问题
I'm running into an issue where reading from a HttpResponseStream fails because the StreamReader that I'm wrapping around in reads faster that the Response Stream gets the actual response. I'm retrieving a reasonably small sized file (around 60k) but the Parser which processes the response into an actual object fails because it hits an unexpected character (Code 65535) which from experience I know to be the character produced when you read from a StreamReader and there are no further characters available.
For the record I know that the content being returned is valid and will parse correctly since the failure occurs at different points in the file each time I run the code. It's the parser.Load() line in the following where it fails.
Is there a way to ensure I've read all the content before attempting to parse it short of copying the response stream into a MemoryStream or string and then processing it?
/// <summary>
/// Makes a Query where the expected Result is an RDF Graph ie. CONSTRUCT and DESCRIBE Queries
/// </summary>
/// <param name="sparqlQuery">SPARQL Query String</param>
/// <returns>RDF Graph</returns>
public Graph QueryWithResultGraph(String sparqlQuery)
{
try
{
//Build the Query URI
StringBuilder queryUri = new StringBuilder();
queryUri.Append(this._endpoint.ToString());
queryUri.Append("?query=");
queryUri.Append(Uri.EscapeDataString(sparqlQuery));
if (!this._defaultGraphUri.Equals(String.Empty))
{
queryUri.Append("&default-graph-uri=");
queryUri.Append(Uri.EscapeUriString(this._defaultGraphUri));
}
//Make the Query via HTTP
HttpWebResponse httpResponse = this.DoQuery(new Uri(queryUri.ToString()),false);
//Set up an Empty Graph ready
Graph g = new Graph();
g.BaseURI = this._endpoint;
//Parse into a Graph based on Content Type
String ctype = httpResponse.ContentType;
IRDFReader parser = MIMETypesHelper.GetParser(ctype);
parser.Load(g, new StreamReader(httpResponse.GetResponseStream()));
return g;
}
catch (UriFormatException uriEx)
{
//URI Format Invalid
throw new Exception("The format of the URI was invalid", uriEx);
}
catch (WebException webEx)
{
//Some sort of HTTP Error occurred
throw new Exception("A HTTP Error occurred", webEx);
}
catch (RDFException)
{
//Some problem with the RDF or Parsing thereof
throw;
}
catch (Exception)
{
//Other Exception
throw;
}
}
/// <summary>
/// Internal Helper Method which executes the HTTP Requests against the SPARQL Endpoint
/// </summary>
/// <param name="target">URI to make Request to</param>
/// <param name="sparqlOnly">Indicates if only SPARQL Result Sets should be accepted</param>
/// <returns>HTTP Response</returns>
private HttpWebResponse DoQuery(Uri target, bool sparqlOnly)
{
//Expect errors in this function to be handled by the calling function
//Set-up the Request
HttpWebRequest httpRequest;
HttpWebResponse httpResponse;
httpRequest = (HttpWebRequest)WebRequest.Create(target);
//Use HTTP GET/POST according to user set preference
if (!sparqlOnly)
{
httpRequest.Accept = MIMETypesHelper.HTTPAcceptHeader();
//For the time being drop the application/json as this doesn't play nice with Virtuoso
httpRequest.Accept = httpRequest.Accept.Replace("," + MIMETypesHelper.JSON[0], String.Empty);
}
else
{
httpRequest.Accept = MIMETypesHelper.HTTPSPARQLAcceptHeader();
}
httpRequest.Method = this._httpMode;
httpRequest.Timeout = this._timeout;
//HTTP Debugging
if (Options.HTTPDebugging)
{
Tools.HTTPDebugRequest(httpRequest);
}
httpResponse = (HttpWebResponse)httpRequest.GetResponse();
//HTTP Debugging
if (Options.HTTPDebugging)
{
Tools.HTTPDebugResponse(httpResponse);
}
return httpResponse;
}
Edit
To clarify what I already stated this is not a bug in the Parser, this is an issue of the StreamReader reading faster than the Response Stream provides data. I can get around this by doing the following but would like suggestions of better or more elegant solutions:
//Parse into a Graph based on Content Type
String ctype = httpResponse.ContentType;
IRDFReader parser = MIMETypesHelper.GetParser(ctype);
Stream response = httpResponse.GetResponseStream();
MemoryStream temp = new MemoryStream();
Tools.StreamCopy(response, temp);
