Improve handling of Directory creation on linux for FileDistributedLocks

Fix #195

src/DistributedLock.FileSystem/FileDistributedLock.cs

@@ -142,11 +142,13 @@ private void EnsureDirectoryExists()
                 System.IO.Directory.CreateDirectory(this.Directory);
                 return;
             }
-            // This can indicate either a transient failure during concurrent creation/deletion or a permissions issue.
-            // If we encounter it, assume it is transient unless it persists.
-            //catch (UnauthorizedAccessException) when (++retryCount <= MaxUnauthorizedAccessExceptionRetries) { }
             catch (Exception ex)
             {
+                // This can indicate either a transient failure during concurrent creation/deletion or a permissions issue.
+                // If we encounter it, assume it is transient unless it persists.
+                // For a long time, I just checked for UnauthorizedAccessException here. However, recent tests on Linux have
+                // shown that in race conditions we can see IOException as well, presumably because there is some period during
+                // directory creation where it presents as a file.
                 if (ex is UnauthorizedAccessException or IOException
                     && retryCount < MaxUnauthorizedAccessExceptionRetries)
                 {

src/DistributedLock.Tests/Tests/FileSystem/FileDistributedLockTest.cs

@@ -120,6 +120,16 @@ public void TestThrowsIfProvidedFileNameIsAlreadyADirectory()
         Assert.That(exception.Message, Does.Contain("because it is already the name of a directory"));
     }
 
+    [Test]
+    public void TestThrowsIfProvidedDirectoryIsAlreadyFile()
+    {
+        var tempFile = Path.GetTempFileName();
+
+        var @lock = new FileDistributedLock(lockFileDirectory: new(tempFile), "some name");
+        var exception = Assert.Throws<InvalidOperationException>(() => @lock.Acquire().Dispose());
+        Assert.That(exception.InnerException!.Message, Does.Match("file .* already exists"));
+    }
+
     [Test]
     public void TestEmptyNameIsAllowed() => AssertCanUseName(string.Empty);