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

(cherry picked from commit 0d25cc6541)

src/libstore/ssh.cc

@@ -240,4 +240,19 @@ Path SSHMaster::startMaster()
 
 #endif
 
+void SSHMaster::Connection::trySetBufferSize(size_t size)
+{
+#ifdef F_SETPIPE_SZ
+    /* This `fcntl` method of doing this takes a positive `int`. Check
+       and convert accordingly.
+
+       The function overall still takes `size_t` because this is more
+       portable for a platform-agnostic interface. */
+    assert(size <= INT_MAX);
+    int pipesize = size;
+    fcntl(in.get(), F_SETPIPE_SZ, pipesize);
+    fcntl(out.get(), F_SETPIPE_SZ, pipesize);
+#endif
+}
+
 }

src/libstore/ssh.hh

@@ -54,6 +54,18 @@ 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. For this implementation, `size` must
+         * convertable to an `int`. In other words, it must be within
+         * `[0, INT_MAX]`.
+         */
+        void trySetBufferSize(size_t size);
     };
 
     /**