Ensure file handles are correctly disposed

test/SeqCli.Tests/Forwarder/Filesystem/InMemoryStoreDirectory.cs

@@ -30,15 +30,26 @@ public InMemoryStoreFile Create(string name, Span<byte> contents)
 
     public override bool TryDelete(string name)
     {
+        var existing = _files[name];
+
+        if (!existing.CanDelete())
+        {
+            throw new InvalidOperationException($"Cannot delete {name} with active handles");
+        }
+
         return _files.Remove(name);
     }
 
     public override InMemoryStoreFile Replace(string toReplace, string replaceWith)
     {
         _files[toReplace] = _files[replaceWith];
-        _files.Remove(replaceWith);
 
-        return _files[toReplace];
+        if (!TryDelete(toReplace))
+        {
+            throw new InvalidOperationException($"Failed to replace {toReplace} with {replaceWith}");
+        }
+
+        return _files[replaceWith];
     }
 
     public override IEnumerable<(string Name, StoreFile File)> List(Func<string, bool> predicate)

test/SeqCli.Tests/Forwarder/Filesystem/InMemoryStoreFile.cs

@@ -10,6 +10,9 @@ class InMemoryStoreFile : StoreFile
 {
     public byte[] Contents { get; private set; } = Array.Empty<byte>();
 
+    int _activeReaders;
+    int _activeWriters;
+
     public override bool TryGetLength([NotNullWhen(true)] out long? length)
     {
         length = Contents.Length;
@@ -29,12 +32,36 @@ public void Append(Span<byte> incoming)
     public override bool TryOpenRead(long length, [NotNullWhen(true)] out StoreFileReader? reader)
     {
         reader = new InMemoryStoreFileReader(this, (int)length);
+        _activeReaders++;
+
         return true;
     }
 
+    internal void CloseReader()
+    {
+        _activeReaders--;
+    }
+
     public override bool TryOpenAppend([NotNullWhen(true)] out StoreFileAppender? appender)
     {
+        if (_activeWriters != 0)
+        {
+            throw new InvalidOperationException("Attempt to open multiple appenders to the same file");
+        }
+
         appender = new InMemoryStoreFileAppender(this);
+        _activeWriters++;
+
         return true;
     }
+
+    internal void CloseAppender()
+    {
+        _activeWriters--;
+    }
+
+    internal bool CanDelete()
+    {
+        return _activeReaders == 0 && _activeWriters == 0;
+    }
 }

test/SeqCli.Tests/Forwarder/Filesystem/InMemoryStoreFileAppender.cs

@@ -35,5 +35,6 @@ public override void Sync()
 
     public override void Dispose()
     {
+        _storeFile.CloseAppender();
     }
 }

test/SeqCli.Tests/Forwarder/Filesystem/InMemoryStoreFileReader.cs

@@ -25,5 +25,6 @@ public override long CopyTo(Span<byte> buffer, long from = 0, long? length = nul
 
     public override void Dispose()
     {
+        _storeFile.CloseReader();
     }
 }