feat(spanner): introduce the RouteToLeaderOption (#10885)

devbww · web-flow · commit 3df984d2438a · 2023-02-21T15:54:07.000-05:00
This will be used in upcoming code that will add a routing header to RPCs
that should be served in the leader region.

The boolean option defaults to on (unset), but the default can be changed with
a suitably-negative value in `${GOOGLE_CLOUD_CPP_SPANNER_ROUTE_TO_LEADER}`.
diff --git a/google/cloud/spanner/internal/defaults.cc b/google/cloud/spanner/internal/defaults.cc
@@ -104,6 +104,19 @@ Options DefaultOptions(Options opts) {
   min_sessions =
       (std::min)(min_sessions, max_sessions_per_channel * num_channels);
 
+  if (!opts.has<spanner::RouteToLeaderOption>()) {
+    if (auto e = internal::GetEnv("GOOGLE_CLOUD_CPP_SPANNER_ROUTE_TO_LEADER")) {
+      for (auto const* disable : {"N", "n", "F", "f", "0", "off"}) {
+        if (*e == disable) {
+          // Change the default from "for RW/PartitionedDml transactions"
+          // to "never".
+          opts.set<spanner::RouteToLeaderOption>(false);
+          break;
+        }
+      }
+    }
+  }
+
   return opts;
 }
 
diff --git a/google/cloud/spanner/internal/defaults_test.cc b/google/cloud/spanner/internal/defaults_test.cc
@@ -55,6 +55,8 @@ TEST(Options, Defaults) {
                                                absl::nullopt);
   testing_util::ScopedEnvironment user_project_env(
       "GOOGLE_CLOUD_CPP_USER_PROJECT", absl::nullopt);
+  testing_util::ScopedEnvironment route_to_leader_env(
+      "GOOGLE_CLOUD_CPP_SPANNER_ROUTE_TO_LEADER", absl::nullopt);
   auto opts = spanner_internal::DefaultOptions();
 
   EXPECT_EQ(opts.get<EndpointOption>(), "spanner.googleapis.com");
@@ -82,6 +84,8 @@ TEST(Options, Defaults) {
   EXPECT_TRUE(opts.has<SpannerRetryPolicyOption>());
   EXPECT_TRUE(opts.has<SpannerBackoffPolicyOption>());
   EXPECT_TRUE(opts.has<spanner_internal::SessionPoolClockOption>());
+
+  EXPECT_FALSE(opts.has<spanner::RouteToLeaderOption>());
 }
 
 TEST(Options, AdminDefaults) {
@@ -150,6 +154,14 @@ TEST(Options, SpannerEmulatorHost) {
   EXPECT_NE(opts.get<GrpcCredentialOption>(), nullptr);
 }
 
+TEST(Options, RouteToLeaderFromEnv) {
+  testing_util::ScopedEnvironment route_to_leader_env(
+      "GOOGLE_CLOUD_CPP_SPANNER_ROUTE_TO_LEADER", "off");
+  auto opts = spanner_internal::DefaultOptions();
+  EXPECT_TRUE(opts.has<spanner::RouteToLeaderOption>());
+  EXPECT_FALSE(opts.get<spanner::RouteToLeaderOption>());
+}
+
 TEST(Options, TracingComponentsFromEnv) {
   testing_util::ScopedEnvironment tracing_components_env(
       "GOOGLE_CLOUD_CPP_ENABLE_TRACING", "c1,c2,c3");
diff --git a/google/cloud/spanner/options.h b/google/cloud/spanner/options.h
@@ -188,6 +188,17 @@ using SessionPoolOptionList =
                SessionPoolActionOnExhaustionOption,
                SessionPoolKeepAliveIntervalOption, SessionPoolLabelsOption>;
 
+/**
+ * Option for `google::cloud::Options` to, when present and false, suppress
+ * adding headers to distinguish requests served by the leader v/s non-leader
+ * region.
+ *
+ * @ingroup spanner-options
+ */
+struct RouteToLeaderOption {
+  using Type = bool;
+};
+
 /**
  * Option for `google::cloud::Options` to set the optimizer version used in an
  * SQL query.
