null) {
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-            newIndex.putIfAbsent(sid, () => []).add(newLine);
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-          }
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-        } catch (_) {}
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+        // Same scan as the index build, for the same reason: the only field
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+        // needed here is short, and decoding the record to reach it makes
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+        // compaction cost the size of the history it exists to bound.
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+        final sid = _sessionIdOf(line);
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+        if (sid != null) {
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+          newIndex.putIfAbsent(sid, () => []).add(newLine);
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+        }
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         newLine++;
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       }
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