Keep a condition's looks in the store, so a look that changes nothing writes no bucket (issue 420)

Every look rewrote the condition's value in the conditions bucket, because the
value carried the count of looks, the last look and the newest evidence: 131 647
writes in four days for twelve open conditions, against ADR 0289's rule that a
bucket is written only when what it describes changed. The looks now live in the
controller's store (migration 0095, condition_looks), the bucket holds what a
condition says and is written only when that changes, and the conditions verb and
the condition events put the looks back.

cmd/mesh-controller/condition_looks.go

@@ -0,0 +1,76 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/novox/mesh-controller/internal/conditions"
+	"github.com/novox/mesh-controller/internal/inventory"
+)
+
+// The looks of the open conditions — how often each was observed, when last, its newest evidence — are
+// kept in the controller's store, not the conditions bucket (novox/hq issue 420, ADR 0289 §1): the bucket
+// is written when what a condition says changes, never for a look that saw it as it was.
+
+// storeLooks keeps the conditions' looks in the inventory.
+type storeLooks struct{ inv *inventory.Inventory }
+
+func (s storeLooks) KeepLooks(ctx context.Context, key string, l conditions.Looks) error {
+	evidence, err := json.Marshal(l.Evidence)
+	if err != nil {
+		return err
+	}
+	return s.inv.KeepConditionLooks(ctx, inventory.ConditionLooks{Key: key, Observations: l.Observations,
+		LastObserved: l.LastObserved, Evidence: evidence})
+}
+
+func (s storeLooks) Looked(ctx context.Context, key string) (conditions.Looks, bool, error) {
+	row, kept, err := s.inv.ConditionLooksOf(ctx, key)
+	if err != nil || !kept {
+		return conditions.Looks{}, kept, err
+	}
+	l, err := looksOf(row)
+	return l, err == nil, err
+}
+
+func (s storeLooks) AllLooks(ctx context.Context) (map[string]conditions.Looks, error) {
+	rows, err := s.inv.AllConditionLooks(ctx)
+	if err != nil {
+		return nil, err
+	}
+	out := make(map[string]conditions.Looks, len(rows))
+	for key, row := range rows {
+		l, err := looksOf(row)
+		if err != nil {
+			return nil, err
+		}
+		out[key] = l
+	}
+	return out, nil
+}
+
+func (s storeLooks) ForgetLooks(ctx context.Context, key string) error {
+	return s.inv.ForgetConditionLooks(ctx, key)
+}
+
+func looksOf(row inventory.ConditionLooks) (conditions.Looks, error) {
+	l := conditions.Looks{Observations: row.Observations, LastObserved: row.LastObserved.UTC()}
+	if len(row.Evidence) > 0 {
+		if err := json.Unmarshal(row.Evidence, &l.Evidence); err != nil {
+			return conditions.Looks{}, fmt.Errorf("the evidence kept for the condition %s cannot be read: %w", row.Key, err)
+		}
+	}
+	return l, nil
+}
+
+// looksHere is where a process that is not the serving controller reads the conditions' looks: the store,
+// opened for it, closed by the function returned. A store that cannot be opened is said on every condition
+// read through it, never read as none.
+func looksHere(ctx context.Context) (conditions.LookStore, func()) {
+	inv, err := openInventory(ctx)
+	if err != nil {
+		return conditions.NoLooks{Err: err}, func() {}
+	}
+	return storeLooks{inv}, inv.Close
+}