Adds LogDnsClient::NotifyWhenNotThrottled

components/certificate_transparency/log_dns_client.cc

Index: components/certificate_transparency/log_dns_client.cc
diff --git a/components/certificate_transparency/log_dns_client.cc b/components/certificate_transparency/log_dns_client.cc
index 1344cb86ccdb682fcdcaeb64919e7e4a9e8776ae..a7601c0d3817ef6815e688cff2feb26fdc08ffb0 100644
--- a/components/certificate_transparency/log_dns_client.cc
+++ b/components/certificate_transparency/log_dns_client.cc
@@ -352,10 +352,16 @@ void LogDnsClient::OnDNSChanged() {
 
 void LogDnsClient::OnInitialDNSConfigRead() {
   UpdateDnsConfig();
 }
 
+void LogDnsClient::NotifyWhenNotThrottled(
+    const NotThrottledCallback& callback) {
+  DCHECK(HasMaxConcurrentQueriesInProgress());
+  not_throttled_callbacks_.push_back(callback);
+}
+
 net::Error LogDnsClient::QueryAuditProof(base::StringPiece domain_for_log,
                                          base::StringPiece leaf_hash,
                                          uint64_t tree_size,
                                          const AuditProofCallback& callback) {
   if (domain_for_log.empty() || leaf_hash.size() != crypto::kSHA256Length) {
@@ -399,10 +405,16 @@ void LogDnsClient::QueryAuditProofComplete(const AuditProofCallback& callback,
   DCHECK(query_iterator != audit_proof_queries_.end());
   audit_proof_queries_.erase(query_iterator);
 
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE, base::Bind(callback, result, base::Passed(&proof)));
+
+  for (const NotThrottledCallback& callback : not_throttled_callbacks_) {
+    base::ThreadTaskRunnerHandle::Get()->PostTask(
+        FROM_HERE, base::Bind(callback, weak_ptr_factory_.GetWeakPtr()));

[Ryan Sleevi, 2016/10/03 23:56:04]

From a design perspective: Why asynchronously? Why not synchronously?

[Rob Percival, 2016/10/04 18:53:03]

An abundance of caution, but actually this can be done synchronously without
issue.

+  }
+  not_throttled_callbacks_.clear();
 }
 
 bool LogDnsClient::HasMaxConcurrentQueriesInProgress() const {
   return max_concurrent_queries_ != 0 &&
          audit_proof_queries_.size() >= max_concurrent_queries_;