77db4b4f35
This commit introduces a breaking change in the diode API in order to hide the diodes package interface. This removes a good number of dependencies introduced by the test framework used by the diodes package.
130 lines
4.0 KiB
Go
130 lines
4.0 KiB
Go
package diodes
|
|
|
|
import (
|
|
"sync/atomic"
|
|
"unsafe"
|
|
)
|
|
|
|
// GenericDataType is the data type the diodes operate on.
|
|
type GenericDataType unsafe.Pointer
|
|
|
|
// Alerter is used to report how many values were overwritten since the
|
|
// last write.
|
|
type Alerter interface {
|
|
Alert(missed int)
|
|
}
|
|
|
|
// AlertFunc type is an adapter to allow the use of ordinary functions as
|
|
// Alert handlers.
|
|
type AlertFunc func(missed int)
|
|
|
|
// Alert calls f(missed)
|
|
func (f AlertFunc) Alert(missed int) {
|
|
f(missed)
|
|
}
|
|
|
|
type bucket struct {
|
|
data GenericDataType
|
|
seq uint64 // seq is the recorded write index at the time of writing
|
|
}
|
|
|
|
// OneToOne diode is meant to be used by a single reader and a single writer.
|
|
// It is not thread safe if used otherwise.
|
|
type OneToOne struct {
|
|
buffer []unsafe.Pointer
|
|
writeIndex uint64
|
|
readIndex uint64
|
|
alerter Alerter
|
|
}
|
|
|
|
// NewOneToOne creates a new diode is meant to be used by a single reader and
|
|
// a single writer. The alerter is invoked on the read's go-routine. It is
|
|
// called when it notices that the writer go-routine has passed it and wrote
|
|
// over data. A nil can be used to ignore alerts.
|
|
func NewOneToOne(size int, alerter Alerter) *OneToOne {
|
|
if alerter == nil {
|
|
alerter = AlertFunc(func(int) {})
|
|
}
|
|
|
|
return &OneToOne{
|
|
buffer: make([]unsafe.Pointer, size),
|
|
alerter: alerter,
|
|
}
|
|
}
|
|
|
|
// Set sets the data in the next slot of the ring buffer.
|
|
func (d *OneToOne) Set(data GenericDataType) {
|
|
idx := d.writeIndex % uint64(len(d.buffer))
|
|
|
|
newBucket := &bucket{
|
|
data: data,
|
|
seq: d.writeIndex,
|
|
}
|
|
d.writeIndex++
|
|
|
|
atomic.StorePointer(&d.buffer[idx], unsafe.Pointer(newBucket))
|
|
}
|
|
|
|
// TryNext will attempt to read from the next slot of the ring buffer.
|
|
// If there is no data available, it will return (nil, false).
|
|
func (d *OneToOne) TryNext() (data GenericDataType, ok bool) {
|
|
// Read a value from the ring buffer based on the readIndex.
|
|
idx := d.readIndex % uint64(len(d.buffer))
|
|
result := (*bucket)(atomic.SwapPointer(&d.buffer[idx], nil))
|
|
|
|
// When the result is nil that means the writer has not had the
|
|
// opportunity to write a value into the diode. This value must be ignored
|
|
// and the read head must not increment.
|
|
if result == nil {
|
|
return nil, false
|
|
}
|
|
|
|
// When the seq value is less than the current read index that means a
|
|
// value was read from idx that was previously written but has since has
|
|
// been dropped. This value must be ignored and the read head must not
|
|
// increment.
|
|
//
|
|
// The simulation for this scenario assumes the fast forward occurred as
|
|
// detailed below.
|
|
//
|
|
// 5. The reader reads again getting seq 5. It then reads again expecting
|
|
// seq 6 but gets seq 2. This is a read of a stale value that was
|
|
// effectively "dropped" so the read fails and the read head stays put.
|
|
// `| 4 | 5 | 2 | 3 |` r: 7, w: 6
|
|
//
|
|
if result.seq < d.readIndex {
|
|
return nil, false
|
|
}
|
|
|
|
// When the seq value is greater than the current read index that means a
|
|
// value was read from idx that overwrote the value that was expected to
|
|
// be at this idx. This happens when the writer has lapped the reader. The
|
|
// reader needs to catch up to the writer so it moves its write head to
|
|
// the new seq, effectively dropping the messages that were not read in
|
|
// between the two values.
|
|
//
|
|
// Here is a simulation of this scenario:
|
|
//
|
|
// 1. Both the read and write heads start at 0.
|
|
// `| nil | nil | nil | nil |` r: 0, w: 0
|
|
// 2. The writer fills the buffer.
|
|
// `| 0 | 1 | 2 | 3 |` r: 0, w: 4
|
|
// 3. The writer laps the read head.
|
|
// `| 4 | 5 | 2 | 3 |` r: 0, w: 6
|
|
// 4. The reader reads the first value, expecting a seq of 0 but reads 4,
|
|
// this forces the reader to fast forward to 5.
|
|
// `| 4 | 5 | 2 | 3 |` r: 5, w: 6
|
|
//
|
|
if result.seq > d.readIndex {
|
|
dropped := result.seq - d.readIndex
|
|
d.readIndex = result.seq
|
|
d.alerter.Alert(int(dropped))
|
|
}
|
|
|
|
// Only increment read index if a regular read occurred (where seq was
|
|
// equal to readIndex) or a value was read that caused a fast forward
|
|
// (where seq was greater than readIndex).
|
|
d.readIndex++
|
|
return result.data, true
|
|
}
|