base/trace_event/trace_logging_minimal_win.h - chromium/src - Git at Google

 // Copyright 2020 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 #ifndef BASE_TRACE_EVENT_TRACE_LOGGING_MINIMAL_WIN_H_
 #define BASE_TRACE_EVENT_TRACE_LOGGING_MINIMAL_WIN_H_

 /*
  * TraceLogging minimal dynamic provider
  *
  * TlmProvider is a simple class that implements an Event Tracing for Windows
  * (ETW) provider that generates TraceLogging events with string fields. Unlike
  * the Windows SDK's TraceLoggingProvider.h, this provider class supports
  * runtime-variable settings for event name, level, keyword, and field name.
  *
  * Note that this approach is not recommended for general use. Support for
  * runtime-variable settings is not normally needed, and it requires extra
  * buffering as compared to the approach used by TraceLoggingProvider.h. It is
  * needed in this case because we're trying to feed data from the existing call
  * sites (which use a runtime-variable function-call syntax) into ETW. If this
  * were new code, it would be better to update each call site to use a syntax
  * compatible with compile-time event settings compatible with structured
  * logging like TraceLoggingProvider.h.
  */

 #include <stdint.h>
 #include <windows.h>
 // Evntprov.h must come after windows.h.
 #include <evntprov.h>
 // TODO(joel@microsoft.com) Update headers and use defined constants instead
 // of magic numbers after crbug.com/1089996 is resolved.

 /*
  * An instance of TlmProvider represents a logger through which data can be
  *  sent to Event Tracing for Windows (ETW). This logger generates
  * TraceLogging-encoded events (compatible with the events generated by the
  * Windows SDK's TraceLoggingProvider.h header). In most cases, a developer
  * would prefer using TraceLoggingProvider.h over TlmProvider
  * (TraceLoggingProvider.h is more efficient and more full-featured), but
  * TlmProvider allows for configuring the event parameters (event name,
  * level, keyword, field names) at runtime (TraceLoggingProvider.h requires
  * these to be set at compile time).
  *
  * Note that the Register/Unregister operations are relatively expensive, so
  * the TlmProvider instance should be a long-lived variable (i.e. global
  * variable, static variable, or field of a long-lived object), not a local
  * variable andnot a field of a short-lived object.
  *
  * Note that provider name and provider GUID are a tightly-bound pair, i.e.
  * they should each uniquely map to each other. Once a provider name and
  * provider GUID have been used together, no other GUID should be used with
  * that name and no other name should be used with that GUID. Normally this
  * goal is achieved by using a hashing algorithm to generate the GUID from
  * a hash of the name.
  *
  * Note that each event should use a non-zero level and a non-zero keyword.
  * Predefined level constants are defined in <evntrace.h>: 0=Always,
  * 1=Critical, 2=Error, 3=Warning, 4=Info, 5=Verbose (other level values can
  * be used but are not well-defined and are not generally useful). A keyword
  * is a bitmask of "category" bits, where each bit indicates whether or not
  * the event belongs in a particular category of event. The low 48 bits are
  * user-defined and the upper 16 bits are Microsoft-defined (in <winmeta.h>).
  *
  * General usage:
  *
  *     // During component initialization (main or DllMain), call Register().
  *     // Note that there is an overload of the TlmProvider constructor that
  *     // calls Register(), but it's often convenient to do this manually
  *     // (i.e. to control the timing of the call to Register).
  *     my_provider.Register(
  *         "MyCompany.MyComponentName",
  *         MyComponentGuid);
  *
  *     // To log an event with minimal code:
  *     my_provider.WriteEvent("MyEventName",
  *         TlmEventDescriptor(
  *             TRACE_LEVEL_VERBOSE, // Level defined in <evntrace.h>
  *             0x20),               // Keyword bits are user-defined.
  *         // Value must not be null for the string fields.
  *         TlmUtf8StringField("MyUtf8Field", GetValue1()),
  *         TlmMbcsStringField("MyAsciiField", GetValue2()));
  *
  *     // Note that the minimal-code example has a bit of overhead, as it
  *     // will make the calls to GetValue1(), GetValue2(), and WriteEvent()
  *     // even if nobody is listening to the event. WriteEvent() will return
        // immediately if nobody is listening, but there is still some
  *     // overhead. To minimize the overhead when nobody is listening,
  *     // add an extra IF condition:
  *     static const auto MyEventDescriptor = TlmEventDescriptor(
  *         TRACE_LEVEL_VERBOSE, // Level defined in <evntrace.h>
  *         0x20);               // Keyword bits are user-defined.
  *     if (my_provider.IsEnabled(MyEventDescriptor))
  *     {
  *         // The IF condition is primarily to prevent unnecessary
  *         // calls to GetValue1() and GetValue2().
  *         my_provider.WriteEvent("MyEventName",
  *             MyEventDescriptor,
  *             // Value must not be null for the string fields.
  *             TlmUtf8StringField("MyUtf8Field", GetValue1()),
  *             TlmMbcsStringField("MyAsciiField", GetValue2()));
  *     }
  *
  *     // During component shutdown (main or DllMain), call Unregister().
  *     // Note that the TlmProvider destructor will also call
  *     // Unregister(), butit's often convenient to do this manually
  *     // (i.e. to control the timingof the call to Unregister).
  *     my_provider.Unregister();
  */

