Bug 1296: Add option+logic to disable CBT learning.

There are now four ways that CBT can be disabled:

1. Network-wide, with the cbtdisabled consensus param.
2. Via config, with "LearnCircuitBuildTimeout 0"
3. Via config, with "AuthoritativeDirectory 1"
4. Via a state file write failure.

2 files changed, 14 insertions, 1 deletions

diff --git a/doc/spec/path-spec.txt b/doc/spec/path-spec.txt
index ae5ab3e3d..2beeedeac 100644
--- a/doc/spec/path-spec.txt
+++ b/doc/spec/path-spec.txt
@@ -382,6 +382,12 @@ of their choices.
    construction. If these parameters are not present in the consensus,
    the listed default values should be used instead.
 
+      cbtdisabled
+        Default: 0
+        Effect: If non-zero, all CircuitBuildTime learning code should be
+                disabled and history should be discarded. For use in
+                emergency situations only.
+
       cbtrecentcount
         Default: 20
         Effect: This is the number of circuit build times to keep track of
diff --git a/doc/tor.1.txt b/doc/tor.1.txt
index 2ae5005d8..e6bce02e3 100644
--- a/doc/tor.1.txt
+++ b/doc/tor.1.txt
@@ -419,9 +419,16 @@ The following options are useful only for clients (that is, if
     fingerprint to look up the bridge descriptor at the bridge authority, if
     it's provided and if UpdateBridgesFromAuthority is set too.
 
+**LearnCircuitBuildTimeout** **0**|**1**::
+    If 0, CircuitBuildTimeout adaptive learning is disabled. (Default: 1)
+
 **CircuitBuildTimeout** __NUM__::
+
     Try for at most NUM seconds when building circuits. If the circuit isn't
-    open in that time, give up on it. (Default: 1 minute.)
+    open in that time, give up on it. If LearnCircuitBuildTimeout is 1, this
+    value serves as the initial value to use before a timeout is learned. If
+    LearnCircuitBuildTimeout is 0, this value is the only value used.
+    (Default: 60 seconds.)
 
 **CircuitIdleTimeout** __NUM__::
     If we have kept a clean (never used) circuit around for NUM seconds, then