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// Copyright (c) 2011 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_THREADING_THREAD_RESTRICTIONS_H_
#define BASE_THREADING_THREAD_RESTRICTIONS_H_
#include "base/base_api.h"
#include "base/basictypes.h"
namespace base {
// Certain behavior is disallowed on certain threads. ThreadRestrictions helps
// enforce these rules. Examples of such rules:
//
// * Do not do blocking IO (makes the thread janky)
// * Do not access Singleton/LazyInstance (may lead to shutdown crashes)
//
// Here's more about how the protection works:
//
// 1) If a thread should not be allowed to make IO calls, mark it:
// base::ThreadRestrictions::SetIOAllowed(false);
// By default, threads *are* allowed to make IO calls.
// In Chrome browser code, IO calls should be proxied to the File thread.
//
// 2) If a function makes a call that will go out to disk, check whether the
// current thread is allowed:
// base::ThreadRestrictions::AssertIOAllowed();
//
// ThreadRestrictions does nothing in release builds; it is debug-only.
//
// Style tip: where should you put AssertIOAllowed checks? It's best
// if you put them as close to the disk access as possible, at the
// lowest level. This rule is simple to follow and helps catch all
// callers. For example, if your function GoDoSomeBlockingDiskCall()
// only calls other functions in Chrome and not fopen(), you should go
// add the AssertIOAllowed checks in the helper functions.
class BASE_API ThreadRestrictions {
public:
// Constructing a ScopedAllowIO temporarily allows IO for the current
// thread. Doing this is almost certainly always incorrect.
class BASE_API ScopedAllowIO {
public:
ScopedAllowIO() { previous_value_ = SetIOAllowed(true); }
~ScopedAllowIO() { SetIOAllowed(previous_value_); }
private:
// Whether IO is allowed when the ScopedAllowIO was constructed.
bool previous_value_;
DISALLOW_COPY_AND_ASSIGN(ScopedAllowIO);
};
// Constructing a ScopedAllowSingleton temporarily allows accessing for the
// current thread. Doing this is almost always incorrect.
class BASE_API ScopedAllowSingleton {
public:
ScopedAllowSingleton() { previous_value_ = SetSingletonAllowed(true); }
~ScopedAllowSingleton() { SetSingletonAllowed(previous_value_); }
private:
// Whether singleton use is allowed when the ScopedAllowSingleton was
// constructed.
bool previous_value_;
DISALLOW_COPY_AND_ASSIGN(ScopedAllowSingleton);
};
#ifndef NDEBUG
// Set whether the current thread to make IO calls.
// Threads start out in the *allowed* state.
// Returns the previous value.
static bool SetIOAllowed(bool allowed);
// Check whether the current thread is allowed to make IO calls,
// and DCHECK if not. See the block comment above the class for
// a discussion of where to add these checks.
static void AssertIOAllowed();
// Set whether the current thread can use singletons. Returns the previous
// value.
static bool SetSingletonAllowed(bool allowed);
// Check whether the current thread is allowed to use singletons (Singleton /
// LazyInstance). DCHECKs if not.
static void AssertSingletonAllowed();
#else
// In Release builds, inline the empty definitions of these functions so
// that they can be compiled out.
static bool SetIOAllowed(bool allowed) { return true; }
static void AssertIOAllowed() {}
static bool SetSingletonAllowed(bool allowed) { return true; }
static void AssertSingletonAllowed() {}
#endif
private:
DISALLOW_IMPLICIT_CONSTRUCTORS(ThreadRestrictions);
};
} // namespace base
#endif // BASE_THREADING_THREAD_RESTRICTIONS_H_
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