response.Close();
temp.Seek(0, SeekOrigin.Begin);
parser.Load(g, new StreamReader(temp));
Edit 2
BlockingStreamReader class as per Eamon's suggestion:
/// <summary>
/// A wrapper to a Stream which does all its Read() and Peek() calls using ReadBlock() to handle slow underlying streams (eg Network Streams)
/// </summary>
public sealed class BlockingStreamReader : StreamReader
{
private bool _peeked = false;
private int _peekChar = -1;
public BlockingStreamReader(StreamReader reader) : base(reader.BaseStream) { }
public BlockingStreamReader(Stream stream) : base(stream) { }
public override int Read()
{
if (this._peeked)
{
this._peeked = false;
return this._peekChar;
}
else
{
if (this.EndOfStream) return -1;
char[] cs = new char[1];
base.ReadBlock(cs, 0, 1);
return cs[0];
}
}
public override int Peek()
{
if (this._peeked)
{
return this._peekChar;
}
else
{
if (this.EndOfStream) return -1;
this._peeked = true;
char[] cs = new char[1];
base.ReadBlock(cs, 0, 1);
this._peekChar = cs[0];
return this._peekChar;
}
}
public new bool EndOfStream
{
get
{
return (base.EndOfStream && !this._peeked);
}
}
}
Edit 3
Here is a much improved solution which can wrap any TextReader and provide an EndOfStream property. It uses an internal buffer which is filled by using ReadBlock() on the wrapped TextReader. All the Read() methods of the reader can the be defined using this buffer, buffer size is configurable:
/// <summary>
/// The BlockingTextReader is an implementation of a <see cref="TextReader">TextReader</see> designed to wrap other readers which may or may not have high latency.
/// </summary>
/// <remarks>
/// <para>
/// This is designed to avoid premature detection of end of input when the input has high latency and the consumer tries to read from the input faster than it can return data. All methods are defined by using an internal buffer which is filled using the <see cref="TextReader.ReadBlock">ReadBlock()</see> method of the underlying <see cref="TextReader">TextReader</see>
/// </para>
/// </remarks>
public sealed class BlockingTextReader : TextReader
{
private char[] _buffer;
private int _pos = -1;
private int _bufferAmount = -1;
private bool _finished = false;
private TextReader _reader;
public const int DefaultBufferSize = 1024;
public BlockingTextReader(TextReader reader, int bufferSize)
{
if (reader == null) throw new ArgumentNullException("reader", "Cannot read from a null TextReader");
if (bufferSize < 1) throw new ArgumentException("bufferSize must be >= 1", "bufferSize");
this._reader = reader;
this._buffer = new char[bufferSize];
}
public BlockingTextReader(TextReader reader)
: this(reader, DefaultBufferSize) { }
public BlockingTextReader(Stream input, int bufferSize)
: this(new StreamReader(input), bufferSize) { }
public BlockingTextReader(Stream input)
: this(new StreamReader(input)) { }
private void FillBuffer()
{
this._pos = -1;
if (this._finished)
{
this._bufferAmount = 0;
}
else
{
this._bufferAmount = this._reader.ReadBlock(this._buffer, 0, this._buffer.Length);
if (this._bufferAmount == 0 || this._bufferAmount < this._buffer.Length) this._finished = true;
}
}
public override int ReadBlock(char[] buffer, int index, int count)
{
if (count == 0) return 0;
if (buffer == null) throw new ArgumentNullException("buffer");
if (index < 0) throw new ArgumentException("index", "Index must be >= 0");
if (count < 0) throw new ArgumentException("count", "Count must be >= 0");
if ((buffer.Length - index) < count) throw new ArgumentException("Buffer too small");
if (this._bufferAmount == -1 || this._pos >= this._bufferAmount)
{
if (!this._finished)
{
this.FillBuffer();
if (this.EndOfStream) return 0;
}
else
{
return 0;
}
}
this._pos = Math.Max(0, this._pos);
if (count <= this._bufferAmount - this._pos)
{
//If we have sufficient things buffered to fufill the request just copy the relevant stuff across
Array.Copy(this._buffer, this._pos, buffer, index, count);
this._pos += count;
return count;
}
else
{
int copied = 0;
while (copied < count)
{
int available = this._bufferAmount - this._pos;
if (count < copied + available)
{
//We can finish fufilling this request this round
