From ed78582847d3932763ad9b7b239f843306fa6fe9 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Wed, 26 Sep 2018 21:19:34 +0200
Subject: sinkToSource(): Start the coroutine lazily

In particular this causes copyStorePath() from HttpBinaryCacheStore to
only start a download if needed. E.g. if the destination LocalStore
goes to sleep waiting for the path lock and another process creates
the path, then LocalStore::addToStore() will never read from the
source so we don't have to do the download.
---
 src/libutil/serialise.cc | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index 31df6fdfd..0e75eeec2 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -169,17 +169,13 @@ std::unique_ptr<Source> sinkToSource(
     {
         typedef boost::coroutines2::coroutine<std::string> coro_t;
 
+        std::function<void(Sink &)> fun;
         std::function<void()> eof;
-        coro_t::pull_type coro;
+        std::experimental::optional<coro_t::pull_type> coro;
+        bool started = false;
 
         SinkToSource(std::function<void(Sink &)> fun, std::function<void()> eof)
-            : eof(eof)
-            , coro([&](coro_t::push_type & yield) {
-                LambdaSink sink([&](const unsigned char * data, size_t len) {
-                    if (len) yield(std::string((const char *) data, len));
-                });
-                fun(sink);
-            })
+            : fun(fun), eof(eof)
         {
         }
 
@@ -188,11 +184,19 @@ std::unique_ptr<Source> sinkToSource(
 
         size_t read(unsigned char * data, size_t len) override
         {
-            if (!coro) { eof(); abort(); }
+            if (!coro)
+                coro = coro_t::pull_type([&](coro_t::push_type & yield) {
+                    LambdaSink sink([&](const unsigned char * data, size_t len) {
+                            if (len) yield(std::string((const char *) data, len));
+                        });
+                    fun(sink);
+                });
+
+            if (!*coro) { eof(); abort(); }
 
             if (pos == cur.size()) {
-                if (!cur.empty()) coro();
-                cur = coro.get();
+                if (!cur.empty()) (*coro)();
+                cur = coro->get();
                 pos = 0;
             }
 
-- 
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