From 6f25dd05af0fadd6bef23cfa30d9a5aefc6234b4 Mon Sep 17 00:00:00 2001
From: Nick Mathewson <nickm@torproject.org>
Date: Wed, 2 Dec 2020 14:09:33 -0500
Subject: [PATCH] Use a better calculation for deciding if we have enough
 paths.

---
 tor-netdir/src/lib.rs           | 41 ++++++++++++++++++++++++---------
 tor-netdoc/src/doc/netstatus.rs | 33 +++++++++++++++++++++++++-
 2 files changed, 62 insertions(+), 12 deletions(-)

diff --git a/tor-netdir/src/lib.rs b/tor-netdir/src/lib.rs
index 3d437343f..69207e66e 100644
--- a/tor-netdir/src/lib.rs
+++ b/tor-netdir/src/lib.rs
@@ -176,22 +176,41 @@ impl NetDir {
     pub fn relays(&self) -> impl Iterator<Item = Relay<'_>> {
         self.all_relays().filter_map(UncheckedRelay::into_relay)
     }
-    /// Return true if there is enough information in this NetDir to build
-    /// multihop circuits.
-    fn have_enough_paths(&self) -> bool {
-        // TODO: Implement the real path-based algorithm.
-        let mut total_bw = 0_u64;
-        let mut have_bw = 0_u64;
+    /// Return the fraction of total bandwidth weight for a given role
+    /// that we have available information for in this NetDir.
+    fn frac_for_role(&self, role: WeightRole) -> f64 {
+        let mut total_weight = 0_u64;
+        let mut have_weight = 0_u64;
+
         for r in self.all_relays() {
-            let w = self.weights.weight_rs_for_role(&r.rs, WeightRole::Middle);
-            total_bw += w;
+            let w = self.weights.weight_rs_for_role(&r.rs, role);
+            total_weight += w;
             if r.is_usable() {
-                have_bw += w;
+                have_weight += w
             }
         }
 
-        // TODO: Do a real calculation here.
-        have_bw > (total_bw / 2)
+        (have_weight as f64) / (total_weight as f64)
+    }
+    /// Return true if there is enough information in this NetDir to build
+    /// multihop circuits.
+    fn have_enough_paths(&self) -> bool {
+        // If we can build a randomly chosen path with at least this
+        // probability, we know enough information to participate
+        // on the network.
+        let min_pct = self
+            .consensus
+            .params()
+            .get_clamped("min_paths_for_circs_pct", 25, 95)
+            .unwrap_or(60);
+        let min_frac_paths = (min_pct as f64) / 100.0;
+
+        // What fraction of paths can we build?
+        let available = self.frac_for_role(WeightRole::Guard)
+            * self.frac_for_role(WeightRole::Middle)
+            * self.frac_for_role(WeightRole::Exit);
+
+        available >= min_frac_paths
     }
     /// Chose a relay at random.
     ///
diff --git a/tor-netdoc/src/doc/netstatus.rs b/tor-netdoc/src/doc/netstatus.rs
index 6a9acb935..f8064c954 100644
--- a/tor-netdoc/src/doc/netstatus.rs
+++ b/tor-netdoc/src/doc/netstatus.rs
@@ -136,7 +136,33 @@ impl<T> NetParams<T> {
         self.params.get(v.as_ref())
     }
 }
-
+impl<T> NetParams<T>
+where
+    T: Copy + Clone,
+{
+    /// Return a given network parameter by value, if it is present.
+    pub fn get_val<A: AsRef<str>>(&self, v: A) -> Option<T> {
+        self.get(v).map(T::clone)
+    }
+}
+impl<T> NetParams<T>
+where
+    T: Copy + Clone + PartialOrd,
+{
+    /// Return a given network parameter by value, if it is
+    /// present. Clamp it so that it lies within the range `low..=high`.
+    pub fn get_clamped<A: AsRef<str>>(&self, v: A, low: T, high: T) -> Option<T> {
+        self.get_val(v).map(|x| {
+            if x < low {
+                low
+            } else if x > high {
+                high
+            } else {
+                x
+            }
+        })
+    }
+}
 /// A list of subprotocol versions that implementors should/must provide.
 #[allow(dead_code)]
 #[derive(Debug, Clone)]
@@ -477,6 +503,11 @@ impl MDConsensus {
     pub fn bandwidth_weights(&self) -> &NetParams<i32> {
         &self.footer.weights
     }
+
+    /// Return the map of network parameters that this consensus advertises.
+    pub fn params(&self) -> &NetParams<u32> {
+        &self.header.hdr.params
+    }
 }
 
 decl_keyword! {