 class TlmProvider {
  public:
   // Initialize a provider in the unregistered state.
   // Note that WriteEvent and Unregister operations on an unregistered
   // provider are safe no-ops.
   constexpr TlmProvider() noexcept = default;

   // Initializes a provider and attempts to register it.
   // If there is an error, provider will be left unregistered.
   // Note that WriteEvent and Unregister operations on an unregistered
   // provider are safe no-ops.
   TlmProvider(const char* provider_name,
               const GUID& provider_guid,
               PENABLECALLBACK enable_callback = nullptr,
               void* enable_callback_context = nullptr) noexcept;

   // If provider is registered, unregisters provider.
   ~TlmProvider();

   // Disable copy operations.
   TlmProvider(const TlmProvider&) = delete;
   TlmProvider& operator=(const TlmProvider&) = delete;

   // Unregisters this provider.
   // Calling Unregister on an unregistered provider is a safe no-op.
   // Not thread safe - caller must ensure serialization between calls to
   // Register() and calls to Unregister().
   void Unregister() noexcept;

   // Registers this provider. Returns Win32 error code or 0 for success.
   // Error code is primarily for debugging and should generally be ignored
   // in production (failure to register means Unregister and WriteEvent are
   // safe no-ops.)
   // Calling Register on an already-registered provider is a fatal error.
   // Not thread safe - caller must ensure serialization between calls to
   // Register() and calls to Unregister().
   int32_t Register(const char* provider_name,
                    const GUID& provider_guid,
                    PENABLECALLBACK enable_callback = nullptr,
                    void* enable_callback_context = nullptr) noexcept;

   // Returns true if any active trace listeners are interested in any events
   // from this provider.
   // Equivalent to IsEnabled(0, 0).
   bool IsEnabled() const noexcept;

   // Returns true if any active trace listeners are interested in events
   // from this provider with the specified level.
   // Equivalent to IsEnabled(level, 0).
   bool IsEnabled(uint8_t level) const noexcept;

   // Returns true if any active trace listeners are interested in events
   // from this provider with the specified level and keyword.
   bool IsEnabled(uint8_t level, uint64_t keyword) const noexcept;

   // Returns true if any active trace listeners are interested in events
   // from this provider with the specified level and keyword.
   // Equivalent to IsEnabled(event_descriptor.level, event_descriptor.keyword).
   bool IsEnabled(const EVENT_DESCRIPTOR& event_descriptor) const noexcept;

