[tor-bugs] #7743 [Tor]: Avoid needless wasted space in cells

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#7743: Avoid needless wasted space in cells
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 Reporter:  nickm                   |          Owner:                    
     Type:  defect                  |         Status:  new               
 Priority:  major                   |      Milestone:  Tor: 0.2.4.x-final
Component:  Tor                     |        Version:                    
 Keywords:  backport-023 tor-relay  |         Parent:                    
   Points:                          |   Actualpoints:                    
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 In #7707, cypherpunks notes that when packaging cells, we can do some
 really dumb things when bandwidth is low, such reading incoming bytes in a
 super-slow trickle, and packaging them into cells aggressively.

 There are two comparatively easy things I want to do to prevent this:

  * We should use FIONREAD to determine the number of cells waiting on an
 incoming edge socket.  If we determine that more reading in the future
 will let us fit more bytes in a cell, we shouldn't package bytes in a cell
 yet.
  * We could avoid packaging incoming bytes from a stream for so long as
 that circuit has any queued cells that originated here, in case more
 incoming bytes arrive before we'd transmit the current cell.

 This feels like a backport candidate to 0.2.3.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/7743>
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