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Draft implementation of Context. (#200)
Context holds information specific to an operation, such as a TagMap and Span. Each thread has a currently active Context. Contexts are conceptually immutable: the contents of a Context cannot be modified in-place.
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‎opencensus/context/BUILD‎

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# OpenCensus C++ Context library.
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# See context.h for details.
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#
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# Copyright 2018, OpenCensus Authors
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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load("//opencensus:copts.bzl", "DEFAULT_COPTS", "TEST_COPTS")
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licenses(["notice"]) # Apache 2.0
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package(default_visibility = ["//visibility:private"])
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cc_library(
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name = "context",
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srcs = [
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"internal/context.cc",
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"internal/with_context.cc",
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],
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hdrs = [
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"context.h",
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"with_context.h",
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],
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copts = DEFAULT_COPTS,
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visibility = ["//visibility:public"],
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deps = [
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"//opencensus/tags",
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"//opencensus/trace",
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],
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)
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# Tests
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# ========================================================================= #
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cc_test(
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name = "context_test",
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srcs = ["internal/context_test.cc"],
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copts = TEST_COPTS,
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deps = [
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":context",
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"@com_google_googletest//:gtest_main",
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],
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)
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cc_test(
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name = "with_context_test",
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srcs = ["internal/with_context_test.cc"],
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copts = TEST_COPTS,
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deps = [
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":context",
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"@com_google_googletest//:gtest_main",
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],
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)

‎opencensus/context/context.h‎

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// Copyright 2018, OpenCensus Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef OPENCENSUS_CONTEXT_CONTEXT_H_
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#define OPENCENSUS_CONTEXT_CONTEXT_H_
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#include <functional>
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#include <string>
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#include "opencensus/tags/tag_map.h"
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#include "opencensus/trace/span.h"
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namespace opencensus {
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namespace context {
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// Context holds information specific to an operation, such as a TagMap and
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// Span. Each thread has a currently active Context. Contexts are conceptually
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// immutable: the contents of a Context cannot be modified in-place.
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//
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// This is a draft implementation of Context, and we chose to depend on TagMap
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// and Span directly. In future, the implementation will change, so only rely
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// on the public API for manipulating Contexts. In future we may support
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// arbitrary keys and values.
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class Context {
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public:
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// Returns a const reference to the current (thread local) Context.
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static const Context& Current();
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// Context is copiable and movable.
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Context(const Context&) = default;
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Context(Context&&) = default;
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Context& operator=(const Context&) = default;
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Context& operator=(Context&&) = default;
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// Returns an std::function wrapped to run with a copy of this Context.
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std::function<void()> Wrap(std::function<void()> fn) const;
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// Returns a human-readable string for debugging. Do not rely on its format or
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// try to parse it. Do not use the DebugString to retrieve Spans or Tags.
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std::string DebugString() const;
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private:
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// Creates a default Context.
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Context();
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static Context* InternalMutableCurrent();
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friend void swap(Context& a, Context& b);
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friend class WithContext;
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opencensus::tags::TagMap tags_;
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opencensus::trace::Span span_;
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};
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} // namespace context
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} // namespace opencensus
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#endif // OPENCENSUS_CONTEXT_CONTEXT_H_

‎opencensus/context/internal/context.cc‎

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// Copyright 2018, OpenCensus Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#include "opencensus/context/context.h"
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#include <functional>
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#include <utility>
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#include "absl/strings/str_cat.h"
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#include "opencensus/context/with_context.h"
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#include "opencensus/tags/tag_map.h"
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#include "opencensus/trace/span.h"
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namespace opencensus {
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namespace context {
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Context::Context()
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: tags_(opencensus::tags::TagMap({})),
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span_(opencensus::trace::Span::BlankSpan()) {}
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// static
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const Context& Context::Current() { return *InternalMutableCurrent(); }
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std::function<void()> Context::Wrap(std::function<void()> fn) const {
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Context copy(Context::Current());
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return [fn, copy]() {
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WithContext wc(copy);
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fn();
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};
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}
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std::string Context::DebugString() const {
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return absl::StrCat("ctx@", absl::Hex(this),
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" span=", span_.context().ToString(),
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", tags=", tags_.DebugString());
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}
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// static
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Context* Context::InternalMutableCurrent() {
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static __thread Context* thread_ctx = nullptr;
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if (thread_ctx == nullptr) thread_ctx = new Context;
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return thread_ctx;
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}
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void swap(Context& a, Context& b) {
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using std::swap;
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swap(a.span_, b.span_);
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swap(a.tags_, b.tags_);
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}
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} // namespace context
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} // namespace opencensus

