Fix race condition in GetExitCodeAsync().

After `WaitForExitAsync`, the git process has written all its output to the pipes and terminated.
However, it's possible that CodeConverter process has not yet read the data from the pipe, or that `OutputDataReceived` event is still executing after `GetExitCodeAsync()` returns.

This can cause an `ObjectDisposedException` if the caller manages to dispose the `StringWriter` before the data is read from the pipe.

The `OutputDataReceived` event is called once with `e.Data == null` to signify the end-of-output. Waiting for that prevents the race condition.

Author: dgrunwald

CommandLine/CodeConv.Shared/Util/ProcessRunner.cs

@@ -36,18 +36,28 @@ private static async Task<int> GetExitCodeAsync(this ProcessStartInfo psi, TextW
             psi.RedirectStandardError = true;
             psi.RedirectStandardOutput = true;
             using var process = new Process() { StartInfo = psi };
-            process.OutputDataReceived += (sender, e) => stdOut.WriteLine(e.Data);
-            process.ErrorDataReceived += (sender, e) => stdErr.WriteLine(e.Data);
+            var stdOutComplete = new TaskCompletionSource<object?>();
+            var stdErrComplete = new TaskCompletionSource<object?>();
+            process.OutputDataReceived += (sender, e) => {
+                if (e.Data != null)
+                    stdOut.WriteLine(e.Data);
+                else
+                    stdOutComplete.SetResult(null);
+            };
+            process.ErrorDataReceived += (sender, e) => {
+                if (e.Data != null)
+                    stdErr.WriteLine(e.Data);
+                else
+                    stdErrComplete.SetResult(null);
+            };
             try {
                 process.Start();
             } catch (Win32Exception win32Exception) {
                 return win32Exception.ErrorCode;
             }
             process.BeginOutputReadLine();
             process.BeginErrorReadLine();
-            await process.WaitForExitAsync();
-            await stdOut.FlushAsync();
-            await stdErr.FlushAsync();
+            await Task.WhenAll(process.WaitForExitAsync(), stdOutComplete.Task, stdErrComplete.Task);
 
             return process.ExitCode;
         }