mesh-sdk/go/views/held_writer.go

package views

import (
	"context"
	"encoding/json"
	"errors"
	"sync"
)

// HeldWriter is the finding writer of a part whose findings are raised as they happen, not judged afresh (design 47 G A-29,
// §686): it holds the findings it has written, seeded from the keys of its state `findings` before it starts, under a lock of
// its own (never the part's: the writer calls Observe under its own lock while the part writes under its), and its Observe
// answers them, so a statement says exactly what the part holds. Write and Clear update the held set and then the writer's.
// It is a Writer[F, C, S, A] with that Observe, so no part writes the adapter.
type HeldWriter[F, C, S, A any] struct {
	*Writer[F, C, S, A]
	mu      sync.Mutex
	held    map[string]F
	state   FindingState
	started bool
	pending map[string]F         // written before Start, written through the writer at Start
	cleared map[string][4]string // cleared before Start, cleared through the writer at Start
}

// NewHeldWriter makes the writer for cfg, whose Observe is none (a given Observe is refused): the findings that stand in
// cfg.State are read at once, each key's value as the writer kept it (a value that is no finding of F is left to the next
// statement, which deletes its key when it is of a kind the writer owns).
func NewHeldWriter[F, C, S, A any](cfg WriterConfig[F, C, S, A]) (*HeldWriter[F, C, S, A], error) {
	if cfg.Observe != nil {
		return nil, errors.New("a held finding writer observes what it holds: it takes no Observe")
	}
	if cfg.State == nil {
		return nil, errors.New("a finding writer needs the state `findings`")
	}
	h := &HeldWriter[F, C, S, A]{held: map[string]F{}}
	h.state, h.pending, h.cleared = cfg.State, map[string]F{}, map[string][4]string{}
	cfg.Observe = func(context.Context) (Observed[F], error) {
		h.mu.Lock()
		defer h.mu.Unlock()
		var o Observed[F]
		for _, f := range h.held {
			o.Findings = append(o.Findings, f)
		}
		return o, nil
	}
	w, err := NewWriter(cfg)
	if err != nil {
		return nil, err
	}
	h.Writer = w
	return h, nil
}

// Write holds the finding and writes it through the writer.
func (h *HeldWriter[F, C, S, A]) Write(ctx context.Context, finding F) error {
	m, err := wireOf(&finding)
	if err != nil {
		return err
	}
	if err := CheckFinding(m); err != nil {
		return err
	}
	if !h.Writer.owns(str(m, "kind")) {
		return h.Writer.Write(ctx, finding) // refused by the writer, naming the kind, and held no more than it is
	}
	h.mu.Lock()
	h.held[keyOf(m)] = finding
	if !h.started {
		h.pending[keyOf(m)] = finding
		delete(h.cleared, keyOf(m))
		h.mu.Unlock()
		return nil
	}
	h.mu.Unlock()
	return h.Writer.Write(ctx, finding)
}

// Clear stops holding the finding of scope, id and kind and clears it through the writer.
func (h *HeldWriter[F, C, S, A]) Clear(ctx context.Context, scope, id, kind, why string) error {
	h.mu.Lock()
	k := FindingKey(scope, id, kind)
	delete(h.held, k)
	if !h.started {
		delete(h.pending, k)
		h.cleared[k] = [4]string{scope, id, kind, why}
		h.mu.Unlock()
		return nil
	}
	h.mu.Unlock()
	return h.Writer.Clear(ctx, scope, id, kind, why)
}

// WriteWire is Write of a finding in its wire form, held as Write holds it (design 47 G A-37): a views reader writing through
// the held writer has its findings in the held set, so in the statement.
func (h *HeldWriter[F, C, S, A]) WriteWire(ctx context.Context, finding map[string]any) error {
	var f F
	if err := fillFrom(&f, finding); err != nil {
		return err
	}
	return h.Write(ctx, f)
}

// Held answers the findings the writer holds, a copy taken under its lock (design 47 G A-38), so a part reads its open findings
// from the writer.
func (h *HeldWriter[F, C, S, A]) Held() []F {
	h.mu.Lock()
	defer h.mu.Unlock()
	out := make([]F, 0, len(h.held))
	for _, f := range h.held {
		out = append(out, f)
	}
	return out
}

// Start seeds what the writer holds from the keys of its state `findings` (design 47 G A-40), what was written before it
// standing over them, writes what was written and clears what was cleared before it, and starts the writer. A part makes the
// writer before its module serves, so nothing is read from the state until now.
func (h *HeldWriter[F, C, S, A]) Start(ctx context.Context) error {
	keys, err := h.state.Keys()
	if err != nil {
		return err
	}
	h.mu.Lock()
	for _, k := range keys {
		e, err := h.state.Get(k)
		if err != nil {
			h.mu.Unlock()
			return err
		}
		if _, ok := h.held[k]; ok {
			continue
		}
		if _, ok := h.cleared[k]; ok {
			continue
		}
		var f F
		if e == nil || json.Unmarshal(e.Value, &f) != nil {
			continue
		}
		if m, err := wireOf(&f); err == nil && keyOf(m) == k {
			h.held[k] = f
		}
	}
	pending, cleared := h.pending, h.cleared
	h.started, h.pending, h.cleared = true, nil, nil
	h.mu.Unlock()
	for _, f := range pending {
		if err := h.Writer.Write(ctx, f); err != nil {
			return err
		}
	}
	for _, c := range cleared {
		if err := h.Writer.Clear(ctx, c[0], c[1], c[2], c[3]); err != nil {
			return err
		}
	}
	return h.Writer.Start(ctx)
}