Add reusePort argument to ServerSocket.

runtime/bin/socket_macos.cc

Index: runtime/bin/socket_macos.cc
diff --git a/runtime/bin/socket_macos.cc b/runtime/bin/socket_macos.cc
index be7beca2f90d80eb73b085164362091457285c8d..d7ee815c42f9f381effc4d3077d75908473a041a 100644
--- a/runtime/bin/socket_macos.cc
+++ b/runtime/bin/socket_macos.cc
@@ -311,7 +311,8 @@ AddressList<InterfaceSocketAddress>* Socket::ListInterfaces(
 intptr_t ServerSocket::CreateBindListen(RawAddr addr,
                                         intptr_t port,
                                         intptr_t backlog,
-                                        bool v6_only) {
+                                        bool v6_only,
+                                        bool reuse_port) {
   intptr_t fd;
 
   fd = TEMP_FAILURE_RETRY(socket(addr.ss.ss_family, SOCK_STREAM, 0));
@@ -320,8 +321,30 @@ intptr_t ServerSocket::CreateBindListen(RawAddr addr,
   FDUtils::SetCloseOnExec(fd);
 
   int optval = 1;
-  VOID_TEMP_FAILURE_RETRY(
-      setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(optval)));
+
+  if (reuse_port) {
+    if (TEMP_FAILURE_RETRY(setsockopt(fd,
+                                      SOL_SOCKET,
+                                      SO_REUSEPORT,
+                                      &optval,
+                                      sizeof(optval))) != 0) {
+      int e = errno;
+      VOID_TEMP_FAILURE_RETRY(close(fd));
+      errno = e;
+      return -1;
+    }
+  } else {
+    optval = 0;
+    VOID_TEMP_FAILURE_RETRY(setsockopt(fd,
+                                       SOL_SOCKET,
+                                       SO_REUSEPORT,
+                                       &optval,
+                                       sizeof(optval)));
+
+    optval = 1;
+    VOID_TEMP_FAILURE_RETRY(
+        setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(optval)));
+  }
 
   if (addr.ss.ss_family == AF_INET6) {
     optval = v6_only ? 1 : 0;