Commits on Apr 11, 2013

1. stream: Fix unshift() race conditions …

   Fix #5272
   
   The consumption of a readable stream is a dance with 3 partners.
   
   1. The specific stream Author (A)
   2. The Stream Base class (B), and
   3. The Consumer of the stream (C)
   
   When B calls the _read() method that A implements, it sets a 'reading'
   flag, so that parallel calls to _read() can be avoided.  When A calls
   stream.push(), B knows that it's safe to start calling _read() again.
   
   If the consumer C is some kind of parser that wants in some cases to
   pass the source stream off to some other party, but not before "putting
   back" some bit of previously consumed data (as in the case of Node's
   websocket http upgrade implementation).  So, stream.unshift() will
   generally *never* be called by A, but *only* called by C.
   
   Prior to this patch, stream.unshift() *also* unset the state.reading
   flag, meaning that C could indicate the end of a read, and B would
   dutifully fire off another _read() call to A.  This is inappropriate.
   In the case of fs streams, and other variably-laggy streams that don't
   tolerate overlapped _read() calls, this causes big problems.
   
   Also, calling stream.shift() after the 'end' event did not raise any
   kind of error, but would cause very strange behavior indeed.  Calling it
   after the EOF chunk was seen, but before the 'end' event was fired would
   also cause weird behavior, and could lead to data being lost, since it
   would not emit another 'readable' event.
   
   This change makes it so that:
   
   1. stream.unshift() does *not* set state.reading = false
   2. stream.unshift() is allowed up until the 'end' event.
   3. unshifting onto a EOF-encountered and zero-length (but not yet
   end-emitted) stream will defer the 'end' event until the new data is
   consumed.
   4. pushing onto a EOF-encountered stream is now an error.
   
   So, if you read(), you have that single tick to safely unshift() data
   back into the stream, even if the null chunk was pushed, and the length
   was 0.

   

   isaacs committed Apr 11, 2013
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