int toCopy = Math.Min(available, count - copied);
Array.Copy(this._buffer, this._pos, buffer, index + copied, toCopy);
copied += toCopy;
this._pos += toCopy;
return copied;
}
else
{
//Copy everything we currently have available
Array.Copy(this._buffer, this._pos, buffer, index + copied, available);
copied += available;
this._pos = this._bufferAmount;
if (!this._finished)
{
//If we haven't reached the end of the input refill our buffer and continue
this.FillBuffer();
if (this.EndOfStream) return copied;
this._pos = 0;
}
else
{
//Otherwise we have reached the end of the input so just return what we've managed to copy
return copied;
}
}
}
return copied;
}
}
public override int Read(char[] buffer, int index, int count)
{
return this.ReadBlock(buffer, index, count);
}
public override int Read()
{
if (this._bufferAmount == -1 || this._pos >= this._bufferAmount - 1)
{
if (!this._finished)
{
this.FillBuffer();
if (this.EndOfStream) return -1;
}
else
{
return -1;
}
}
this._pos++;
return (int)this._buffer[this._pos];
}
public override int Peek()
{
if (this._bufferAmount == -1 || this._pos >= this._bufferAmount - 1)
{
if (!this._finished)
{
this.FillBuffer();
if (this.EndOfStream) return -1;
}
else
{
return -1;
}
}
return (int)this._buffer[this._pos + 1];
}
public bool EndOfStream
{
get
{
return this._finished && (this._pos >= this._bufferAmount - 1);
}
}
public override void Close()
{
this._reader.Close();
}
protected override void Dispose(bool disposing)
{
this.Close();
this._reader.Dispose();
base.Dispose(disposing);
}
}
回答1:
Without knowing the specifics the parser you're using, I can only guess at the bug, but there's a fairly easy to make bug the .NET framework I/O libs almost encourage you to make...
Are you aware of the fact that Streams and TextReaders may read fewer bytes/characters than requested?
In particular, TextReader.Read(char[] buffer, int index, int count)'s docs say:
Return Value
Type: System..::.Int32
The number of characters that have been read. The number will be less than or equal to count, depending on whether the data is available within the stream. This method returns zero if called when no more characters are left to read.
Emphasis mine.
For example, if you call reader.Read(buffer, 0, 100) you cannot assume that 100 characters have been read.
Edit: It's very likely that the parser does assume this; and this explains your observed behavior: if you fully cache the stream in a MemoryStream, there will always be enough characters to fullfill the request - but if you don't, the parser will receive fewer characters than requested at unpredictable times whenever the underlying stream is "slow".
Edit2: You can fix your bug by replacing all instances of TextReader.Read() in the parser with TextReader.ReadBlock().
回答2:
To support a blocking read scenario, rather than subclassing StreamReader, you can subclass TextReader: this avoids issues with EndOfStream, and it means you can make any reader blocking - not just StreamReaders:
public sealed class BlockingReader : TextReader
{
bool hasPeeked;
int peekChar;
readonly TextReader reader;
public BlockingReader(TextReader reader) { this.reader = reader; }
public override int Read()
{
if (!hasPeeked)
return reader.Read();
hasPeeked = false;
return peekChar;
}
public override int Peek()
{
if (!hasPeeked)
{
peekChar = reader.Read();
hasPeeked = true;
}
return peekChar;
}
public override int Read(char[] buffer, int index, int count)
{
if (buffer == null)
throw new ArgumentNullException("buffer");
if (index < 0)
throw new ArgumentOutOfRangeException("index");
if (count < 0)
throw new ArgumentOutOfRangeException("count");
if ((buffer.Length - index) < count)
throw new ArgumentException("Buffer too small");
int peekCharsRead = 0;
if (hasPeeked)
{
buffer[index] = (char)peekChar;
hasPeeked = false;
index++;
count--;
peekCharsRead++;
}
return peekCharsRead + reader.ReadBlock(buffer, index, count);
}
protected override void Dispose(bool disposing)
{
try
{
if (disposing)
reader.Dispose();
}
finally
{
base.Dispose(disposing);
}
}
}
来源：https://stackoverflow.com/questions/1264952/reading-from-a-httpresponsestream-fails