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4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
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22 #include <folly/Function.h>
23 #include <folly/SharedMutex.h>
24 #include <folly/Synchronized.h>
28 // Some request context that follows an async request through a process
29 // Everything in the context must be thread safe
33 virtual ~RequestData() = default;
34 // Avoid calling RequestContext::setContextData, setContextDataIfAbsent, or
35 // clearContextData from these callbacks. Doing so will cause deadlock. We
36 // could fix these deadlocks, but only at significant performance penalty, so
38 virtual void onSet() {}
39 virtual void onUnset() {}
44 // If you do not call create() to create a unique request context,
45 // this default request context will always be returned, and is never
46 // copied between threads.
47 class RequestContext {
49 using Provider = folly::Function<std::shared_ptr<RequestContext>&()>;
51 // Create a unique request context for this request.
52 // It will be passed between queues / threads (where implemented),
53 // so it should be valid for the lifetime of the request.
54 static void create() {
55 setContext(std::make_shared<RequestContext>());
58 // Get the current context.
59 static RequestContext* get();
61 // The following API may be used to set per-request data in a thread-safe way.
62 // This access is still performance sensitive, so please ask if you need help
63 // profiling any use of these functions.
65 const std::string& val,
66 std::unique_ptr<RequestData> data);
68 // Unlike setContextData, this method does not panic if the key is already
69 // present. Returns true iff the new value has been inserted.
70 bool setContextDataIfAbsent(
71 const std::string& val,
72 std::unique_ptr<RequestData> data);
74 bool hasContextData(const std::string& val) const;
76 RequestData* getContextData(const std::string& val);
77 const RequestData* getContextData(const std::string& val) const;
82 void clearContextData(const std::string& val);
84 // The following API is used to pass the context through queues / threads.
85 // saveContext is called to get a shared_ptr to the context, and
86 // setContext is used to reset it on the other side of the queue.
88 // Whenever possible, use RequestContextScopeGuard instead of setContext
89 // to make sure that RequestContext is reset to the original value when
92 // A shared_ptr is used, because many request may fan out across
93 // multiple threads, or do post-send processing, etc.
94 static std::shared_ptr<RequestContext> setContext(
95 std::shared_ptr<RequestContext> ctx);
97 static std::shared_ptr<RequestContext> saveContext() {
98 return getStaticContext();
101 // This API allows one to override the default behavior of getStaticContext()
102 // by providing a custom RequestContext provider. The old provider is
103 // returned, and the user must restore the old provider via a subsequent call
104 // to setRequestContextProvider() once the new provider is no longer needed.
106 // Using custom RequestContext providers can be more efficient than having to
107 // setContext() whenever context must be switched. This is especially true in
108 // applications that do not actually use RequestContext, but where library
109 // code must still support RequestContext for other use cases. See
110 // FiberManager for an example of how a custom RequestContext provider can
111 // reduce calls to setContext().
112 static Provider setRequestContextProvider(Provider f);
115 static std::shared_ptr<RequestContext>& getStaticContext();
116 static Provider& requestContextProvider();
118 using Data = std::map<std::string, std::unique_ptr<RequestData>>;
119 folly::Synchronized<Data, folly::SharedMutex> data_;
122 class RequestContextScopeGuard {
124 std::shared_ptr<RequestContext> prev_;
127 RequestContextScopeGuard(const RequestContextScopeGuard&) = delete;
128 RequestContextScopeGuard& operator=(const RequestContextScopeGuard&) = delete;
129 RequestContextScopeGuard(RequestContextScopeGuard&&) = delete;
130 RequestContextScopeGuard& operator=(RequestContextScopeGuard&&) = delete;
132 // Create a new RequestContext and reset to the original value when
133 // this goes out of scope.
134 RequestContextScopeGuard() : prev_(RequestContext::saveContext()) {
135 RequestContext::create();
138 // Set a RequestContext that was previously captured by saveContext(). It will
139 // be automatically reset to the original value when this goes out of scope.
140 explicit RequestContextScopeGuard(std::shared_ptr<RequestContext> ctx)
141 : prev_(RequestContext::setContext(std::move(ctx))) {
144 ~RequestContextScopeGuard() {
145 RequestContext::setContext(std::move(prev_));