[Statistics] Record FTP exceptions to logs (#463)

This also tweaks the retry pattern inside of `FtpDownloadStream`. Previously, `FtpDownloadStream` would dispose itself if a download failed. It will now disposes just the stream that failed to download.

Author: loic-sharma

src/Stats.CollectAzureCdnLogs/Ftp/FtpDownloadStream.cs

@@ -16,7 +16,6 @@ internal sealed class FtpDownloadStream
         private readonly Uri _uri;
         private Stream _stream;
         private int _totalDone;
-        private bool _disposing;
 
         public Exception CaughtException { get; private set; }
 
@@ -31,12 +30,11 @@ public override async Task<int> ReadAsync(byte[] buffer, int offset, int count,
             var attempts = 0;
             while (attempts++ < 5)
             {
-                var uriString = _uri.ToString();
                 if (_stream == null)
                 {
                     _stream = await _client.StartOrResumeFtpDownload(_uri, _totalDone);
 
-                    _client.Logger.LogInformation("Finishing download from '{FtpBlobUri}'", uriString);
+                    _client.Logger.LogInformation("Finishing download from '{FtpBlobUri}'", _uri);
                 }
                 try
                 {
@@ -51,11 +49,11 @@ public override async Task<int> ReadAsync(byte[] buffer, int offset, int count,
                 {
                     CaughtException = ex;
 
-                    _client.Logger.LogError("Failed to download file.", ex);
+                    _client.Logger.LogError(0, ex, "Failed to download file after {Attempts} attempts", attempts);
 
-                    // Close ftp resources if possible.
-                    // Set instances to null to force restart.
-                    Close();
+                    // Close ftp resources and set instance to null to restart the download.
+                    _stream?.Dispose();
+                    _stream = null;
                 }
             }
             return 0;
@@ -86,34 +84,22 @@ public override int Read(byte[] buffer, int offset, int count)
                 {
                     CaughtException = ex;
 
-                    // Close ftp resources if possible. Set instances to null to force restart.
-                    Close();
+                    _client.Logger.LogError(0, ex, "Failed to download file after {Attempts}", attempts);
+
+                    // Close ftp resources and set instance to null to restart the download.
+                    _stream?.Dispose();
+                    _stream = null;
                 }
             }
             return 0;
         }
 
-        public override void Close()
+        protected override void Dispose(bool disposing)
         {
-            if (_disposing)
-            {
-                return;
-            }
-
-            _disposing = true;
-
-            if (_stream != null)
+            if (disposing)
             {
-                try
-                {
-                    _stream.Close();
-                }
-                catch
-                {
-                    // No action required
-                }
+                _stream?.Dispose();
             }
-            _stream = null;
         }
 
         public override void Flush()

src/Stats.CollectAzureCdnLogs/Ftp/FtpRawLogClient.cs

@@ -186,12 +186,11 @@ internal async Task<Stream> StartOrResumeFtpDownload(Uri uri, int contentOffset
         {
             if (contentOffset == 0)
             {
-                Logger.LogInformation("Beginning download from '{FtpBlobUri}'.", uri.ToString());
                 Logger.LogInformation("Downloading file '{FtpBlobUri}'.", uri);
             }
             else
             {
-                Logger.LogInformation("Resuming download from '{FtpBlobUri}' at content offset {ContentOffset}.", uri.ToString(), contentOffset);
+                Logger.LogInformation("Resuming download from '{FtpBlobUri}' at content offset {ContentOffset}.", uri, contentOffset);
             }
 
             var request = CreateRequest(uri);