From a331bc28822963c19a72af6d39278677b6ebd3d1 Mon Sep 17 00:00:00 2001
From: Jason Volk <jason@zemos.net>
Date: Thu, 9 May 2019 19:03:30 -0700
Subject: [PATCH] ircd::m::dbs: Add interposing system for transaction
 self-references.

---
 include/ircd/m/dbs/write_opts.h | 13 ++++++++++++
 ircd/m_dbs.cc                   | 35 ++++++++++++++++++++++++++-------
 2 files changed, 41 insertions(+), 7 deletions(-)

diff --git a/include/ircd/m/dbs/write_opts.h b/include/ircd/m/dbs/write_opts.h
index 17039ec64..73771aec9 100644
--- a/include/ircd/m/dbs/write_opts.h
+++ b/include/ircd/m/dbs/write_opts.h
@@ -56,4 +56,17 @@ struct ircd::m::dbs::write_opts
 	/// Whether the event.source can be used directly for event_json. Defaults
 	/// to false unless the caller wants to avoid a redundant re-stringify.
 	bool json_source {false};
+
+	/// Data in this db::txn is used as a primary source in some cases where
+	/// indexers make a database query. This is useful when the sought data
+	/// has not even been written to the database, and this may even point to
+	/// the same db::txn as the result being composed in the first place. By
+	/// default a database query is made as a fallback after using this.
+	const db::txn *interpose {nullptr};
+
+	/// Whether indexers are allowed to make database queries when composing
+	/// the transaction. note: database queries may yield the ircd::ctx and
+	/// made indepdently; this is slow and requires external synchronization
+	/// to not introduce inconsistent data into the txn.
+	bool allow_queries {true};
 };
diff --git a/ircd/m_dbs.cc b/ircd/m_dbs.cc
index d59bb4e21..f4ba7750c 100644
--- a/ircd/m_dbs.cc
+++ b/ircd/m_dbs.cc
@@ -257,6 +257,11 @@ ircd::m::dbs::write(db::txn &txn,
 // Internal interface
 //
 
+namespace ircd::m::dbs
+{
+	static event::idx find_event_idx(const event::id &, const write_opts &);
+}
+
 void
 ircd::m::dbs::_index_event(db::txn &txn,
                            const event &event,
@@ -438,7 +443,7 @@ ircd::m::dbs::_index_event_refs_prev(db::txn &txn,
 
 		const event::idx &prev_idx
 		{
-			m::index(prev_id, std::nothrow)  // query
+			find_event_idx(prev_id, opts)
 		};
 
 		if(opts.appendix.test(appendix::EVENT_HORIZON) && !prev_idx)
@@ -496,7 +501,7 @@ ircd::m::dbs::_index_event_refs_auth(db::txn &txn,
 
 		const event::idx &auth_idx
 		{
-			m::index(auth_id, std::nothrow)  // query
+			find_event_idx(auth_id, opts)
 		};
 
 		if(unlikely(!auth_idx))
@@ -562,7 +567,9 @@ ircd::m::dbs::_index_event_refs_state(db::txn &txn,
 
 	const event::idx &prev_state_idx
 	{
-		state.get(std::nothrow, at<"type"_>(event), at<"state_key"_>(event)) // query
+		opts.allow_queries?
+			state.get(std::nothrow, at<"type"_>(event), at<"state_key"_>(event)): // query
+			0UL
 	};
 
 	if(!prev_state_idx)
@@ -631,7 +638,7 @@ ircd::m::dbs::_index_event_refs_m_receipt_m_read(db::txn &txn,
 
 	const event::idx &event_idx
 	{
-		m::index(event_id, std::nothrow) // query
+		find_event_idx(event_id, opts)
 	};
 
 	if(opts.appendix.test(appendix::EVENT_HORIZON) && !event_idx)
@@ -728,7 +735,7 @@ ircd::m::dbs::_index_event_refs_m_relates_m_reply(db::txn &txn,
 
 	const event::idx &event_idx
 	{
-		m::index(event_id, std::nothrow) // query
+		find_event_idx(event_id, opts)
 	};
 
 	if(opts.appendix.test(appendix::EVENT_HORIZON) && !event_idx)
@@ -786,7 +793,7 @@ ircd::m::dbs::_index_event_refs_m_room_redaction(db::txn &txn,
 
 	const event::idx &event_idx
 	{
-		m::index(event_id, std::nothrow) // query
+		find_event_idx(event_id, opts)
 	};
 
 	if(opts.appendix.test(appendix::EVENT_HORIZON) && !event_idx)
@@ -1052,7 +1059,7 @@ try
 
 	const m::event::idx target_idx
 	{
-		m::index(target_id, std::nothrow)
+		find_event_idx(target_id, opts)
 	};
 
 	if(unlikely(!target_idx))
@@ -1302,6 +1309,20 @@ ircd::m::dbs::_index__room_state(db::txn &txn,
 	};
 }
 
+ircd::m::event::idx
+ircd::m::dbs::find_event_idx(const event::id &event_id,
+                             const write_opts &wopts)
+{
+	event::idx ret{0};
+	if(wopts.interpose)
+		ret = wopts.interpose->val(db::op::SET, "_event_idx", event_id, 0UL);
+
+	if(wopts.allow_queries && !ret)
+		ret = m::index(event_id, std::nothrow); // query
+
+	return ret;
+}
+
 ircd::string_view
 ircd::m::dbs::state_root(const mutable_buffer &out,
                          const event &event)