   // If any active trace listeners are interested in events from this provider
   // with the specified level and keyword, packs the data into an event and
   // sends it to ETW. Returns Win32 error code or 0 for success.
   template <class... FieldTys>
   int32_t WriteEvent(const char* event_name,
                      const EVENT_DESCRIPTOR& event_descriptor,
                      const FieldTys&... event_fields) const noexcept {
     if (!IsEnabled(event_descriptor)) {
       // If nobody is listening, report success.
       return 0;
     }
     // Pack the event metadata.
     char metadata[kMaxEventMetadataSize];
     uint16_t metadata_index;
     metadata_index = EventBegin(metadata, event_name);
     {  // scope for dummy array (simulates a C++17 comma-fold expression)
       bool dummy[sizeof...(FieldTys) == 0 ? 1 : sizeof...(FieldTys)] = {
           EventAddField(metadata, &metadata_index, event_fields.in_type_,
                         event_fields.out_type_, event_fields.Name())...};
       DCHECK(dummy);
     }

     // Pack the event data.
     constexpr uint8_t kDescriptorsCount =
         2 + DataDescCountSum<FieldTys...>::value;
     EVENT_DATA_DESCRIPTOR descriptors[kDescriptorsCount];
     uint8_t descriptors_index = 2;
     {  // scope for dummy array (simulates a C++17 comma-fold expression)
       bool dummy[sizeof...(FieldTys) == 0 ? 1 : sizeof...(FieldTys)] = {
           EventDescriptorFill(descriptors, &descriptors_index,
                               event_fields)...};
       DCHECK(dummy);
     }

     // Finalize event and call EventWrite.
     return EventEnd(metadata, metadata_index, descriptors, descriptors_index,
                     event_descriptor);
   }

  private:
   // Size of the buffer used for provider metadata (field within the
   // TlmProvider object). Provider metadata consists of the nul-terminated
   // provider name plus a few sizes and flags, so this buffer needs to be
   // just a few bytes larger than the largest expected provider name.
   static constexpr uint16_t kMaxProviderMetadataSize = 128;

   // Size of the buffer used for event metadata (stack-allocated in the
   // WriteEvent method). Event metadata consists of nul-terminated event
   // name, nul-terminated field names, field types (1 or 2 bytes per field),
   // and a few bytes for sizes and flags.
   static constexpr uint16_t kMaxEventMetadataSize = 256;

   template <class... FieldTys>
   struct DataDescCountSum;  // undefined

   template <>
   struct DataDescCountSum<> {
     static constexpr uint8_t value = 0;
   };

   template <class FieldTy1, class... FieldTyRest>
   struct DataDescCountSum<FieldTy1, FieldTyRest...> {
     static constexpr uint8_t value =
         FieldTy1::data_desc_count_ + DataDescCountSum<FieldTyRest...>::value;
   };

   template <class FieldTy>
   static char EventDescriptorFill(EVENT_DATA_DESCRIPTOR* descriptors,
                                   uint8_t* pdescriptors_index,
                                   const FieldTy& event_field) noexcept {
     event_field.FillEventDescriptor(&descriptors[*pdescriptors_index]);
     *pdescriptors_index += FieldTy::data_desc_count_;
     return 0;
   }

   // This is called from the OS, so use the required call type.
   static void __stdcall StaticEnableCallback(
       const GUID* source_id,
       ULONG is_enabled,
       UCHAR level,
       ULONGLONG match_any_keyword,
       ULONGLONG match_all_keyword,
       PEVENT_FILTER_DESCRIPTOR FilterData,
       PVOID callback_context);

   // Returns initial value of metadata_index.
   uint16_t EventBegin(char* metadata, const char* event_name) const noexcept;

   char EventAddField(char* metadata,
                      uint16_t* metadata_index,
                      uint8_t in_type,
                      uint8_t out_type,
                      const char* field_name) const noexcept;

   // Returns Win32 error code, or 0 for success.
   int32_t EventEnd(char* metadata,
                    uint16_t metadata_index,
                    EVENT_DATA_DESCRIPTOR* descriptors,
                    uint32_t descriptors_index,
                    const EVENT_DESCRIPTOR& event_descriptor) const noexcept;

   bool KeywordEnabled(uint64_t keyword) const noexcept;

   uint16_t AppendNameToMetadata(char* metadata,
                                 uint16_t metadata_size,
                                 uint16_t metadata_index,
                                 const char* name) const noexcept;

   uint32_t level_plus1_ = 0;
   uint32_t provider_metadata_size_ = 0;
   uint64_t keyword_any_ = 0;
   uint64_t keyword_all_ = 0;
   uint64_t reg_handle_ = 0;
   PENABLECALLBACK enable_callback_ = nullptr;
   void* enable_callback_context_ = nullptr;
   char provider_metadata_[kMaxProviderMetadataSize] = {};
 };

