Stop frobbing timestamp_dirty as our sole means to mark circuits unusable

In a number of places, we decrement timestamp_dirty by
MaxCircuitDirtiness in order to mark a stream as "unusable for any
new connections.

This pattern sucks for a few reasons:
  * It is nonobvious.
  * It is error-prone: decrementing 0 can be a bad choice indeed.
  * It really wants to have a function.

It can also introduce bugs if the system time jumps backwards, or if
MaxCircuitDirtiness is increased.

So in this patch, I add an unusable_for_new_conns flag to
origin_circuit_t, make it get checked everywhere it should (I looked
for things that tested timestamp_dirty), and add a new function to
frob it.

For now, the new function does still frob timestamp_dirty (after
checking for underflow and whatnot), in case I missed any cases that
should be checking unusable_for_new_conns.

Fixes bug 6174. We first used this pattern in 516ef41ac1fd26f338c,
which I think was in 0.0.2pre26 (but it could have been 0.0.2pre27).

1 files changed, 1 insertions, 0 deletions

diff --git a/src/or/circuituse.h b/src/or/circuituse.h
index d4d68aad92..11e5a64163 100644
--- a/src/or/circuituse.h
+++ b/src/or/circuituse.h
@@ -55,6 +55,7 @@ void circuit_change_purpose(circuit_t *circ, uint8_t new_purpose);
 
 int hostname_in_track_host_exits(const or_options_t *options,
                                  const char *address);
+void mark_circuit_unusable_for_new_conns(origin_circuit_t *circ);
 
 #endif