fix(audience): drop DiskStore._cachedCount, read file count from disk

Removes the cached on-disk file counter and the BumpCount delta
tracking that maintained it. Count() now calls Directory.GetFiles
each time.

Why
- File.Delete silently succeeds when the path is missing, so
  TryDelete returned true regardless of whether a real file went
  away. Two concurrent flushes that both called Delete on the same
  paths each got true back and each called BumpCount(-1), drifting
  the cached count negative.
- Shutdown's force-clear of _sendInFlight followed by an ungated
  final SendBatchAsync gave the race a real opening (visible as
  the StatusBar_QueueSizeIncrementsAfterTrack flake on Linux Unity 6,
  where finalCount went past zero into "got -2").
- The cached counter was already documented as approximate ("Tests
  that plant files outside the DiskStore API will drift this and
  should assert on filesystem state, not Count()"), so callers
  could not rely on it anyway.

Trade-off: Count() is now O(N) on the queue directory. The only
caller is ImmutableAudience.QueueSize, read from a 500 ms UI poll
and a couple of test assertions, so this is fine for any realistic
queue size and removes an entire class of drift.

TryDelete drops its bool return because nothing needs to know
whether a real file was removed once we are not counting deltas.

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/Transport/DiskStore.cs

@@ -5,7 +5,6 @@
 using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
-using System.Threading;
 
 namespace Immutable.Audience
 {
@@ -15,17 +14,10 @@ internal sealed class DiskStore
     {
         private readonly string _queueDir;
 
-        // Cached queue file count: on-disk count at construction, plus
-        // tracked deltas from Write/Delete. Tests that plant files
-        // outside the DiskStore API will drift this and should assert
-        // on filesystem state, not Count().
-        private int _cachedCount;
-
         internal DiskStore(string persistentDataPath)
         {
             _queueDir = Path.Combine(persistentDataPath, "imtbl_audience", "queue");
             Directory.CreateDirectory(_queueDir);
-            _cachedCount = Directory.GetFiles(_queueDir, "*.json").Length;
         }
 
         // Atomically writes json as a new event file.
@@ -37,7 +29,6 @@ internal void Write(string json)
 
             File.WriteAllText(tmpPath, json);
 
-            var replaced = false;
             try
             {
                 File.Move(tmpPath, finalPath);
@@ -46,10 +37,7 @@ internal void Write(string json)
             {
                 File.Delete(finalPath);
                 File.Move(tmpPath, finalPath);
-                replaced = true;
             }
-
-            if (!replaced) BumpCount(+1);
         }
 
         // Returns up to maxSize file paths, oldest first. Stale files
@@ -86,7 +74,7 @@ internal IReadOnlyList<string> ReadBatch(int maxSize)
                     var fileTime = new DateTime(ticks, DateTimeKind.Utc);
                     if (fileTime < cutoff)
                     {
-                        if (TryDelete(path)) BumpCount(-1);
+                        TryDelete(path);
                         continue;
                     }
                 }
@@ -101,30 +89,34 @@ internal IReadOnlyList<string> ReadBatch(int maxSize)
         internal void Delete(IEnumerable<string> paths)
         {
             foreach (var path in paths)
-                if (TryDelete(path)) BumpCount(-1);
+                TryDelete(path);
         }
 
-        // Total number of event files currently on disk. Reads the cached
-        // count seeded at construction; mutating ops maintain it.
-        internal int Count() => Volatile.Read(ref _cachedCount);
-
-        private void BumpCount(int delta) => Interlocked.Add(ref _cachedCount, delta);
+        // Total number of event files currently on disk. Reads the filesystem
+        // each call so concurrent Write / Delete from any thread is reflected
+        // without a separately maintained counter that could drift.
+        internal int Count()
+        {
+            try { return Directory.GetFiles(_queueDir, "*.json").Length; }
+            catch (DirectoryNotFoundException) { return 0; }
+        }
 
-        private static bool TryDelete(string path)
+        private static void TryDelete(string path)
         {
-            try { File.Delete(path); return true; }
-            catch (DirectoryNotFoundException) { return true; }
-            catch (IOException) { return false; }
-            catch (UnauthorizedAccessException) { return false; }
+            try { File.Delete(path); }
+            catch (DirectoryNotFoundException) { }
+            catch (IOException) { }
+            catch (UnauthorizedAccessException) { }
         }
+
         internal void DeleteAll()
         {
             string[] paths;
             try { paths = Directory.GetFiles(_queueDir, "*.json"); }
             catch (DirectoryNotFoundException) { return; }
 
             foreach (var path in paths)
-                if (TryDelete(path)) BumpCount(-1);
+                TryDelete(path);
         }
 
         // Drops queued identify/alias files, strips userId from track files.
@@ -145,13 +137,13 @@ private void ApplyAnonymousDowngradeToFile(string path)
                 !msg.TryGetValue(MessageFields.Type, out var typeObj) ||
                 !(typeObj is string type))
             {
-                if (TryDelete(path)) BumpCount(-1);
+                TryDelete(path);
                 return;
             }
 
             if (IsIdentityMessage(type))
             {
-                if (TryDelete(path)) BumpCount(-1);
+                TryDelete(path);
                 return;
             }
 
@@ -195,11 +187,11 @@ private void RewriteTrackWithoutUserId(string path, Dictionary<string, object> m
             catch (IOException)
             {
                 // Delete rather than leave the old userId-bearing payload.
-                if (TryDelete(path)) BumpCount(-1);
+                TryDelete(path);
             }
             catch (UnauthorizedAccessException)
             {
-                if (TryDelete(path)) BumpCount(-1);
+                TryDelete(path);
             }
         }