From 346b79d3b7dfe649dc33efe582a096418e354450 Mon Sep 17 00:00:00 2001
From: Michael Davidsaver <mdavidsaver@gmail.com>
Date: Mon, 19 Oct 2020 16:41:19 -0700
Subject: [PATCH] Add Subscription::shared_from_this()

Allow event() handler to acquire a shared_ptr<Subscription>
to eg. queue for later processing.
---
 src/clientmon.cpp | 6 ++++++
 src/pvxs/client.h | 5 +++++
 2 files changed, 11 insertions(+)

diff --git a/src/clientmon.cpp b/src/clientmon.cpp
index 7716455..2e6b2f1 100644
--- a/src/clientmon.cpp
+++ b/src/clientmon.cpp
@@ -39,6 +39,7 @@ struct SubscriptionImpl : public OperationBase, public Subscription
     evevent ackTick;
 
     // const after exec()
+    std::weak_ptr<SubscriptionImpl> self; // internal
     std::function<void (const Value&)> onInit;
     std::function<void(Subscription&)> event;
     Value pvRequest;
@@ -173,6 +174,10 @@ struct SubscriptionImpl : public OperationBase, public Subscription
         return ret;
     }
 
+    virtual std::shared_ptr<Subscription> shared_from_this() const override final {
+        return std::shared_ptr<Subscription>(self);
+    }
+
     virtual bool cancel() override final {
         auto context = chan->context;
         decltype (event) junk;
@@ -552,6 +557,7 @@ std::shared_ptr<Subscription> MonitorBuilder::exec()
         auto chan = Channel::build(ctx->shared_from_this(), _name);
 
         auto op = std::make_shared<SubscriptionImpl>(Operation::Monitor, chan);
+        op->self = op;
         op->event = std::move(_event);
         op->onInit = std::move(_onInit);
         op->pvRequest = _buildReq();
diff --git a/src/pvxs/client.h b/src/pvxs/client.h
index 2a69c21..f8bda66 100644
--- a/src/pvxs/client.h
+++ b/src/pvxs/client.h
@@ -193,6 +193,11 @@ public:
      * @endcode
      */
     virtual Value pop() =0;
+
+    //! Return strong internal reference which will not prevent
+    //! implicit cancellation when the last reference returned
+    //! by exec() is released.
+    virtual std::shared_ptr<Subscription> shared_from_this() const =0;
 };
 
 //! Handle for entry in Channel cache