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/**
* Copyright (C) 2018-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <memory>
#include <string>
#include "mongo/base/string_data.h"
#include "mongo/util/static_immortal.h"
namespace mongo {
/**
* A nullable handle pinning a ref-counted immutable string.
* Copies of a ThreadNameString refer to the same string object.
* Equality comparisons consider only that string's identity, not its value.
*
* This class is just a kind of refcounted string handle and does not itself
* interact with the OS or with thread storage.
*
* Presents a pointer-like API with `get()`, and dereference operators, and
* explicit bool conversion. Dereferencing yields a reference to a
* string value if nonempty. Dereferencing an empty reference is allowed and
* yields the singleton string value "-".
*
* Copyable and movable, with the usual refcounting semantics. Copies refer
* to the same string and will compare equal to each other.
*
*/
class ThreadNameRef {
public:
/** An empty ref (empty refs still stringify as "-"). */
ThreadNameRef() = default;
/** A ref to the string value `name`. */
explicit ThreadNameRef(std::string name)
: _ptr{std::make_shared<std::string>(std::move(name))} {}
/**
* Dereferences this. If nonempty, returns its string value.
* Otherwise, returns a singleton "-" string.
*/
const std::string* get() const {
if (_ptr)
return &*_ptr;
static const StaticImmortal whenEmpty = std::string("-");
return &*whenEmpty;
}
const std::string* operator->() const {
return get();
}
const std::string& operator*() const {
return *get();
}
/** Returns true if nonempty. */
explicit operator bool() const {
return !!_ptr;
}
operator StringData() const {
return **this;
}
/**
* Two ThreadNameRef are equal if and only if they are copies of the same
* original ThreadNameRef object. Equality of string value is insufficient.
*/
friend bool operator==(const ThreadNameRef& a, const ThreadNameRef& b) noexcept {
return a._ptr == b._ptr;
}
friend bool operator!=(const ThreadNameRef& a, const ThreadNameRef& b) noexcept {
return !(a == b);
}
private:
std::shared_ptr<const std::string> _ptr;
};
/**
* Returns the name reference attached to current thread.
* The empty ThreadNameRef still has a valid string value of "-".
*
* This string is not limited in length, so it will be a better name
* than the name the OS uses to refer to the same thread.
*/
ThreadNameRef getThreadNameRef();
/**
* Swaps in a new active name, returns the old one if it was active.
*
* The active thread name is used for:
* - Setting the thread name in the OS. As an optimization, clearing
* the thread name in the OS is performed lazily.
* - Populating the "ctx" field for log lines.
* - Providing a thread name to GDB.
*/
ThreadNameRef setThreadNameRef(ThreadNameRef name);
/**
* Marks the ThreadNameRef attached to the current thread as inactive.
* - The inactive thread name remains attached to the thread.
* - The thread name according to the OS is not changed.
* - A subsequent `setThreadNameRef` call will not return it.
* - An immediately subsequent `setThreadNameRef` call with the same name will
* cheaply reactivate it, saving two OS thread rename operations.
* This is an optimization on the assumption that a thread name will be
* temporarily set to the same `ThreadNameRef` repeatedly, so setting it and
* resetting it with the OS on each change would be wasteful.
*/
void releaseThreadNameRef();
/**
* Sets the name of the current thread.
*/
inline void setThreadName(std::string name) {
setThreadNameRef(ThreadNameRef{std::move(name)});
}
/**
* Returns current thread's name, as previously set, or "main", or
* "thread#" if no name was previously set.
*
* Used by the MongoDB GDB pretty printer extentions in `gdb/mongo.py`.
*/
inline StringData getThreadName() {
return *getThreadNameRef();
}
} // namespace mongo
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