Prop210: Add schedules for simultaneous client consensus downloads

Prop210: Add attempt-based connection schedules

Existing tor schedules increment the schedule position on failure,
then retry the connection after the scheduled time.

To make multiple simultaneous connections, we need to increment the
schedule position when making each attempt, then retry a (potentially
simultaneous) connection after the scheduled time.

(Also change find_dl_schedule_and_len to find_dl_schedule, as it no
longer takes or returns len.)

Prop210: Add multiple simultaneous consensus downloads for clients

Make connections on TestingClientBootstrapConsensus*DownloadSchedule,
incrementing the schedule each time the client attempts to connect.

Check if the number of downloads is less than
TestingClientBootstrapConsensusMaxInProgressTries before trying any
more connections.

1 files changed, 9 insertions, 0 deletions

diff --git a/changes/bug4483-multiple-consensus-downloads b/changes/bug4483-multiple-consensus-downloads
new file mode 100644
index 0000000000..23d22a89c4
--- /dev/null
+++ b/changes/bug4483-multiple-consensus-downloads
@@ -0,0 +1,9 @@
+  o Major features (consensus downloads):
+    - Schedule multiple in-progress consensus downloads during client
+      bootstrap. Use the first one that starts downloading, close the
+      rest. This reduces failures when authorities are slow or down.
+      With #15775, it reduces failures due to fallback churn.
+      Implements #4483 (reduce failures when authorities are down).
+      Patch by "teor".
+      Implements IPv4 portions of proposal #210 by "mikeperry" and
+      "teor".