ref.go

// Copyright (c) 2016 Uber Technologies, Inc.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

package checked

import (
	"fmt"
	"reflect"
	"runtime"
	"sync"
	"sync/atomic"
	"unsafe"

	xresource "github.com/m3db/m3/src/x/resource"
)

// RefCount is an embeddable checked.Ref.
type RefCount struct {
	ref           int32
	reads         int32
	writes        int32
	onFinalize    unsafe.Pointer
	finalizeState refCountFinalizeState
}

type refCountFinalizeState struct {
	sync.Mutex
	called   bool
	delayRef int32
}

// IncRef increments the reference count to this entity.
func (c *RefCount) IncRef() {
	n := atomic.AddInt32(&c.ref, 1)
	tracebackEvent(c, int(n), incRefEvent)
}

// DecRef decrements the reference count to this entity.
func (c *RefCount) DecRef() {
	n := atomic.AddInt32(&c.ref, -1)
	tracebackEvent(c, int(n), decRefEvent)

	if n < 0 {
		err := fmt.Errorf("negative ref count, ref=%d", n)
		panicRef(c, err)
	}
}

// MoveRef signals a move of the ref to this entity.
func (c *RefCount) MoveRef() {
	tracebackEvent(c, c.NumRef(), moveRefEvent)
}

// NumRef returns the reference count to this entity.
func (c *RefCount) NumRef() int {
	return int(atomic.LoadInt32(&c.ref))
}

// Finalize will call the finalizer if any, ref count must be zero.
func (c *RefCount) Finalize() {
	n := c.NumRef()
	tracebackEvent(c, n, finalizeEvent)

	if n != 0 {
		err := fmt.Errorf("finalize before zero ref count, ref=%d", n)
		panicRef(c, err)
	}

	c.finalizeState.Lock()
	c.finalizeState.called = true
	if c.finalizeState.delayRef == 0 {
		c.finalizeWithLock()
	}
	c.finalizeState.Unlock()
}

func (c *RefCount) finalizeWithLock() {
	// Reset the finalize called state for reuse.
	c.finalizeState.called = false
	if f := c.OnFinalize(); f != nil {
		f.OnFinalize()
	}
}

// DelayFinalizer will delay calling the finalizer on this entity
// until the closer returned by the method is called at least once.
// This is useful for dependent resources requiring the lifetime of this
// entityt to be extended.
func (c *RefCount) DelayFinalizer() xresource.SimpleCloser {
	c.finalizeState.Lock()
	c.finalizeState.delayRef++
	c.finalizeState.Unlock()
	return c
}

// Close implements xresource.SimpleCloser for the purpose of use with DelayFinalizer.
func (c *RefCount) Close() {
	c.finalizeState.Lock()
	c.finalizeState.delayRef--
	if c.finalizeState.called && c.finalizeState.delayRef == 0 {
		c.finalizeWithLock()
	}
	c.finalizeState.Unlock()
}

// OnFinalize returns the finalizer callback if any or nil otherwise.
func (c *RefCount) OnFinalize() OnFinalize {
	finalizerPtr := (*OnFinalize)(atomic.LoadPointer(&c.onFinalize))
	if finalizerPtr == nil {
		return nil
	}
	return *finalizerPtr
}

// SetOnFinalize sets the finalizer callback.
func (c *RefCount) SetOnFinalize(f OnFinalize) {
	atomic.StorePointer(&c.onFinalize, unsafe.Pointer(&f))
}

// IncReads increments the reads count to this entity.
func (c *RefCount) IncReads() {
	tracebackEvent(c, c.NumRef(), incReadsEvent)
	n := atomic.AddInt32(&c.reads, 1)

	if ref := c.NumRef(); n > 0 && ref < 1 {
		err := fmt.Errorf("read after free: reads=%d, ref=%d", n, ref)
		panicRef(c, err)
	}
}

// DecReads decrements the reads count to this entity.
func (c *RefCount) DecReads() {
	tracebackEvent(c, c.NumRef(), decReadsEvent)
	n := atomic.AddInt32(&c.reads, -1)

	if ref := c.NumRef(); ref < 1 {
		err := fmt.Errorf("read finish after free: reads=%d, ref=%d", n, ref)
		panicRef(c, err)
	}
}

// NumReaders returns the active reads count to this entity.
func (c *RefCount) NumReaders() int {
	return int(atomic.LoadInt32(&c.reads))
}

// IncWrites increments the writes count to this entity.
func (c *RefCount) IncWrites() {
	tracebackEvent(c, c.NumRef(), incWritesEvent)
	n := atomic.AddInt32(&c.writes, 1)
	ref := c.NumRef()

	if n > 0 && ref < 1 {
		err := fmt.Errorf("write after free: writes=%d, ref=%d", n, ref)
		panicRef(c, err)
	}

	if n > 1 {
		err := fmt.Errorf("double write: writes=%d, ref=%d", n, ref)
		panicRef(c, err)
	}
}

// DecWrites decrements the writes count to this entity.
func (c *RefCount) DecWrites() {
	tracebackEvent(c, c.NumRef(), decWritesEvent)
	n := atomic.AddInt32(&c.writes, -1)

	if ref := c.NumRef(); ref < 1 {
		err := fmt.Errorf("write finish after free: writes=%d, ref=%d", n, ref)
		panicRef(c, err)
	}
}

// NumWriters returns the active writes count to this entity.
func (c *RefCount) NumWriters() int {
	return int(atomic.LoadInt32(&c.writes))
}

// TrackObject sets up the initial internal state of the Ref for
// leak detection.
func (c *RefCount) TrackObject(v interface{}) {
	if !leakDetectionFlag {
		return
	}

	var size int

	switch v := reflect.ValueOf(v); v.Kind() {
	case reflect.Ptr:
		size = int(v.Type().Elem().Size())
	case reflect.Array, reflect.Slice, reflect.Chan:
		size = int(v.Type().Elem().Size()) * v.Cap()
	case reflect.String:
		size = v.Len()
	default:
		size = int(v.Type().Size())
	}

	runtime.SetFinalizer(c, func(c *RefCount) {
		if c.NumRef() == 0 {
			return
		}

		origin := getDebuggerRef(c).String()

		leaks.Lock()
		// Keep track of bytes leaked, not objects.
		leaks.m[origin] += uint64(size)
		leaks.Unlock()
	})
}