docs(pubsub): document RetryPolicy interface

google/cloud/pubsub/retry_policy.h

@@ -18,6 +18,8 @@
 #include "google/cloud/pubsub/version.h"
 #include "google/cloud/internal/retry_policy_impl.h"
 #include "google/cloud/status.h"
+#include <chrono>
+#include <memory>
 
 namespace google {
 namespace cloud {
@@ -40,19 +42,129 @@ GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_END
 namespace pubsub {
 GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_BEGIN
 
-/// The base class for retry policies.
-using RetryPolicy = ::google::cloud::internal::TraitBasedRetryPolicy<
-    pubsub_internal::RetryTraits>;
+/// The base class for the Pub/Sub library retry policies.
+class RetryPolicy : public google::cloud::RetryPolicy {
+ public:
+  /// Creates a new instance of the policy, reset to the initial state.
+  virtual std::unique_ptr<RetryPolicy> clone() const = 0;
+};
+
+/**
+ * A retry policy based on counting errors.
+ *
+ * This policy stops retrying if:
+ * - An RPC returns a non-transient error.
+ * - More than a prescribed number of transient failures is detected.
+ *
+ * In this class the following status codes are treated as transient errors:
+ * - [`kAborted`](@ref google::cloud::StatusCode)
+ * - [`kInternal`](@ref google::cloud::StatusCode)
+ * - [`kUnavailable`](@ref google::cloud::StatusCode)
+ * - [`kResourceExhausted`](@ref google::cloud::StatusCode)
+ */
+class LimitedErrorCountRetryPolicy : public RetryPolicy {
+ public:
+  /**
+   * Create an instance that tolerates up to @p maximum_failures transient
+   * errors.
+   *
+   * @note Disable the retry loop by providing an instance of this policy with
+   *     @p maximum_failures == 0.
+   */
+  explicit LimitedErrorCountRetryPolicy(int maximum_failures)
+      : impl_(maximum_failures) {}
+
+  LimitedErrorCountRetryPolicy(LimitedErrorCountRetryPolicy&& rhs) noexcept
+      : LimitedErrorCountRetryPolicy(rhs.maximum_failures()) {}
+  LimitedErrorCountRetryPolicy(LimitedErrorCountRetryPolicy const& rhs) noexcept
+      : LimitedErrorCountRetryPolicy(rhs.maximum_failures()) {}
+
+  int maximum_failures() const { return impl_.maximum_failures(); }
+
+  bool OnFailure(Status const& s) override { return impl_.OnFailure(s); }
+  bool IsExhausted() const override { return impl_.IsExhausted(); }
+  bool IsPermanentFailure(Status const& s) const override {
+    return impl_.IsPermanentFailure(s);
+  }
+  std::unique_ptr<RetryPolicy> clone() const override {
+    return std::make_unique<LimitedErrorCountRetryPolicy>(
+        impl_.maximum_failures());
+  }
+
+  // This is provided only for backwards compatibility.
+  using BaseType = RetryPolicy;
+
+ private:
+  google::cloud::internal::LimitedErrorCountRetryPolicy<
+      pubsub_internal::RetryTraits>
+      impl_;
+};
+
+/**
+ * A retry policy based on elapsed time.
+ *
+ * This policy stops retrying if:
+ * - An RPC returns a non-transient error.
+ * - The elapsed time in the retry loop exceeds a prescribed duration.
+ *
+ * In this class the following status codes are treated as transient errors:
+ * - [`kAborted`](@ref google::cloud::StatusCode)
+ * - [`kInternal`](@ref google::cloud::StatusCode)
+ * - [`kUnavailable`](@ref google::cloud::StatusCode)
+ * - [`kResourceExhausted`](@ref google::cloud::StatusCode)
+ */
+class LimitedTimeRetryPolicy : public RetryPolicy {
+ public:
+  /**
+   * Constructor given a `std::chrono::duration<>` object.
+   *
+   * @tparam DurationRep a placeholder to match the `Rep` tparam for
+   *     @p maximum_duration's type. The semantics of this template parameter
+   *     are documented in `std::chrono::duration<>`. In brief, the underlying
+   *     arithmetic type used to store the number of ticks. For our purposes it
+   *     is simply a formal parameter.
+   * @tparam DurationPeriod a placeholder to match the `Period` tparam for
+   *     @p maximum_duration's type. The semantics of this template parameter
+   *     are documented in `std::chrono::duration<>`. In brief, the length of
+   *     the tick in seconds, expressed as a `std::ratio<>`. For our purposes it
+   *     is simply a formal parameter.
+   * @param maximum_duration the maximum time allowed before the policy expires,
+   *     while the application can express this time in any units they desire,
+   *     the class truncates to milliseconds.
+   *
+   * @see https://en.cppreference.com/w/cpp/chrono/duration for more details
+   *     about `std::chrono::duration`.
+   */
+  template <typename DurationRep, typename DurationPeriod>
+  explicit LimitedTimeRetryPolicy(
+      std::chrono::duration<DurationRep, DurationPeriod> maximum_duration)
+      : impl_(maximum_duration) {}
 
-/// A retry policy that limits based on time.
-using LimitedTimeRetryPolicy =
-    ::google::cloud::internal::LimitedTimeRetryPolicy<
-        pubsub_internal::RetryTraits>;
+  LimitedTimeRetryPolicy(LimitedTimeRetryPolicy&& rhs) noexcept
+      : LimitedTimeRetryPolicy(rhs.maximum_duration()) {}
+  LimitedTimeRetryPolicy(LimitedTimeRetryPolicy const& rhs) noexcept
+      : LimitedTimeRetryPolicy(rhs.maximum_duration()) {}
 
-/// A retry policy that limits the number of times a request can fail.
-using LimitedErrorCountRetryPolicy =
-    google::cloud::internal::LimitedErrorCountRetryPolicy<
-        pubsub_internal::RetryTraits>;
+  std::chrono::milliseconds maximum_duration() const {
+    return impl_.maximum_duration();
+  }
+
+  bool OnFailure(Status const& s) override { return impl_.OnFailure(s); }
+  bool IsExhausted() const override { return impl_.IsExhausted(); }
+  bool IsPermanentFailure(Status const& s) const override {
+    return impl_.IsPermanentFailure(s);
+  }
+  std::unique_ptr<RetryPolicy> clone() const override {
+    return std::make_unique<LimitedTimeRetryPolicy>(impl_.maximum_duration());
+  }
+
+  // This is provided only for backwards compatibility.
+  using BaseType = RetryPolicy;
+
+ private:
+  google::cloud::internal::LimitedTimeRetryPolicy<pubsub_internal::RetryTraits>
+      impl_;
+};
 
 GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_END
 }  // namespace pubsub