threadsafe port picker for remote bazel

test/core/util/port_isolated_runtime_environment.cc

@@ -20,6 +20,8 @@
  * runs in a separate container) the framework takes a round-robin pick of a
  * port within certain range. There is no need to recycle ports.
  */
+#include <grpc/support/atm.h>
+#include <grpc/support/log.h>
 #include <grpc/support/time.h>
 #include <stdlib.h>
 #include "src/core/lib/iomgr/port.h"
@@ -31,23 +33,22 @@
 #define MIN_PORT 49152
 #define MAX_PORT 65535
 
-int get_random_starting_port() {
+static int get_random_port_offset() {
   srand(gpr_now(GPR_CLOCK_REALTIME).tv_nsec);
   double rnd = static_cast<double>(rand()) /
                (static_cast<double>(RAND_MAX) + 1.0);  // values from [0,1)
-  return static_cast<int>(rnd * (MAX_PORT - MIN_PORT + 1)) + MIN_PORT;
+  return static_cast<int>(rnd * (MAX_PORT - MIN_PORT + 1));
 }
 
-static int s_allocated_port = get_random_starting_port();
+static int s_initial_offset = get_random_port_offset();
+static gpr_atm s_pick_counter = 0;
 
 int grpc_pick_unused_port_or_die(void) {
-  // TODO(jtattermusch): protect by mutex
-  int allocated_port = s_allocated_port++;
-  if (s_allocated_port == MAX_PORT + 1) {
-    s_allocated_port = MIN_PORT;
-  }
-
-  return allocated_port;
+  int orig_counter_val =
+      static_cast<int>(gpr_atm_full_fetch_add(&s_pick_counter, 1));
+  GPR_ASSERT(orig_counter_val < (MAX_PORT - MIN_PORT + 1));
+  return MIN_PORT +
+         (s_initial_offset + orig_counter_val) % (MAX_PORT - MIN_PORT + 1);
 }
 
 void grpc_recycle_unused_port(int port) { (void)port; }