WIP: extract logic for WaitForTrigger, Trigger, Reset, Poll into a reusable abstraction

daemon/job/trigger/trigger.go

@@ -0,0 +1,187 @@
+//
+//
+// Alternative Design (in "RustGo")
+//
+//	enum InternalMsg {
+//	    Trigger((), chan (TriggerResponse, error)),
+//	    Poll(PollRequest, chan PollResponse),
+//	    Reset(ResetRequest, chan (ResetResponse, error)),
+//	}
+//	
+//	enum State {
+//	    Running{
+//	        invocationId: u32,
+//	        cancelCurrentInvocation: context.CancelFunc
+//	    }
+//	    Waiting{
+//	        nextInvocationId: u32,
+//	    }
+//	}
+//	
+//	for msg := <- t.internalMsgs {
+//	    match (msg, state) {
+//	        ...
+//	    }
+//	}
+package trigger
+
+import (
+	"context"
+	"fmt"
+	"math"
+	"sync"
+
+	"github.com/zrepl/zrepl/daemon/logging"
+	"github.com/zrepl/zrepl/logger"
+)
+
+type T struct {
+	mtx sync.Mutex
+	cv  sync.Cond
+
+	nextInvocationId        uint64
+	activeInvocationId      uint64 // 0 <=> inactive
+	triggerPending          bool
+	contextDone             bool
+	reset                   chan uint64
+	stopWaitForReset        chan struct{}
+	cancelCurrentInvocation context.CancelFunc
+}
+
+func New() *T {
+	t := &T{
+		activeInvocationId: math.MaxUint64,
+		nextInvocationId:   1,
+	}
+	t.cv.L = &t.mtx
+	return t
+}
+
+func (t *T) WaitForTrigger(ctx context.Context) (rctx context.Context, err error) {
+	t.mtx.Lock()
+	defer t.mtx.Unlock()
+
+	if t.activeInvocationId == 0 {
+		return nil, fmt.Errorf("must be running when calling this function")
+	}
+	t.activeInvocationId = 0
+	t.cancelCurrentInvocation = nil
+
+	if t.contextDone == true {
+		panic("implementation error: this variable is only true while in WaitForTrigger, and that's a mutually exclusive function")
+	}
+	stopWaitingForDone := make(chan struct{})
+	go func() {
+		select {
+		case <-stopWaitingForDone:
+		case <-ctx.Done():
+			t.mtx.Lock()
+			t.contextDone = true
+			t.cv.Broadcast()
+			t.mtx.Unlock()
+		}
+	}()
+
+	defer func() {
+		t.triggerPending = false
+		t.contextDone = false
+	}()
+	for !t.triggerPending && !t.contextDone {
+		t.cv.Wait()
+	}
+	close(stopWaitingForDone)
+	if t.contextDone {
+		if ctx.Err() == nil {
+			panic("implementation error: contextDone <=> ctx.Err() != nil")
+		}
+		return nil, ctx.Err()
+	}
+
+	t.activeInvocationId = t.nextInvocationId
+	t.nextInvocationId++
+	rctx, t.cancelCurrentInvocation = context.WithCancel(ctx)
+
+	return rctx, nil
+}
+
+type TriggerResponse struct {
+	InvocationId uint64
+}
+
+func (t *T) Trigger() (TriggerResponse, error) {
+	t.mtx.Lock()
+	defer t.mtx.Unlock()
+	var invocationId uint64
+	if t.activeInvocationId != 0 {
+		invocationId = t.activeInvocationId
+	} else {
+		invocationId = t.nextInvocationId
+	}
+	// non-blocking send (.Run() must not hold mutex while waiting for signals)
+	t.triggerPending = true
+	t.cv.Broadcast()
+	return TriggerResponse{InvocationId: invocationId}, nil
+}
+
+type PollRequest struct {
+	InvocationId uint64
+}
+
+type PollResponse struct {
+	Done         bool
+	InvocationId uint64
+}
+
+func (t *T) Poll(req PollRequest) (res PollResponse) {
+	t.mtx.Lock()
+	defer t.mtx.Unlock()
+
+	waitForId := req.InvocationId
+	if req.InvocationId == 0 {
+		// handle the case where the client doesn't know what the current invocation id is
+		if t.activeInvocationId != 0 {
+			waitForId = t.activeInvocationId
+		} else {
+			waitForId = t.nextInvocationId
+		}
+	}
+
+	var done bool
+	if t.activeInvocationId == 0 {
+		done = waitForId < t.nextInvocationId
+	} else {
+		done = waitForId < t.activeInvocationId
+	}
+	return PollResponse{Done: done, InvocationId: waitForId}
+}
+
+type ResetRequest struct {
+	InvocationId uint64
+}
+
+type ResetResponse struct {
+	InvocationId uint64
+}
+
+func (t *T) Reset(req ResetRequest) (*ResetResponse, error) {
+	t.mtx.Lock()
+	defer t.mtx.Unlock()
+
+	resetId := req.InvocationId
+	if req.InvocationId == 0 {
+		// handle the case where the client doesn't know what the current invocation id is
+		resetId = t.activeInvocationId
+	}
+
+	if resetId == 0 {
+		return nil, fmt.Errorf("no active invocation")
+	}
+
+	if resetId != t.activeInvocationId {
+		return nil, fmt.Errorf("active invocation (%d) is not the invocation requested for reset (%d); (active invocation '0' indicates no active invocation)", t.activeInvocationId, resetId)
+	}
+
+	t.cancelCurrentInvocation()
+
+	return &ResetResponse{InvocationId: resetId}, nil
+}