Add SSHMaster::Connection::trySetBufferSize

It is unused in Nix currently, but will be used in Hydra. This reflects
what Hydra does in https://github.com/NixOS/hydra/pull/1387.

We may probalby to use it more widely for better SSH store performance,
but this needs to be subject to more testing before we do that.

src/libstore/ssh.cc

@@ -194,4 +194,14 @@ Path SSHMaster::startMaster()
 
 #endif
 
+void SSHMaster::Connection::trySetBufferSize(size_t size)
+{
+#ifdef F_SETPIPE_SZ
+    // convert type
+    int pipesize = size;
+    fcntl(in.get(), F_SETPIPE_SZ, pipesize);
+    fcntl(out.get(), F_SETPIPE_SZ, pipesize);
+#endif
+}
+
 }

src/libstore/ssh.hh

@@ -51,6 +51,16 @@ public:
         Pid sshPid;
 #endif
         AutoCloseFD out, in;
+
+        /**
+         * Try to set the buffer size in both directions to the
+         * designated amount, if possible. If not possible, does
+         * nothing.
+         *
+         * Current implementation is to use `fcntl` with `F_SETPIPE_SZ`,
+         * which is Linux-only.
+         */
+        void trySetBufferSize(size_t size);
     };
 
     /**