Wrap rand.Reader in a bufio.Reader.

rand.Read() results in a syscall to the host on every call instead
we can wrap it with a bufio.Reader to buffer and reduce number of syscalls.
This is especially important for TCP where every newly created endpoint
reads random data to initialize the timestamp offsets for the endpoint.

Updates #231

PiperOrigin-RevId: 301501607

pkg/rand/rand_linux.go

@@ -45,12 +45,18 @@ func (r *reader) Read(p []byte) (int, error) {
 	return rand.Read(p)
 }
 
+// mu protects the global Reader below.
+var mu sync.Mutex
+
 // Reader is the default reader.
 var Reader io.Reader = &reader{}
 
 // Read reads from the default reader.
 func Read(b []byte) (int, error) {
-	return io.ReadFull(Reader, b)
+	mu.Lock()
+	n, err := io.ReadFull(Reader, b)
+	mu.Unlock()
+	return n, err
 }
 
 // Init can be called to make sure /dev/urandom is pre-opened on kernels that