 // Base class for field types.
 template <uint8_t data_desc_count,
           uint8_t in_type,
           uint8_t out_type = 0>  // Default out_type is TlgOutNULL
 class TlmFieldBase {
  public:
   constexpr const char* Name() const noexcept { return name_; }

  protected:
   explicit constexpr TlmFieldBase(const char* name) noexcept : name_(name) {}

  private:
   friend class TlmProvider;

   static constexpr uint8_t data_desc_count_ = data_desc_count;
   static constexpr uint8_t in_type_ = in_type;
   static constexpr uint8_t out_type_ = out_type;

   const char* name_;
 };

 // Class that represents an event field containing nul-terminated MBCS data.
 class TlmMbcsStringField
     : public TlmFieldBase<1, 2>  // 1 data descriptor, Type = TlgInANSISTRING
 {
  public:
   // name is a utf-8 nul-terminated string.
   // value is MBCS nul-terminated string (assumed to be in system's default code
   // page).
   TlmMbcsStringField(const char* name, const char* value) noexcept;

   const char* Value() const noexcept;

   void FillEventDescriptor(EVENT_DATA_DESCRIPTOR* descriptors) const noexcept;

  private:
   const char* value_;
 };

 // Class that represents an event field containing nul-terminated UTF-8 data.
 class TlmUtf8StringField
     : public TlmFieldBase<1, 2, 35>  // 1 data descriptor, Type =
                                      // TlgInANSISTRING + TlgOutUTF8
 {
  public:
   // name and value are utf-8 nul-terminated strings.
   TlmUtf8StringField(const char* name, const char* value) noexcept;

   const char* Value() const noexcept;

   void FillEventDescriptor(EVENT_DATA_DESCRIPTOR* descriptors) const noexcept;

  private:
   const char* value_;
 };

 // Helper for creating event descriptors for use with WriteEvent.
 constexpr EVENT_DESCRIPTOR TlmEventDescriptor(uint8_t level,
                                               uint64_t keyword) noexcept {
   return {

       // Id
       // TraceLogging generally uses the event's Name instead of Id+Version,
       // so Id is normally set to 0 for TraceLogging events.
       0,

       // Version
       // TraceLogging generally uses the event's Name instead of Id+Version,
       // so Version is normally set to 0 for TraceLogging events.
       0,

       // Channel (WINEVENT_CHANNEL_*)
       // Setting Channel = 11 allows TraceLogging events to be decoded
       // correctly even if they were collected on older operating systems.
       // If a TraceLogging event sets channel to a value other than 11, the
       // event will only decode correctly if it was collected on an
       // operating system that has built-in TraceLogging support, i.e.
       // Windows 7sp1 + patch, Windows 8.1 + patch, or Windows 10+.
       11,  // = WINEVENT_CHANNEL_TRACELOGGING

       // Level (WINEVENT_LEVEL_*)
       // 0=always, 1=fatal, 2=error, 3=warning, 4=info, 5=verbose.
       // Levels higher than 5 are for user-defined debug levels.
       level,

       // Opcode (WINEVENT_OPCODE_*)
       // Set Opcode for special semantics such as starting/ending an
       // activity.
       0,  // = WINEVENT_OPCODE_INFO

       // Task
       // Set Task for user-defined semantics.
       0,  // = WINEVENT_TASK_NONE

       // Keyword
       // A keyword is a 64-bit value used for filtering events. Each bit of
       // the keyword indicates whether the event belongs to a particular
       // category of events. The top 16 bits of keyword have
       // Microsoft-defined semantics and should be set to 0. The low 48 bits
       // of keyword have user-defined semantics. All events should use a
       // nonzero keyword to support effective event filtering (events with
       // keyword set to 0 always pass keyword filtering).
       keyword};
 }

