Promise map operator (#26917)

* Poll type for promises library

* Library to talk about things that look like promises if you squint

* Promise helpers, and basic type erasure

* Promise map operator - change return type via a function

* build

* Changes to sync required for promise activities

* sanitized

* Automated change: Fix sanity tests

* fixes

* fixes

* Automated change: Fix sanity tests

* fix build

* review feedback

* Automated change: Fix sanity tests

* comment

Co-authored-by: ctiller <ctiller@users.noreply.github.com>

BUILD

@@ -856,6 +856,17 @@ grpc_cc_library(
     deps = ["gpr_platform"],
 )
 
+grpc_cc_library(
+    name = "map",
+    language = "c++",
+    public_hdrs = ["src/core/lib/promise/map.h"],
+    deps = [
+        "gpr_platform",
+        "poll",
+        "promise_like",
+    ],
+)
+
 grpc_cc_library(
     name = "promise",
     external_deps = [

CMakeLists.txt

@@ -916,6 +916,7 @@ if(gRPC_BUILD_TESTS)
   add_dependencies(buildtests_cxx popularity_count_test)
   add_dependencies(buildtests_cxx port_sharing_end2end_test)
   add_dependencies(buildtests_cxx promise_factory_test)
+  add_dependencies(buildtests_cxx promise_map_test)
   add_dependencies(buildtests_cxx promise_test)
   add_dependencies(buildtests_cxx proto_server_reflection_test)
   add_dependencies(buildtests_cxx proto_utils_test)
@@ -13048,6 +13049,42 @@ target_link_libraries(promise_factory_test
 )
 
 
+endif()
+if(gRPC_BUILD_TESTS)
+
+add_executable(promise_map_test
+  test/core/promise/map_test.cc
+  third_party/googletest/googletest/src/gtest-all.cc
+  third_party/googletest/googlemock/src/gmock-all.cc
+)
+
+target_include_directories(promise_map_test
+  PRIVATE
+    ${CMAKE_CURRENT_SOURCE_DIR}
+    ${CMAKE_CURRENT_SOURCE_DIR}/include
+    ${_gRPC_ADDRESS_SORTING_INCLUDE_DIR}
+    ${_gRPC_RE2_INCLUDE_DIR}
+    ${_gRPC_SSL_INCLUDE_DIR}
+    ${_gRPC_UPB_GENERATED_DIR}
+    ${_gRPC_UPB_GRPC_GENERATED_DIR}
+    ${_gRPC_UPB_INCLUDE_DIR}
+    ${_gRPC_XXHASH_INCLUDE_DIR}
+    ${_gRPC_ZLIB_INCLUDE_DIR}
+    third_party/googletest/googletest/include
+    third_party/googletest/googletest
+    third_party/googletest/googlemock/include
+    third_party/googletest/googlemock
+    ${_gRPC_PROTO_GENS_DIR}
+)
+
+target_link_libraries(promise_map_test
+  ${_gRPC_PROTOBUF_LIBRARIES}
+  ${_gRPC_ALLTARGETS_LIBRARIES}
+  absl::optional
+  absl::variant
+)
+
+
 endif()
 if(gRPC_BUILD_TESTS)
 

build_autogenerated.yaml

@@ -6077,6 +6077,21 @@ targets:
   - absl/types:variant
   - absl/utility:utility
   uses_polling: false
+- name: promise_map_test
+  gtest: true
+  build: test
+  language: c++
+  headers:
+  - src/core/lib/promise/detail/promise_like.h
+  - src/core/lib/promise/map.h
+  - src/core/lib/promise/poll.h
+  - src/core/lib/promise/promise.h
+  src:
+  - test/core/promise/map_test.cc
+  deps:
+  - absl/types:optional
+  - absl/types:variant
+  uses_polling: false
 - name: promise_test
   gtest: true
   build: test

