RemoteStore::addToStore(): Fix race between stderrThread and NAR writer

As pointed out by @B4dM4n, the call to to.flush() on stderrThread is
unsafe because the NAR writer thread is also writing to 'to'.

Fixes #3943.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 69ae0874a..8f17bc34c 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -23,7 +23,8 @@ struct Sink
 };
 
 
-/* A buffered abstract sink. */
+/* A buffered abstract sink. Warning: a BufferedSink should not be
+   used from multiple threads concurrently. */
 struct BufferedSink : virtual Sink
 {
     size_t bufSize, bufPos;
@@ -66,7 +67,8 @@ struct Source
 };
 
 
-/* A buffered abstract source. */
+/* A buffered abstract source. Warning: a BufferedSink should not be
+   used from multiple threads concurrently. */
 struct BufferedSource : Source
 {
     size_t bufSize, bufPosIn, bufPosOut;