 #endif  // BASE_TRACE_EVENT_TRACE_LOGGING_MINIMAL_WIN_H_
	// Copyright 2020 The Chromium Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	#ifndef BASE_TRACE_EVENT_TRACE_LOGGING_MINIMAL_WIN_H_
	#define BASE_TRACE_EVENT_TRACE_LOGGING_MINIMAL_WIN_H_

	/*
	* TraceLogging minimal dynamic provider
	*
	* TlmProvider is a simple class that implements an Event Tracing for Windows
	* (ETW) provider that generates TraceLogging events with string fields. Unlike
	* the Windows SDK's TraceLoggingProvider.h, this provider class supports
	* runtime-variable settings for event name, level, keyword, and field name.
	*
	* Note that this approach is not recommended for general use. Support for
	* runtime-variable settings is not normally needed, and it requires extra
	* buffering as compared to the approach used by TraceLoggingProvider.h. It is
	* needed in this case because we're trying to feed data from the existing call
	* sites (which use a runtime-variable function-call syntax) into ETW. If this
	* were new code, it would be better to update each call site to use a syntax
	* compatible with compile-time event settings compatible with structured
	* logging like TraceLoggingProvider.h.
	*/

	#include <stdint.h>
	#include <windows.h>
	// Evntprov.h must come after windows.h.
	#include <evntprov.h>
	// TODO(joel@microsoft.com) Update headers and use defined constants instead
	// of magic numbers after crbug.com/1089996 is resolved.

	/*
	* An instance of TlmProvider represents a logger through which data can be
	* sent to Event Tracing for Windows (ETW). This logger generates
	* TraceLogging-encoded events (compatible with the events generated by the
	* Windows SDK's TraceLoggingProvider.h header). In most cases, a developer
	* would prefer using TraceLoggingProvider.h over TlmProvider
	* (TraceLoggingProvider.h is more efficient and more full-featured), but
	* TlmProvider allows for configuring the event parameters (event name,
	* level, keyword, field names) at runtime (TraceLoggingProvider.h requires
	* these to be set at compile time).
	*
	* Note that the Register/Unregister operations are relatively expensive, so
	* the TlmProvider instance should be a long-lived variable (i.e. global
	* variable, static variable, or field of a long-lived object), not a local
	* variable andnot a field of a short-lived object.
	*
	* Note that provider name and provider GUID are a tightly-bound pair, i.e.
	* they should each uniquely map to each other. Once a provider name and
	* provider GUID have been used together, no other GUID should be used with
	* that name and no other name should be used with that GUID. Normally this
	* goal is achieved by using a hashing algorithm to generate the GUID from
	* a hash of the name.
	*
	* Note that each event should use a non-zero level and a non-zero keyword.
	* Predefined level constants are defined in <evntrace.h>: 0=Always,
	* 1=Critical, 2=Error, 3=Warning, 4=Info, 5=Verbose (other level values can
	* be used but are not well-defined and are not generally useful). A keyword
	* is a bitmask of "category" bits, where each bit indicates whether or not
	* the event belongs in a particular category of event. The low 48 bits are
	* user-defined and the upper 16 bits are Microsoft-defined (in <winmeta.h>).
	*
	* General usage:
	*
	* // During component initialization (main or DllMain), call Register().
	* // Note that there is an overload of the TlmProvider constructor that
	* // calls Register(), but it's often convenient to do this manually
	* // (i.e. to control the timing of the call to Register).
	* my_provider.Register(
	* "MyCompany.MyComponentName",
	* MyComponentGuid);
	*
	* // To log an event with minimal code:
	* my_provider.WriteEvent("MyEventName",
	* TlmEventDescriptor(
	* TRACE_LEVEL_VERBOSE, // Level defined in <evntrace.h>
	* 0x20), // Keyword bits are user-defined.
	* // Value must not be null for the string fields.
	* TlmUtf8StringField("MyUtf8Field", GetValue1()),
	* TlmMbcsStringField("MyAsciiField", GetValue2()));
	*
	* // Note that the minimal-code example has a bit of overhead, as it
	* // will make the calls to GetValue1(), GetValue2(), and WriteEvent()
	* // even if nobody is listening to the event. WriteEvent() will return
	// immediately if nobody is listening, but there is still some
	* // overhead. To minimize the overhead when nobody is listening,
	* // add an extra IF condition:
	* static const auto MyEventDescriptor = TlmEventDescriptor(
	* TRACE_LEVEL_VERBOSE, // Level defined in <evntrace.h>
	* 0x20); // Keyword bits are user-defined.
	* if (my_provider.IsEnabled(MyEventDescriptor))
	* {
	* // The IF condition is primarily to prevent unnecessary
	* // calls to GetValue1() and GetValue2().
	* my_provider.WriteEvent("MyEventName",
	* MyEventDescriptor,
	* // Value must not be null for the string fields.
	* TlmUtf8StringField("MyUtf8Field", GetValue1()),
	* TlmMbcsStringField("MyAsciiField", GetValue2()));
	* }
	*
	* // During component shutdown (main or DllMain), call Unregister().
	* // Note that the TlmProvider destructor will also call
	* // Unregister(), butit's often convenient to do this manually
	* // (i.e. to control the timingof the call to Unregister).
	* my_provider.Unregister();
	*/

