From 441a692dfca59cec9f6b61c3b9aea75b25815347 Mon Sep 17 00:00:00 2001
From: Jason Volk <jason@zemos.net>
Date: Sat, 24 Mar 2018 17:41:16 -0700
Subject: [PATCH] ircd::ctx: Device to allow context switching in exception
 handler.

---
 include/ircd/ctx/ctx.h | 22 ++++++++++++++++++++++
 ircd/ctx.cc            | 18 ++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/include/ircd/ctx/ctx.h b/include/ircd/ctx/ctx.h
index 829be48aa..665820212 100644
--- a/include/ircd/ctx/ctx.h
+++ b/include/ircd/ctx/ctx.h
@@ -63,6 +63,7 @@ namespace ircd::ctx { inline namespace this_ctx
 /// Interface to the currently running context
 {
 	struct critical_assertion;                   // Assert no yielding for a section
+	struct exception_handler;                    // Must be present to yield in a handler
 
 	/// Points to the currently running context or null for main stack (do not modify)
 	extern __thread struct ctx *current;
@@ -147,6 +148,27 @@ class ircd::ctx::this_ctx::critical_assertion
 	~critical_assertion() noexcept;
 };
 
+/// An instance of exception_handler must be present to allow a context
+/// switch inside a catch block. This is due to ABI limitations that stack
+/// exceptions with thread-local assumptions and don't expect catch blocks
+/// on the same thread to interleave when we switch the stack.
+///
+/// We first increment the refcount for the caught exception so it remains
+/// intuitively accessible for the rest of the catch block. Then the presence
+/// of this object makes the ABI believe the catch block has ended.
+///
+/// The exception cannot then be rethrown. DO NOT RETHROW THE EXCEPTION.
+///
+struct ircd::ctx::this_ctx::exception_handler
+:std::exception_ptr
+{
+	exception_handler() noexcept;
+	exception_handler(exception_handler &&) = delete;
+	exception_handler(const exception_handler &) = delete;
+	exception_handler &operator=(exception_handler &&) = delete;
+	exception_handler &operator=(const exception_handler &) = delete;
+};
+
 /// This overload matches ::sleep() and acts as a drop-in for ircd contexts.
 /// interruption point.
 inline void
diff --git a/ircd/ctx.cc b/ircd/ctx.cc
index f62d74828..4f645b0b7 100644
--- a/ircd/ctx.cc
+++ b/ircd/ctx.cc
@@ -9,6 +9,7 @@
 // full license for this software is available in the LICENSE file.
 
 #include <RB_INC_X86INTRIN_H
+#include <cxxabi.h>
 #include <ircd/asio.h>
 
 /// Internal context implementation
@@ -480,6 +481,20 @@ ircd::ctx::id(const ctx &ctx)
 	return ctx.id;
 }
 
+//
+// exception_handler
+//
+
+ircd::ctx::this_ctx::exception_handler::exception_handler()
+noexcept
+:std::exception_ptr{std::current_exception()}
+{
+	assert(bool(*this));
+	assert(!std::uncaught_exceptions());
+	__cxa_end_catch();
+	assert(!std::current_exception());
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 //
 // ctx/continuation.h
@@ -519,6 +534,9 @@ ircd::ctx::continuation::continuation(ctx *const &self)
 	assert(self != nullptr);
 	assert(self->notes <= 1);
 
+	// Note: Construct an instance of ctx::exception_handler to enable yielding
+	// in your catch block.
+	//
 	// GNU cxxabi uses a singly-linked forward list (aka the 'exception
 	// stack') for pending exception activities. Due to this limitation we
 	// cannot interleave _cxa_begin_catch() and __cxa_end_catch() by yielding