From f11282f5836f6971423e889852f008c07374e019 Mon Sep 17 00:00:00 2001
From: Jason Volk <jason@zemos.net>
Date: Mon, 4 Feb 2019 19:36:43 -0800
Subject: [PATCH] ircd::db: Fix prefixed iterator seek to pos::BACK.

---
 include/ircd/db/index.h |  3 ++-
 ircd/db.cc              | 33 ++++++++++++++++++++++++++-------
 2 files changed, 28 insertions(+), 8 deletions(-)

diff --git a/include/ircd/db/index.h b/include/ircd/db/index.h
index 1f764d890..13f8914f6 100644
--- a/include/ircd/db/index.h
+++ b/include/ircd/db/index.h
@@ -66,7 +66,8 @@ struct ircd::db::index::const_iterator_base
 
 	using column::const_iterator_base::const_iterator_base;
 
-	template<class pos> friend bool seek(index::const_iterator_base &, const pos &);
+	friend bool seek(index::const_iterator_base &, const string_view &);
+	friend bool seek(index::const_iterator_base &, const pos &);
 };
 
 struct ircd::db::index::const_iterator
diff --git a/ircd/db.cc b/ircd/db.cc
index 9e5f6aa98..f6846ef76 100644
--- a/ircd/db.cc
+++ b/ircd/db.cc
@@ -4086,16 +4086,38 @@ ircd::db::index::applied_opts
 	{ get::PREFIX }
 };
 
-template<class pos>
 bool
 ircd::db::seek(index::const_iterator_base &it,
                const pos &p)
+{
+	switch(p)
+	{
+		case pos::BACK:
+		{
+			// This is inefficient as per RocksDB's prefix impl. unknown why
+			// a seek to NEXT is still needed after walking back one.
+			while(seek(it, pos::NEXT));
+			if(seek(it, pos::PREV))
+				seek(it, pos::NEXT);
+
+			return bool(it);
+		}
+
+		default:
+			break;
+	}
+
+	it.opts |= index::applied_opts;
+	return seek(static_cast<column::const_iterator_base &>(it), p);
+}
+
+bool
+ircd::db::seek(index::const_iterator_base &it,
+               const string_view &p)
 {
 	it.opts |= index::applied_opts;
 	return seek(static_cast<column::const_iterator_base &>(it), p);
 }
-template bool ircd::db::seek<ircd::db::pos>(index::const_iterator_base &, const pos &);
-template bool ircd::db::seek<ircd::string_view>(index::const_iterator_base &, const string_view &);
 
 ircd::db::index::const_iterator
 ircd::db::index::begin(const string_view &key,
@@ -4141,10 +4163,7 @@ ircd::db::index::rbegin(const string_view &key,
 	};
 
 	if(seek(ret, key))
-	{
-		while(seek(ret, pos::NEXT));
-		seek(ret, pos::PREV);
-	}
+		seek(ret, pos::BACK);
 
 	return ret;
 }