	class TlmProvider {
	public:
	// Initialize a provider in the unregistered state.
	// Note that WriteEvent and Unregister operations on an unregistered
	// provider are safe no-ops.
	constexpr TlmProvider() noexcept = default;

	// Initializes a provider and attempts to register it.
	// If there is an error, provider will be left unregistered.
	// Note that WriteEvent and Unregister operations on an unregistered
	// provider are safe no-ops.
	TlmProvider(const char* provider_name,
	const GUID& provider_guid,
	PENABLECALLBACK enable_callback = nullptr,
	void* enable_callback_context = nullptr) noexcept;

	// If provider is registered, unregisters provider.
	~TlmProvider();

	// Disable copy operations.
	TlmProvider(const TlmProvider&) = delete;
	TlmProvider& operator=(const TlmProvider&) = delete;

	// Unregisters this provider.
	// Calling Unregister on an unregistered provider is a safe no-op.
	// Not thread safe - caller must ensure serialization between calls to
	// Register() and calls to Unregister().
	void Unregister() noexcept;

	// Registers this provider. Returns Win32 error code or 0 for success.
	// Error code is primarily for debugging and should generally be ignored
	// in production (failure to register means Unregister and WriteEvent are
	// safe no-ops.)
	// Calling Register on an already-registered provider is a fatal error.
	// Not thread safe - caller must ensure serialization between calls to
	// Register() and calls to Unregister().
	int32_t Register(const char* provider_name,
	const GUID& provider_guid,
	PENABLECALLBACK enable_callback = nullptr,
	void* enable_callback_context = nullptr) noexcept;

	// Returns true if any active trace listeners are interested in any events
	// from this provider.
	// Equivalent to IsEnabled(0, 0).
	bool IsEnabled() const noexcept;

	// Returns true if any active trace listeners are interested in events
	// from this provider with the specified level.
	// Equivalent to IsEnabled(level, 0).
	bool IsEnabled(uint8_t level) const noexcept;

	// Returns true if any active trace listeners are interested in events
	// from this provider with the specified level and keyword.
	bool IsEnabled(uint8_t level, uint64_t keyword) const noexcept;

	// Returns true if any active trace listeners are interested in events
	// from this provider with the specified level and keyword.
	// Equivalent to IsEnabled(event_descriptor.level, event_descriptor.keyword).
	bool IsEnabled(const EVENT_DESCRIPTOR& event_descriptor) const noexcept;

