back: add qver package to track qid versions

1 files changed, 178 insertions, 0 deletions

diff --git a/back/qver/version.go b/back/qver/version.go
new file mode 100644
index 0000000..8c815cf
--- /dev/null
+++ b/back/qver/version.go
@@ -0,0 +1,178 @@
+// Package qver keeps track of 9P qid versions.
+package qver
+
+// Version is a 9P qid.version.
+//
+// A Version may have several child Versions, representing a directory
+// hierarchy. The root version is incremented whenever any of the
+// children's versions change.
+//
+// Version is safe to share between goroutines.
+type Version struct {
+	req <-chan struct{}
+	res <-chan message
+
+	bump     chan<- struct{}
+	regChild chan<- childReg
+	kill     chan<- struct{}
+}
+
+type Bumper interface {
+	// Bump increments the version.
+	Bump()
+}
+
+type version struct {
+	req chan<- struct{}
+	res chan<- message
+
+	bump     <-chan struct{}
+	regChild <-chan childReg
+	kill     <-chan struct{}
+
+	v uint32
+	UpdateFunc
+	state    interface{}
+	children []*child
+}
+
+type childReg struct {
+	child *Version
+	done  chan<- struct{}
+}
+
+type child struct {
+	*Version
+	last uint32
+}
+
+type message struct {
+	ver uint32
+	err error
+}
+
+func New(opts ...Option) *Version {
+	o := parse(opts...)
+
+	reqc := make(chan struct{})
+	resc := make(chan message)
+	bumpc := make(chan struct{})
+	childc := make(chan childReg)
+	killc := make(chan struct{})
+
+	go (&version{
+		reqc,
+		resc,
+		bumpc,
+		childc,
+		killc,
+		0,
+		o.UpdateFunc,
+		o.state,
+		nil,
+	}).run()
+
+	v := &Version{reqc, resc, bumpc, childc, killc}
+	if o.parent != nil {
+		o.parent.register(v)
+	}
+	return v
+}
+
+func (v *version) run() {
+	defer close(v.req)
+	defer close(v.res)
+
+	for {
+		select {
+		case v.req <- struct{}{}:
+			err := v.update()
+			v.res <- message{v.v, err}
+
+		case <-v.bump:
+			v.v++
+
+		case creg := <-v.regChild:
+			cver, _ := creg.child.Get()
+			v.children = append(v.children, &child{creg.child, cver})
+			close(creg.done)
+
+		case <-v.kill:
+			return
+		}
+	}
+}
+
+func (v *version) update() error {
+	if v.UpdateFunc != nil {
+		var err error
+		v.v, v.state, err = v.UpdateFunc(v.v, v.state)
+		if err != nil {
+			return err
+		}
+	}
+
+	var modified bool
+	for _, c := range v.children {
+		mod, err := c.update()
+		if err != nil {
+			return err
+		}
+		modified = modified || mod
+	}
+	if modified {
+		v.v++
+	}
+
+	return nil
+}
+
+// Returns true if modified.
+func (c *child) update() (bool, error) {
+	_, ok := <-c.req
+	if !ok {
+		// closed
+		return false, nil
+	}
+
+	msg := <-c.res
+	if msg.err != nil {
+		return false, msg.err
+	}
+	if msg.ver != c.last {
+		// modified
+		c.last = msg.ver
+		return true, nil
+	}
+
+	// not modified
+	return false, nil
+}
+
+func (v *Version) register(child *Version) {
+	done := make(chan struct{})
+	v.regChild <- childReg{child, done}
+	<-done
+}
+
+func (v *Version) Close() {
+	v.kill <- struct{}{}
+	close(v.kill)
+	close(v.bump)
+	close(v.regChild)
+}
+
+// Get returns the current version.
+//
+// If the Version was created with the Update option, Get calls the
+// UpdateFunc and returns the new version.
+func (v *Version) Get() (uint32, error) {
+	<-v.req
+	msg := <-v.res
+	return msg.ver, msg.err
+}
+
+// Bump increments the version.
+func (v *Version) Bump() {
+	v.bump <- struct{}{}
+}