Shut down write side before draining the read side

This is important if the remote side *does* execute
nix-store/nix-daemon successfully, but stdout is polluted
(e.g. because the remote user's bashrc script prints something to
stdout). In that case we have to shutdown the write side to force the
remote nix process to exit.

src/libstore/legacy-ssh-store.cc

@@ -89,6 +89,8 @@ struct LegacySSHStore : public virtual LegacySSHStoreConfig, public virtual Stor
                 if (magic != SERVE_MAGIC_2)
                     throw Error("'nix-store --serve' protocol mismatch from '%s'", host);
             } catch (SerialisationError & e) {
+                /* In case the other side is waiting for our input,
+                   close it. */
                 conn->sshConn->in.close();
                 auto msg = conn->from.drain();
                 throw Error("'nix-store --serve' protocol mismatch from '%s', got '%s'",

src/libstore/remote-store.cc

@@ -169,6 +169,9 @@ void RemoteStore::initConnection(Connection & conn)
             if (magic != WORKER_MAGIC_2)
                 throw Error("protocol mismatch");
         } catch (SerialisationError & e) {
+            /* In case the other side is waiting for our input, close
+               it. */
+            conn.closeWrite();
             auto msg = conn.from.drain();
             throw Error("protocol mismatch, got '%s'", chomp(*saved.s + msg));
         }

src/libstore/remote-store.hh

@@ -125,7 +125,6 @@ public:
 
     struct Connection
     {
-        AutoCloseFD fd;
         FdSink to;
         FdSource from;
         unsigned int daemonVersion;
@@ -133,6 +132,8 @@ public:
 
         virtual ~Connection();
 
+        virtual void closeWrite() = 0;
+
         std::exception_ptr processStderr(Sink * sink = 0, Source * source = 0, bool flush = true);
     };
 

src/libstore/ssh-store.cc

@@ -57,6 +57,11 @@ private:
     struct Connection : RemoteStore::Connection
     {
         std::unique_ptr<SSHMaster::Connection> sshConn;
+
+        void closeWrite() override
+        {
+            sshConn->in.close();
+        }
     };
 
     ref<RemoteStore::Connection> openConnection() override;

src/libstore/uds-remote-store.cc

@@ -45,6 +45,12 @@ std::string UDSRemoteStore::getUri()
 }
 
 
+void UDSRemoteStore::Connection::closeWrite()
+{
+    shutdown(fd.get(), SHUT_WR);
+}
+
+
 ref<RemoteStore::Connection> UDSRemoteStore::openConnection()
 {
     auto conn = make_ref<Connection>();

src/libstore/uds-remote-store.hh

@@ -40,6 +40,12 @@ public:
 
 private:
 
+    struct Connection : RemoteStore::Connection
+    {
+        AutoCloseFD fd;
+        void closeWrite() override;
+    };
+
     ref<RemoteStore::Connection> openConnection() override;
     std::optional<std::string> path;
 };