	// If any active trace listeners are interested in events from this provider
	// with the specified level and keyword, packs the data into an event and
	// sends it to ETW. Returns Win32 error code or 0 for success.
	template <class... FieldTys>
	int32_t WriteEvent(const char* event_name,
	const EVENT_DESCRIPTOR& event_descriptor,
	const FieldTys&... event_fields) const noexcept {
	if (!IsEnabled(event_descriptor)) {
	// If nobody is listening, report success.
	return 0;
	}
	// Pack the event metadata.
	char metadata[kMaxEventMetadataSize];
	uint16_t metadata_index;
	metadata_index = EventBegin(metadata, event_name);
	{ // scope for dummy array (simulates a C++17 comma-fold expression)
	bool dummy[sizeof...(FieldTys) == 0 ? 1 : sizeof...(FieldTys)] = {
	EventAddField(metadata, &metadata_index, event_fields.in_type_,
	event_fields.out_type_, event_fields.Name())...};
	DCHECK(dummy);
	}

	// Pack the event data.
	constexpr uint8_t kDescriptorsCount =
	2 + DataDescCountSum<FieldTys...>::value;
	EVENT_DATA_DESCRIPTOR descriptors[kDescriptorsCount];
	uint8_t descriptors_index = 2;
	{ // scope for dummy array (simulates a C++17 comma-fold expression)
	bool dummy[sizeof...(FieldTys) == 0 ? 1 : sizeof...(FieldTys)] = {
	EventDescriptorFill(descriptors, &descriptors_index,
	event_fields)...};
	DCHECK(dummy);
	}

	// Finalize event and call EventWrite.
	return EventEnd(metadata, metadata_index, descriptors, descriptors_index,
	event_descriptor);
	}

	private:
	// Size of the buffer used for provider metadata (field within the
	// TlmProvider object). Provider metadata consists of the nul-terminated
	// provider name plus a few sizes and flags, so this buffer needs to be
	// just a few bytes larger than the largest expected provider name.
	static constexpr uint16_t kMaxProviderMetadataSize = 128;

	// Size of the buffer used for event metadata (stack-allocated in the
	// WriteEvent method). Event metadata consists of nul-terminated event
	// name, nul-terminated field names, field types (1 or 2 bytes per field),
	// and a few bytes for sizes and flags.
	static constexpr uint16_t kMaxEventMetadataSize = 256;

	template <class... FieldTys>
	struct DataDescCountSum; // undefined

	template <>
	struct DataDescCountSum<> {
	static constexpr uint8_t value = 0;
	};

	template <class FieldTy1, class... FieldTyRest>
	struct DataDescCountSum<FieldTy1, FieldTyRest...> {
	static constexpr uint8_t value =
	FieldTy1::data_desc_count_ + DataDescCountSum<FieldTyRest...>::value;
	};

	template <class FieldTy>
	static char EventDescriptorFill(EVENT_DATA_DESCRIPTOR* descriptors,
	uint8_t* pdescriptors_index,
	const FieldTy& event_field) noexcept {
	event_field.FillEventDescriptor(&descriptors[*pdescriptors_index]);
	*pdescriptors_index += FieldTy::data_desc_count_;
	return 0;
	}

	// This is called from the OS, so use the required call type.
	static void __stdcall StaticEnableCallback(
	const GUID* source_id,
	ULONG is_enabled,
	UCHAR level,
	ULONGLONG match_any_keyword,
	ULONGLONG match_all_keyword,
	PEVENT_FILTER_DESCRIPTOR FilterData,
	PVOID callback_context);

	// Returns initial value of metadata_index.
	uint16_t EventBegin(char* metadata, const char* event_name) const noexcept;

	char EventAddField(char* metadata,
	uint16_t* metadata_index,
	uint8_t in_type,
	uint8_t out_type,
	const char* field_name) const noexcept;

	// Returns Win32 error code, or 0 for success.
	int32_t EventEnd(char* metadata,
	uint16_t metadata_index,
	EVENT_DATA_DESCRIPTOR* descriptors,
	uint32_t descriptors_index,
	const EVENT_DESCRIPTOR& event_descriptor) const noexcept;

	bool KeywordEnabled(uint64_t keyword) const noexcept;

	uint16_t AppendNameToMetadata(char* metadata,
	uint16_t metadata_size,
	uint16_t metadata_index,
	const char* name) const noexcept;

	uint32_t level_plus1_ = 0;
	uint32_t provider_metadata_size_ = 0;
	uint64_t keyword_any_ = 0;
	uint64_t keyword_all_ = 0;
	uint64_t reg_handle_ = 0;
	PENABLECALLBACK enable_callback_ = nullptr;
	void* enable_callback_context_ = nullptr;
	char provider_metadata_[kMaxProviderMetadataSize] = {};
	};

