Slow clients can't bootstrap because they expire their consensus fetch but then receive all the bytes from it anyway, making them expire their next fetch, putting them in a terrible loop

If you start your Tor client with no cached directory info, and on a slow (high latency) link, you get:

$ tail -f tordebug-log|grep connected
Aug 09 16:17:12.299 [info] circuit_handle_first_hop(): Next router is $7EA6EAD6FD83083C538F44038BBFA077587DD755~7EA6EAD6FD83083C538 at 194.109.206.212: Not connected. Connecting.
Aug 09 16:17:12.826 [info] connection_edge_process_relay_cell_not_open(): 'connected' received for circid 2747423797 streamid 16685 after 0 seconds.
Aug 09 16:21:57.298 [info] circuit_handle_first_hop(): Next router is $9695DFC35FFEB861329B9F1AB04C46397020CE31~9695DFC35FFEB861329 at 128.31.0.39: Not connected. Connecting.
Aug 09 16:21:59.099 [info] connection_edge_process_relay_cell_not_open(): 'connected' received for circid 4248917890 streamid 42612 after 1 seconds.
Aug 09 16:22:09.711 [info] circuit_handle_first_hop(): Next router is $332CD489177F202570A7021328A17A91BF823889~332CD489177F202570A at 192.150.94.49: Not connected. Connecting.
Aug 09 16:22:09.711 [info] circuit_handle_first_hop(): Next router is $90E9E44FD74B98F87F7573F917AE4AF651B86B4C~90E9E44FD74B98F87F7 at 5.102.146.106: Not connected. Connecting.
Aug 09 16:22:09.712 [info] circuit_handle_first_hop(): Next router is $547C1CDB516798EC66A01F04A5884DCE1A151919~547C1CDB516798EC66A at 87.72.85.217: Not connected. Connecting.
Aug 09 16:22:12.499 [info] connection_edge_process_relay_cell_not_open(): 'connected' received for circid 2850575558 streamid 43745 after 0 seconds.
Aug 09 16:23:33.901 [info] connection_edge_process_relay_cell(): 'connected' received on circid 2850575558 for streamid 43746, no conn attached anymore. Ignoring.
Aug 09 16:24:11.599 [info] connection_edge_process_relay_cell_not_open(): 'connected' received for circid 4148503990 streamid 17036 after 0 seconds.
Aug 09 16:25:34.306 [info] connection_edge_process_relay_cell(): 'connected' received on circid 4148503990 for streamid 17037, no conn attached anymore. Ignoring.
Aug 09 16:26:29.559 [info] connection_edge_process_relay_cell_not_open(): 'connected' received for circid 2948078868 streamid 42748 after 0 seconds.
[...]

Oh hey, what's this "no conn attached anymore" issue?

$ grep 43746 tordebug-log
Aug 09 16:22:12.302 [info] connection_ap_handshake_send_begin(): Sending relay cell 1 on circ 2850575558 to begin stream 43746.
Aug 09 16:22:22.299 [debug] circuit_detach_stream(): Removing stream 43746 from circ 2850575558
Aug 09 16:23:33.901 [debug] connection_edge_process_relay_cell(): Now seen 1433 relay cells here (command 4, stream 43746).
Aug 09 16:23:33.901 [info] connection_edge_process_relay_cell(): 'connected' received on circid 2850575558 for streamid 43746, no conn attached anymore. Ignoring.
Aug 09 16:23:35.799 [debug] connection_edge_process_relay_cell(): Now seen 1434 relay cells here (command 2, stream 43746).
Aug 09 16:23:35.799 [info] connection_edge_process_relay_cell(): data cell dropped, unknown stream (streamid 43746).
[...]

We're hitting the 10-second stream timeout in connection_ap_expire_beginning(), and detaching the stream from the circuit, and presumably trying it again elsewhere.

But that last line above, "data cell dropped", is especially bad news for us -- we get the whole answer, and ignore it all, since we sent an 'end' cell changing our mind but the answer was already coming at us.

This situation comes up because of the optimistic data feature -- we get the answer to our request bundled with the 'connected' cell, which is a feature except in the case where we canceled (and then forgot about) the stream.

For people trying to bootstrap their Tor on a low-bandwidth high-latency network connection, I bet this landmine will be especially frustrating, since you will be clogging your network connection with directory information that you will discard, which in turn will delay the receipt of the other directory information.

You can reproduce the bug on your own, even if you have a great network connection, by starting your Tor with "bandwidthrate 2000 bandwidthburst 5000".