Fix StreamReader to pass cancellation token into ReadBufferAsync (#27464) (#27501)

ReadAsyncInternal is correctly passing its token into the two call sites where it delegates directly to the underlying stream, but in one place it calls through ReadBufferAsync, which is currently not defined to take a token.  Fix that, and pass the token through.

Author: stephentoub

src/System.Private.CoreLib/shared/System/IO/StreamReader.cs

@@ -852,7 +852,7 @@ private int ReadBuffer(Span<char> userBuffer, out bool readToUserBuffer)
 
         private async Task<string?> ReadLineAsyncInternal()
         {
-            if (_charPos == _charLen && (await ReadBufferAsync().ConfigureAwait(false)) == 0)
+            if (_charPos == _charLen && (await ReadBufferAsync(CancellationToken.None).ConfigureAwait(false)) == 0)
             {
                 return null;
             }
@@ -888,7 +888,7 @@ private int ReadBuffer(Span<char> userBuffer, out bool readToUserBuffer)
 
                         _charPos = tmpCharPos = i + 1;
 
-                        if (ch == '\r' && (tmpCharPos < tmpCharLen || (await ReadBufferAsync().ConfigureAwait(false)) > 0))
+                        if (ch == '\r' && (tmpCharPos < tmpCharLen || (await ReadBufferAsync(CancellationToken.None).ConfigureAwait(false)) > 0))
                         {
                             tmpCharPos = _charPos;
                             if (_charBuffer[tmpCharPos] == '\n')
@@ -909,7 +909,7 @@ private int ReadBuffer(Span<char> userBuffer, out bool readToUserBuffer)
                     sb = new StringBuilder(i + 80);
                 }
                 sb.Append(tmpCharBuffer, tmpCharPos, i);
-            } while (await ReadBufferAsync().ConfigureAwait(false) > 0);
+            } while (await ReadBufferAsync(CancellationToken.None).ConfigureAwait(false) > 0);
 
             return sb.ToString();
         }
@@ -943,7 +943,7 @@ private async Task<string> ReadToEndAsyncInternal()
                 int tmpCharPos = _charPos;
                 sb.Append(_charBuffer, tmpCharPos, _charLen - tmpCharPos);
                 _charPos = _charLen;  // We consumed these characters
-                await ReadBufferAsync().ConfigureAwait(false);
+                await ReadBufferAsync(CancellationToken.None).ConfigureAwait(false);
             } while (_charLen > 0);
 
             return sb.ToString();
@@ -976,7 +976,7 @@ public override Task<int> ReadAsync(char[] buffer, int index, int count)
             ThrowIfDisposed();
             CheckAsyncTaskInProgress();
 
-            Task<int> task = ReadAsyncInternal(new Memory<char>(buffer, index, count), default).AsTask();
+            Task<int> task = ReadAsyncInternal(new Memory<char>(buffer, index, count), CancellationToken.None).AsTask();
             _asyncReadTask = task;
 
             return task;
@@ -1003,7 +1003,7 @@ public override ValueTask<int> ReadAsync(Memory<char> buffer, CancellationToken
 
         internal override async ValueTask<int> ReadAsyncInternal(Memory<char> buffer, CancellationToken cancellationToken)
         {
-            if (_charPos == _charLen && (await ReadBufferAsync().ConfigureAwait(false)) == 0)
+            if (_charPos == _charLen && (await ReadBufferAsync(cancellationToken).ConfigureAwait(false)) == 0)
             {
                 return 0;
             }
@@ -1233,7 +1233,7 @@ public override ValueTask<int> ReadBlockAsync(Memory<char> buffer, CancellationT
             return new ValueTask<int>(t);
         }
 
-        private async ValueTask<int> ReadBufferAsync()
+        private async ValueTask<int> ReadBufferAsync(CancellationToken cancellationToken)
         {
             _charLen = 0;
             _charPos = 0;
@@ -1250,7 +1250,7 @@ private async ValueTask<int> ReadBufferAsync()
                 {
                     Debug.Assert(_bytePos <= _encoding.Preamble.Length, "possible bug in _compressPreamble. Are two threads using this StreamReader at the same time?");
                     int tmpBytePos = _bytePos;
-                    int len = await tmpStream.ReadAsync(new Memory<byte>(tmpByteBuffer, tmpBytePos, tmpByteBuffer.Length - tmpBytePos)).ConfigureAwait(false);
+                    int len = await tmpStream.ReadAsync(new Memory<byte>(tmpByteBuffer, tmpBytePos, tmpByteBuffer.Length - tmpBytePos), cancellationToken).ConfigureAwait(false);
                     Debug.Assert(len >= 0, "Stream.Read returned a negative number!  This is a bug in your stream class.");
 
                     if (len == 0)
@@ -1272,7 +1272,7 @@ private async ValueTask<int> ReadBufferAsync()
                 else
                 {
                     Debug.Assert(_bytePos == 0, "_bytePos can be non zero only when we are trying to _checkPreamble. Are two threads using this StreamReader at the same time?");
-                    _byteLen = await tmpStream.ReadAsync(new Memory<byte>(tmpByteBuffer)).ConfigureAwait(false);
+                    _byteLen = await tmpStream.ReadAsync(new Memory<byte>(tmpByteBuffer), cancellationToken).ConfigureAwait(false);
                     Debug.Assert(_byteLen >= 0, "Stream.Read returned a negative number!  Bug in stream class.");
 
                     if (_byteLen == 0)  // We're at EOF