src/core/lib/promise/map.h

@@ -0,0 +1,64 @@
+// Copyright 2021 gRPC authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef GRPC_CORE_LIB_PROMISE_MAP_H
+#define GRPC_CORE_LIB_PROMISE_MAP_H
+
+#include <grpc/impl/codegen/port_platform.h>
+
+#include "absl/types/variant.h"
+#include "src/core/lib/promise/detail/promise_like.h"
+#include "src/core/lib/promise/poll.h"
+
+namespace grpc_core {
+
+namespace promise_detail {
+
+// Implementation of mapping combinator - use this via the free function below!
+// Promise is the type of promise to poll on, Fn is a function that takes the
+// result of Promise and maps it to some new type.
+template <typename Promise, typename Fn>
+class Map {
+ public:
+  Map(Promise promise, Fn fn)
+      : promise_(std::move(promise)), fn_(std::move(fn)) {}
+
+  using Result = typename PromiseLike<Promise>::Result;
+
+  Poll<Result> operator()() {
+    Poll<Result> r = promise_();
+    if (auto* p = absl::get_if<kPollReadyIdx>(&r)) {
+      return fn_(std::move(*p));
+    }
+    return Pending();
+  }
+
+ private:
+  PromiseLike<Promise> promise_;
+  Fn fn_;
+};
+
+}  // namespace promise_detail
+
+// Mapping combinator.
+// Takes a promise, and a synchronous function to mutate its result, and
+// returns a promise.
+template <typename Promise, typename Fn>
+promise_detail::Map<Promise, Fn> Map(Promise promise, Fn fn) {
+  return promise_detail::Map<Promise, Fn>(std::move(promise), std::move(fn));
+}
+
+}  // namespace grpc_core
+
+#endif  // GRPC_CORE_LIB_PROMISE_MAP_H

test/core/promise/BUILD

@@ -42,6 +42,24 @@ grpc_cc_test(
     ],
 )
 
+grpc_cc_test(
+    # Why promise_map_test and not map_test?
+    # third_party/benchmark defines a map_test in its cmakefile, and we depend
+    # on that from our cmakefile, and cmake wants to have a flat namespace to
+    # deal with xcode/visual studio limitations... sooo... promise_map_test it
+    # is.
+    name = "promise_map_test",
+    srcs = ["map_test.cc"],
+    external_deps = ["gtest"],
+    language = "c++",
+    uses_polling = False,
+    deps = [
+        "//:map",
+        "//:promise",
+        "//test/core/util:grpc_suppressions",
+    ],
+)
+
 grpc_cc_test(
     name = "promise_factory_test",
     srcs = ["promise_factory_test.cc"],

test/core/promise/map_test.cc

@@ -0,0 +1,31 @@
+// Copyright 2021 gRPC authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "src/core/lib/promise/map.h"
+#include <gtest/gtest.h>
+#include "src/core/lib/promise/promise.h"
+
+namespace grpc_core {
+
+TEST(MapTest, Works) {
+  Promise<int> x = Map([]() { return 42; }, [](int i) { return i / 2; });
+  EXPECT_EQ(x(), Poll<int>(21));
+}
+
+}  // namespace grpc_core
+
+int main(int argc, char** argv) {
+  ::testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+}

tools/run_tests/generated/tests.json

@@ -5681,6 +5681,30 @@
     ],
     "uses_polling": false
   },
+  {
+    "args": [],
+    "benchmark": false,
+    "ci_platforms": [
+      "linux",
+      "mac",
+      "posix",
+      "windows"
+    ],
+    "cpu_cost": 1.0,
+    "exclude_configs": [],
+    "exclude_iomgrs": [],
+    "flaky": false,
+    "gtest": true,
+    "language": "c++",
+    "name": "promise_map_test",
+    "platforms": [
+      "linux",
+      "mac",
+      "posix",
+      "windows"
+    ],
+    "uses_polling": false
+  },
   {
     "args": [],
     "benchmark": false,