Only bootstrap a compact range once per log

As opposed to doing a new bootstrap with get-proof-by-hash every time
the next root is constructed.  Bootstrapping the compact range from a
get-proof-by-hash query works for the most part, but fails if the log
included a duplicate entry and gives us the index for that instead.  Log
operators with duplicate entries include Cloudflare and Digicert.

If bootstrap fails (unlucky), we try to bootstrap again once the log's
signed tree head moved forward (hoping the last entry has no duplicate).

The more reliable way to bootstrap a compact range would be to use the
get-entry-and-proof endpoint.  This does not work in practise because
some logs are not implementing this endpoint.  Digicert has such logs.

1 files changed, 5 insertions, 2 deletions

diff --git a/internal/monitor/monitor.go b/internal/monitor/monitor.go
index 6de7193..2fe4d88 100644
--- a/internal/monitor/monitor.go
+++ b/internal/monitor/monitor.go
@@ -11,6 +11,7 @@ package monitor
 
 import (
 	"context"
+	"crypto/sha256"
 	"crypto/x509"
 	"encoding/base64"
 	"fmt"
@@ -32,10 +33,12 @@ type MonitoredLog struct {
 }
 
 // State is the latest append-only state the monitor observed from its local
-// vantage point.  The next entry to download is specified by NextIndex.
+// vantage point.  The compact range covers [0, NextIndex).  The next entry to
+// download from the log is at index NextIndex.
 type State struct {
 	ct.SignedTreeHead `json:"latest_sth"`
-	NextIndex         uint64 `json:"next_index"`
+	CompactRange      [][sha256.Size]byte `json:"compact_range"`
+	NextIndex         uint64              `json:"next_index"`
 }
 
 // Event carries the latest consistent monitor state, found matches, as well as