[backoff] cap initial backoff at max backoff (grpc#38239)

Closes grpc#38239

COPYBARA_INTEGRATE_REVIEW=grpc#38239 from markdroth:backoff_clamp_initial_to_max bd8b099
PiperOrigin-RevId: 703370103

BUILD

@@ -3386,6 +3386,7 @@ grpc_cc_library(
     visibility = ["@grpc:alt_grpc_base_legacy"],
     deps = [
         "gpr_platform",
+        "//src/core:experiments",
         "//src/core:time",
     ],
 )

src/core/lib/experiments/experiments.yaml

@@ -40,6 +40,11 @@
 
 # This file only defines the experiments. Refer to rollouts.yaml for the rollout
 # state of each experiment.
+- name: backoff_cap_initial_at_max
+  description: Backoff library applies max_backoff even on initial_backoff.
+  expiry: 2025/03/01
+  owner: [email protected]
+  test_tags: []
 - name: call_tracer_in_transport
   description: Transport directly passes byte counts to CallTracer.
   expiry: 2025/02/01

src/core/lib/experiments/rollouts.yaml

@@ -40,6 +40,8 @@
 #
 # Supported platforms: ios, windows, posix
 
+- name: backoff_cap_initial_at_max
+  default: true
 - name: call_tracer_in_transport
   default: true
 - name: call_v3

src/core/util/backoff.cc

@@ -22,16 +22,27 @@
 
 #include <algorithm>
 
+#include "src/core/lib/experiments/experiments.h"
+
 namespace grpc_core {
 
 BackOff::BackOff(const Options& options) : options_(options) { Reset(); }
 
 Duration BackOff::NextAttemptDelay() {
-  if (initial_) {
-    initial_ = false;
+  if (IsBackoffCapInitialAtMaxEnabled()) {
+    if (initial_) {
+      initial_ = false;
+    } else {
+      current_backoff_ *= options_.multiplier();
+    }
+    current_backoff_ = std::min(current_backoff_, options_.max_backoff());
   } else {
-    current_backoff_ = std::min(current_backoff_ * options_.multiplier(),
-                                options_.max_backoff());
+    if (initial_) {
+      initial_ = false;
+    } else {
+      current_backoff_ = std::min(current_backoff_ * options_.multiplier(),
+                                  options_.max_backoff());
+    }
   }
   const double jitter =
       absl::Uniform(rand_gen_, 1 - options_.jitter(), 1 + options_.jitter());

test/core/end2end/tests/retry_cancel_during_delay.cc

@@ -46,7 +46,7 @@ void TestRetryCancelDuringDelay(
           "    \"retryPolicy\": {\n"
           "      \"maxAttempts\": 3,\n"
           "      \"initialBackoff\": \"%ds\",\n"
-          "      \"maxBackoff\": \"120s\",\n"
+          "      \"maxBackoff\": \"1000s\",\n"
           "      \"backoffMultiplier\": 1.6,\n"
           "      \"retryableStatusCodes\": [ \"ABORTED\" ]\n"
           "    }\n"

test/core/util/backoff_test.cc

@@ -25,6 +25,7 @@
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "src/core/lib/experiments/experiments.h"
 #include "src/core/util/time.h"
 #include "test/core/test_util/test_config.h"
 
@@ -50,6 +51,27 @@ TEST(BackOffTest, ConstantBackOff) {
   EXPECT_EQ(backoff.NextAttemptDelay(), kInitialBackoff);
 }
 
+TEST(BackOffTest, InitialBackoffCappedByMaxBackoff) {
+  if (!IsBackoffCapInitialAtMaxEnabled()) {
+    GTEST_SKIP() << "test requires backoff_cap_initial_at_max experiment";
+  }
+  const auto kInitialBackoff = Duration::Seconds(2);
+  const auto kMaxBackoff = Duration::Seconds(1);
+  const double kMultiplier = 1.0;
+  const double kJitter = 0.0;
+  BackOff::Options options;
+  options.set_initial_backoff(kInitialBackoff)
+      .set_multiplier(kMultiplier)
+      .set_jitter(kJitter)
+      .set_max_backoff(kMaxBackoff);
+  BackOff backoff(options);
+  EXPECT_EQ(backoff.NextAttemptDelay(), kMaxBackoff);
+  EXPECT_EQ(backoff.NextAttemptDelay(), kMaxBackoff);
+  EXPECT_EQ(backoff.NextAttemptDelay(), kMaxBackoff);
+  EXPECT_EQ(backoff.NextAttemptDelay(), kMaxBackoff);
+  EXPECT_EQ(backoff.NextAttemptDelay(), kMaxBackoff);
+}
+
 TEST(BackOffTest, NoJitterBackOff) {
   const auto kInitialBackoff = Duration::Milliseconds(2);
   const double kMultiplier = 2.0;