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 62db1755f754b1ad28c7fe3151f8df6a073bc862..07a1ef8005fa97e83957092f9b54198407e1fe7e 100644
--- a/components/certificate_transparency/log_dns_client.cc
+++ b/components/certificate_transparency/log_dns_client.cc
@@ -406,10 +406,15 @@ void LogDnsClient::OnDNSChanged() {
 
 void LogDnsClient::OnInitialDNSConfigRead() {
   UpdateDnsConfig();
 }
 
+void LogDnsClient::NotifyWhenNotThrottled(const base::Closure& 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) {
@@ -451,10 +456,15 @@ 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 base::Closure& callback : not_throttled_callbacks_) {
+    callback.Run();
+  }
+  not_throttled_callbacks_.clear();

[Ryan Sleevi, 2016/10/07 15:22:55]

Because the base::Callback could result in deletion of |this| (since it's a
callback and you have no guarantee over what the callback behaviour will be),
I'd just recommend the following

std::list<base::Closure> callbacks = std::move(not_throttled_callbacks_);
for (const auto& callback : callbacks) {
  callback.Run();
}

That is, force it into a stack temporary

[Rob Percival, 2016/10/20 12:43:13]

Done.

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