	// Base class for field types.
	template <uint8_t data_desc_count,
	uint8_t in_type,
	uint8_t out_type = 0> // Default out_type is TlgOutNULL
	class TlmFieldBase {
	public:
	constexpr const char* Name() const noexcept { return name_; }

	protected:
	explicit constexpr TlmFieldBase(const char* name) noexcept : name_(name) {}

	private:
	friend class TlmProvider;

	static constexpr uint8_t data_desc_count_ = data_desc_count;
	static constexpr uint8_t in_type_ = in_type;
	static constexpr uint8_t out_type_ = out_type;

	const char* name_;
	};

	// Class that represents an event field containing nul-terminated MBCS data.
	class TlmMbcsStringField
	: public TlmFieldBase<1, 2> // 1 data descriptor, Type = TlgInANSISTRING
	{
	public:
	// name is a utf-8 nul-terminated string.
	// value is MBCS nul-terminated string (assumed to be in system's default code
	// page).
	TlmMbcsStringField(const char* name, const char* value) noexcept;

	const char* Value() const noexcept;

	void FillEventDescriptor(EVENT_DATA_DESCRIPTOR* descriptors) const noexcept;

	private:
	const char* value_;
	};

	// Class that represents an event field containing nul-terminated UTF-8 data.
	class TlmUtf8StringField
	: public TlmFieldBase<1, 2, 35> // 1 data descriptor, Type =
	// TlgInANSISTRING + TlgOutUTF8
	{
	public:
	// name and value are utf-8 nul-terminated strings.
	TlmUtf8StringField(const char* name, const char* value) noexcept;

	const char* Value() const noexcept;

	void FillEventDescriptor(EVENT_DATA_DESCRIPTOR* descriptors) const noexcept;

	private:
	const char* value_;
	};

	// Helper for creating event descriptors for use with WriteEvent.
	constexpr EVENT_DESCRIPTOR TlmEventDescriptor(uint8_t level,
	uint64_t keyword) noexcept {
	return {

	// Id
	// TraceLogging generally uses the event's Name instead of Id+Version,
	// so Id is normally set to 0 for TraceLogging events.
	0,

	// Version
	// TraceLogging generally uses the event's Name instead of Id+Version,
	// so Version is normally set to 0 for TraceLogging events.
	0,

	// Channel (WINEVENT_CHANNEL_*)
	// Setting Channel = 11 allows TraceLogging events to be decoded
	// correctly even if they were collected on older operating systems.
	// If a TraceLogging event sets channel to a value other than 11, the
	// event will only decode correctly if it was collected on an
	// operating system that has built-in TraceLogging support, i.e.
	// Windows 7sp1 + patch, Windows 8.1 + patch, or Windows 10+.
	11, // = WINEVENT_CHANNEL_TRACELOGGING

	// Level (WINEVENT_LEVEL_*)
	// 0=always, 1=fatal, 2=error, 3=warning, 4=info, 5=verbose.
	// Levels higher than 5 are for user-defined debug levels.
	level,

	// Opcode (WINEVENT_OPCODE_*)
	// Set Opcode for special semantics such as starting/ending an
	// activity.
	0, // = WINEVENT_OPCODE_INFO

	// Task
	// Set Task for user-defined semantics.
	0, // = WINEVENT_TASK_NONE

	// Keyword
	// A keyword is a 64-bit value used for filtering events. Each bit of
	// the keyword indicates whether the event belongs to a particular
	// category of events. The top 16 bits of keyword have
	// Microsoft-defined semantics and should be set to 0. The low 48 bits
	// of keyword have user-defined semantics. All events should use a
	// nonzero keyword to support effective event filtering (events with
	// keyword set to 0 always pass keyword filtering).
	keyword};
	}

	#endif // BASE_TRACE_EVENT_TRACE_LOGGING_MINIMAL_WIN_H_