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16132132
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2013-04-21T14:23:29.770
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2013-04-21T14:23:29.770
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Your code accepts one connection and then relays one block of data on one direction and then never looks at that connection again. Your second chunk of code has a variety of problems, but the big one is that it can deadlock. During your call to <code>B.sendall</code>, if <code>B</code> refuses to read any data until it is able to finish sending to <code>A</code>, which may be legal in the protocol you're relaying, your proxy and <code>B</code> will be waiting for each other forever because your proxy refuses to read any data from <code>B</code> until it finishes sending. A proper relays must attempt to do four things at once and may not wait for one thing to finish before starting another. These four things are: 1) Read from connection A. (Unless a large amount of data is already backed up.) 2) Read from connection B. (Unless a large amount of data is already backed up.) 3) Write to connection A. (Unless there is no unrelayed data that has been received from connection B.) 4) Write to connection B. (unless there is no unrelayed data that has been received from connection A.) In particular, you need two buffers, one for unsent data in each direction. You cannot alternate directions because that will require you to wait for one of the nodes to receive data, which can deadlock if that node is waiting for you to receive data. Generally, the simplest way to implement a proxy is with two threads per connection, one for each direction.
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1. POPython Simple TCP relay
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1. PTAnswer
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1. USDavid Schwartz
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1. COI don't really follow much. I'll try to look into your answer more. Sorry I'm new at this. Examples would be great too.
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2. CO@MajidArifSiddiqui: The short version is this: You must be extremely careful anytime you wait for one thing to finish before you do something else. If it is possible that one end of the proxy can wait for you to do that something else before it lets that one thing finish, then the proxy and one of its endpoints wait for each other forever.
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