mesh-controller/internal/chain/chain.go

package chain

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"regexp"
	"strings"

	"github.com/novox/mesh-controller/internal/broker"
)

// Bucket is one kind of record of the chain, typed. Reads need nothing; **every write asks first whether this
// controller holds the lease** and writes nothing when it does not (ADR 0408 §2).
type Bucket[T any] struct {
	b     Backend
	holds Holds
}

// Holds says whether this controller may write now: nil when it holds the lease, and why not when it does not. The
// serving controller answers from its lease, and a command from the holder's (the epoch gate).
type Holds func(ctx context.Context) error

// Kept is a record as the bucket holds it: with its revision, the one a write must find.
type Kept[T any] struct {
	Key      string
	Value    T
	Revision uint64
}

// validKey is what the bus takes as a key: letters, digits and - _ / = . with no dot at either end.
var validKey = regexp.MustCompile(`^[-/_=.a-zA-Z0-9]+$`)

// ErrBadKey is a key the bus does not take: refused loudly, never rewritten into another.
var ErrBadKey = errors.New("the bus takes no key of that spelling")

// ValidKey says the bus takes the key: letters, digits and - _ / = . with no dot at either end.
func ValidKey(key string) bool {
	return validKey.MatchString(key) && !strings.HasPrefix(key, ".") && !strings.HasSuffix(key, ".")
}

func (s Bucket[T]) guard(ctx context.Context, key string, value []byte) error {
	if s.holds == nil {
		return ErrNotHeld
	}
	if err := s.holds(ctx); err != nil {
		return fmt.Errorf("%w: %v", ErrNotHeld, err)
	}
	if !ValidKey(key) {
		return fmt.Errorf("%w: %q", ErrBadKey, key)
	}
	if len(value) > broker.ChainValueBytes {
		return fmt.Errorf("%w: %d bytes, the most is %d", ErrTooLarge, len(value), broker.ChainValueBytes)
	}
	return nil
}

func decode[T any](e Entry) (Kept[T], error) {
	var v T
	if err := json.Unmarshal(e.Value, &v); err != nil {
		return Kept[T]{}, fmt.Errorf("the value of %s cannot be read: %w", e.Key, err)
	}
	return Kept[T]{Key: e.Key, Value: v, Revision: e.Revision}, nil
}

// Get is one record; false when the key holds none.
func (s Bucket[T]) Get(ctx context.Context, key string) (Kept[T], bool, error) {
	e, ok, err := s.b.Get(ctx, key)
	if err != nil || !ok {
		return Kept[T]{}, false, err
	}
	e.Key = key
	k, err := decode[T](e)
	return k, err == nil, err
}

// Create writes a key that holds nothing: ErrExists when it holds something. It returns the revision.
func (s Bucket[T]) Create(ctx context.Context, key string, v T) (uint64, error) {
	body, err := json.Marshal(v)
	if err != nil {
		return 0, err
	}
	if err := s.guard(ctx, key, body); err != nil {
		return 0, err
	}
	return s.b.Create(ctx, key, body)
}

// Update writes a key at the revision it was read at: ErrMoved when it moved, the key unchanged.
func (s Bucket[T]) Update(ctx context.Context, key string, v T, revision uint64) (uint64, error) {
	body, err := json.Marshal(v)
	if err != nil {
		return 0, err
	}
	if err := s.guard(ctx, key, body); err != nil {
		return 0, err
	}
	return s.b.Update(ctx, key, body, revision)
}

// All is every record, by key, read in one pass of a watch. An error is an error: never an empty bucket.
func (s Bucket[T]) All(ctx context.Context) (map[string]Kept[T], error) {
	ctx, stop := context.WithCancel(ctx)
	defer stop()
	w, err := s.b.Watch(ctx)
	if err != nil {
		return nil, err
	}
	out := map[string]Kept[T]{}
	for {
		select {
		case <-ctx.Done():
			return nil, fmt.Errorf("reading the bucket: %w", ctx.Err())
		case e, open := <-w:
			if !open {
				return nil, errors.New("the bucket stopped answering before it was read whole")
			}
			if e.Caught {
				return out, nil
			}
			k, err := decode[T](e)
			if err != nil {
				return nil, err
			}
			out[e.Key] = k
		}
	}
}

// History is a key's earlier records and its current one, oldest first.
func (s Bucket[T]) History(ctx context.Context, key string) ([]Kept[T], error) {
	all, err := s.b.History(ctx, key)
	if err != nil {
		return nil, err
	}
	out := make([]Kept[T], 0, len(all))
	for _, e := range all {
		e.Key = key
		k, err := decode[T](e)
		if err != nil {
			return nil, err
		}
		out = append(out, k)
	}
	return out, nil
}

// Keys is every key of the bucket.
func (s Bucket[T]) Keys(ctx context.Context) ([]string, error) { return s.b.Keys(ctx) }

// Watch is the bucket's raw entries: its current values, a Caught mark, then each write.
func (s Bucket[T]) Watch(ctx context.Context) (<-chan Entry, error) { return s.b.Watch(ctx) }

// GetRaw is one key's value as the bucket holds it, for a writer that changes a field of a record and must keep the rest
// exactly as it is.
func (s Bucket[T]) GetRaw(ctx context.Context, key string) (Entry, bool, error) {
	e, ok, err := s.b.Get(ctx, key)
	e.Key = key
	return e, ok, err
}

// CreateRaw writes a key's first value as bytes, with the same guards as Create.
func (s Bucket[T]) CreateRaw(ctx context.Context, key string, value []byte) (uint64, error) {
	if err := s.guard(ctx, key, value); err != nil {
		return 0, err
	}
	return s.b.Create(ctx, key, value)
}

// UpdateRaw writes a key's value as bytes, at the revision it was read at, with the same guards as Update.
func (s Bucket[T]) UpdateRaw(ctx context.Context, key string, value []byte, revision uint64) (uint64, error) {
	if err := s.guard(ctx, key, value); err != nil {
		return 0, err
	}
	return s.b.Update(ctx, key, value, revision)
}

// Chain is the three buckets of the chain.
type Chain struct {
	Builds      Bucket[Build]
	Rollouts    Bucket[Rollout]
	Assignments Bucket[Assignment]

	rollouts Backend
	holds    Holds
}

// RolloutsOf is the rollouts bucket typed as the controller's plan, which carries more than the chain's Rollout names:
// the same keys and values, read and written as the plan.
func RolloutsOf[T any](c *Chain) Bucket[T] { return Bucket[T]{c.rollouts, c.holds} }

// New is a chain over three backends, written only while holds says this controller may write. A nil holds writes nothing.
func New(builds, rollouts, assignments Backend, holds Holds) *Chain {
	return &Chain{
		Builds:      Bucket[Build]{builds, holds},
		Rollouts:    Bucket[Rollout]{rollouts, holds},
		Assignments: Bucket[Assignment]{assignments, holds},
		rollouts:    rollouts,
		holds:       holds,
	}
}

// InMemory is a chain held in this process, for tests, written whenever holds says so; nil always holds.
func InMemory(holds Holds) *Chain {
	if holds == nil {
		holds = func(context.Context) error { return nil }
	}
	return New(NewMemory(), NewMemory(), NewMemory(), holds)
}