lnd.xprv/multimutex/multimutex.go

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2018-01-23 18:22:07 +03:00
package multimutex
import (
"fmt"
"sync"
)
// cntMutex is a struct that wraps a counter and a mutex, and is used
// to keep track of the number of goroutines waiting for access to the
// mutex, such that we can forget about it when the counter is zero.
type cntMutex struct {
cnt int
sync.Mutex
}
// Mutex is a struct that keeps track of a set of mutexes with
// a given ID. It can be used for making sure only one goroutine
// gets given the mutex per ID.
type Mutex struct {
// mutexes is a map of IDs to a cntMutex. The cntMutex for
// a given ID will hold the mutex to be used by all
// callers requesting access for the ID, in addition to
// the count of callers.
mutexes map[uint64]*cntMutex
// mapMtx is used to give synchronize concurrent access
// to the mutexes map.
mapMtx sync.Mutex
}
// NewMutex creates a new Mutex.
func NewMutex() *Mutex {
return &Mutex{
mutexes: make(map[uint64]*cntMutex),
}
}
// Lock locks the mutex by the given ID. If the mutex is already
// locked by this ID, Lock blocks until the mutex is available.
func (c *Mutex) Lock(id uint64) {
c.mapMtx.Lock()
mtx, ok := c.mutexes[id]
if ok {
// If the mutex already existed in the map, we
// increment its counter, to indicate that there
// now is one more goroutine waiting for it.
mtx.cnt++
} else {
// If it was not in the map, it means no other
// goroutine has locked the mutex for this ID,
// and we can create a new mutex with count 1
// and add it to the map.
mtx = &cntMutex{
cnt: 1,
}
c.mutexes[id] = mtx
}
c.mapMtx.Unlock()
// Acquire the mutex for this ID.
mtx.Lock()
}
// Unlock unlocks the mutex by the given ID. It is a run-time
// error if the mutex is not locked by the ID on entry to Unlock.
func (c *Mutex) Unlock(id uint64) {
// Since we are done with all the work for this
// update, we update the map to reflect that.
c.mapMtx.Lock()
mtx, ok := c.mutexes[id]
if !ok {
// The mutex not existing in the map means
// an unlock for an ID not currently locked
// was attempted.
panic(fmt.Sprintf("double unlock for id %v",
id))
}
// Decrement the counter. If the count goes to
// zero, it means this caller was the last one
// to wait for the mutex, and we can delete it
// from the map. We can do this safely since we
// are under the mapMtx, meaning that all other
// goroutines waiting for the mutex already
// have incremented it, or will create a new
// mutex when they get the mapMtx.
mtx.cnt--
if mtx.cnt == 0 {
delete(c.mutexes, id)
}
c.mapMtx.Unlock()
// Unlock the mutex for this ID.
mtx.Unlock()
}