Avoid using harmful function rand()

Author: vitlibar

src/core/lib/transport/bdp_estimator.cc

@@ -37,6 +37,7 @@ BdpEstimator::BdpEstimator(absl::string_view name)
       stable_estimate_count_(0),
       ping_state_(PingState::UNSCHEDULED),
       bw_est_(0),
+      seed_(0),
       name_(name) {}
 
 Timestamp BdpEstimator::CompletePing() {
@@ -63,7 +64,7 @@ Timestamp BdpEstimator::CompletePing() {
     if (stable_estimate_count_ >= 2) {
       // if the ping estimate is steady, slowly ramp down the probe time
       inter_ping_delay_ += Duration::Milliseconds(
-          100 + static_cast<int>(rand() * 100.0 / RAND_MAX));
+          100 + static_cast<int>(rand_r(&seed_) * 100.0 / RAND_MAX));
     }
   }
   if (start_inter_ping_delay != inter_ping_delay_) {

src/core/lib/transport/bdp_estimator.h

@@ -65,6 +65,9 @@ class BdpEstimator {
     CHECK(ping_state_ == PingState::SCHEDULED);
     ping_state_ = PingState::STARTED;
     ping_start_time_ = gpr_now(GPR_CLOCK_MONOTONIC);
+    if (!seed_) {
+      seed_ = ping_start_time_.tv_nsec;
+    }
   }
 
   // Completes a previously started ping, returns when to schedule the next one
@@ -83,6 +86,7 @@ class BdpEstimator {
   int stable_estimate_count_;
   PingState ping_state_;
   double bw_est_;
+  unsigned int seed_;
   absl::string_view name_;
 };