‎opencensus/context/internal/context_test.cc‎

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// Copyright 2018, OpenCensus Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#include "opencensus/context/context.h"
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#include <iostream>
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#include "gtest/gtest.h"
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// Not in namespace ::opencensus::context in order to better reflect what user
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// code should look like.
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namespace {
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void LogCurrentContext() {
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const std::string s = opencensus::context::Context::Current().DebugString();
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std::cout << " current: " << s << "\n";
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}
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TEST(ContextTest, DefaultContext) { LogCurrentContext(); }
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void Callback1() {
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std::cout << " inside function\n";
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LogCurrentContext();
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}
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TEST(ContextTest, Wrap) {
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std::function<void()> fn =
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opencensus::context::Context::Current().Wrap(Callback1);
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fn();
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}
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TEST(ContextTest, WrapDoesNotLeak) {
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{
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std::function<void()> fn =
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opencensus::context::Context::Current().Wrap(Callback1);
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}
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// We never call fn().
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}
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TEST(ContextTest, WrappedFnIsCopiable) {
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std::function<void()> fn2;
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{
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std::function<void()> fn1 =
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opencensus::context::Context::Current().Wrap(Callback1);
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fn2 = fn1;
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fn1();
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}
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fn2();
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}
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} // namespace

‎opencensus/context/internal/with_context.cc‎

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// Copyright 2018, OpenCensus Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#include "opencensus/context/with_context.h"
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#include <utility>
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#include "opencensus/context/context.h"
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namespace opencensus {
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namespace context {
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WithContext::WithContext(const Context& ctx) : swapped_context_(ctx) {
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using std::swap;
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swap(*Context::InternalMutableCurrent(), swapped_context_);
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}
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WithContext::WithContext(Context&& ctx) : swapped_context_(std::move(ctx)) {
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using std::swap;
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swap(*Context::InternalMutableCurrent(), swapped_context_);
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}
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WithContext::~WithContext() {
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using std::swap;
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swap(*Context::InternalMutableCurrent(), swapped_context_);
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}
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} // namespace context
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} // namespace opencensus

‎opencensus/context/internal/with_context_test.cc‎

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// Copyright 2018, OpenCensus Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#include "opencensus/context/with_context.h"
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#include <iostream>
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#include "gtest/gtest.h"
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#include "opencensus/context/context.h"
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// Not in namespace ::opencensus::context in order to better reflect what user
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// code should look like.
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namespace {
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TEST(WithContextTest, WithContext) {
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opencensus::context::Context ctx = opencensus::context::Context::Current();
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opencensus::context::WithContext wc(ctx);
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}
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TEST(WithContextTest, WithContextMovable) {
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opencensus::context::Context ctx = opencensus::context::Context::Current();
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opencensus::context::WithContext wc(std::move(ctx));
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}
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} // namespace

‎opencensus/context/with_context.h‎

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// Copyright 2018, OpenCensus Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef OPENCENSUS_CONTEXT_WITH_CONTEXT_H_
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#define OPENCENSUS_CONTEXT_WITH_CONTEXT_H_
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#include "opencensus/context/context.h"
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namespace opencensus {
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namespace context {
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// WithContext is a scoped object that sets the current Context to the given
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// one, until the WithContext object is destroyed.
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//
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// Because it changes the current (thread local) context, NEVER allocate a
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// WithContext in one thread and deallocate in another. A simple way to ensure
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// this is to only ever stack-allocate it.
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class WithContext {
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public:
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explicit WithContext(const Context& ctx);
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explicit WithContext(Context&& ctx);
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~WithContext();
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private:
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WithContext() = delete;
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WithContext(const WithContext&) = delete;
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WithContext(WithContext&&) = delete;
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WithContext& operator=(const WithContext&) = delete;
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WithContext& operator=(WithContext&&) = delete;
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Context swapped_context_;
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};
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} // namespace context
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} // namespace opencensus
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#endif // OPENCENSUS_CONTEXT_WITH_CONTEXT_H_

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