diff --git a/include/ircd/ctx/parallel.h b/include/ircd/ctx/parallel.h
index 529b701e4..3cb3878d3 100644
--- a/include/ircd/ctx/parallel.h
+++ b/include/ircd/ctx/parallel.h
@@ -33,7 +33,6 @@ struct ircd::ctx::parallel
 	void rethrow_any_exception();
 	void receiver() noexcept;
 	void sender() noexcept;
-	void sender(const arg &a) noexcept;
 
   public:
 	void wait_avail();
@@ -43,6 +42,8 @@ struct ircd::ctx::parallel
 	void operator()(const arg &a);
 
 	parallel(pool &, const vector_view<arg> &, closure);
+	parallel(parallel &&) = delete;
+	parallel(const parallel &) = delete;
 	~parallel() noexcept;
 };
 
@@ -70,8 +71,11 @@ void
 ircd::ctx::parallel<arg>::operator()(const arg &a)
 {
 	rethrow_any_exception();
-	sender(a);
+	assert(snd < this->a.size());
+	this->a.at(snd) = a;
+	sender();
 	wait_avail();
+	assert(out < this->a.size());
 }
 
 template<class arg>
@@ -81,27 +85,7 @@ ircd::ctx::parallel<arg>::operator()()
 	rethrow_any_exception();
 	sender();
 	wait_avail();
-}
-
-template<class arg>
-void
-ircd::ctx::parallel<arg>::sender(const arg &a)
-noexcept
-{
-	auto &p(*this->p);
-	auto func
-	{
-		std::bind(&parallel::receiver, this)
-	};
-
-	this->a.at(snd++) = a;
-	snd %= this->a.size();
-	out++;
-
-	if(likely(p.size()))
-		p(std::move(func));
-	else
-		func();
+	assert(out < this->a.size());
 }
 
 template<class arg>
@@ -109,16 +93,18 @@ void
 ircd::ctx::parallel<arg>::sender()
 noexcept
 {
+	++snd;
+	snd %= this->a.size();
+
+	++out;
+	assert(out <= this->a.size());
+
 	auto &p(*this->p);
 	auto func
 	{
 		std::bind(&parallel::receiver, this)
 	};
 
-	snd++;
-	snd %= this->a.size();
-	out++;
-
 	if(likely(p.size()))
 		p(std::move(func));
 	else
@@ -130,8 +116,8 @@ void
 ircd::ctx::parallel<arg>::receiver()
 noexcept
 {
-	auto &a(this->a.at(rcv % this->a.size()));
-	rcv++;
+	auto &a{this->a.at(rcv % this->a.size())};
+	++rcv;
 
 	if(!this->eptr) try
 	{
@@ -142,7 +128,9 @@ noexcept
 		this->eptr = std::current_exception();
 	}
 
-	out--;
+	assert(out <= this->a.size());
+	assert(out > 0);
+	--out;
 	d.notify_one();
 }