Carry the power draw to the bar over the bus, not through a file (hq ADR 0260)

Modules talk over the bus. The power module's bundle puts each fresh reading of its own
sampler in its state draw, keyed by the machine; the bar's block shows that state, which
the bar's bundle watches under the read the mesh grants it. A new catalogue check refuses
a module naming another module's path under /run or /var/lib.

modules/i3status-rust/cmd/i3status-rust-tools/values.go

@@ -0,0 +1,118 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"regexp"
+	"sort"
+	"strconv"
+	"time"
+
+	stdio "git.novox.be/novox/mesh-sdk/go"
+)
+
+// The values other modules keep on the bus for a block they contributed (novox/hq ADR 0260).
+//
+// A block that `shows` a state is rendered as a custom block running `i3status-value <state> <key>`.
+// i3status-rs cannot reach the bus, and this bundle can: the node's runtime launched it, and the mesh
+// granted this module a read of every state the modules on its machine offer the bar. So it watches each
+// state its bars name and leaves the value's text in this module's own directory, where the block's
+// command reads it. No file of another module is read, and nothing but the bus passes between them.
+
+// valueCall is how a bar names a value: the command the template renders.
+var valueCall = regexp.MustCompile(`i3status-value ([a-z0-9][a-z0-9-]*\.[a-z0-9][a-z0-9-]*) ([^\s"]+)`)
+
+// Wanted is one state and key a bar shows.
+type Wanted struct{ State, Key string }
+
+func (w Wanted) file() string { return w.State + "@" + w.Key }
+
+// wantedIn is every value the bars' files name, each once, in order.
+func wantedIn(files []string) []Wanted {
+	seen := map[Wanted]bool{}
+	for _, f := range files {
+		raw, err := os.ReadFile(f)
+		if err != nil {
+			continue
+		}
+		for _, m := range valueCall.FindAllStringSubmatch(string(raw), -1) {
+			seen[Wanted{State: m[1], Key: m[2]}] = true
+		}
+	}
+	out := make([]Wanted, 0, len(seen))
+	for w := range seen {
+		out = append(out, w)
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].file() < out[j].file() })
+	return out
+}
+
+// valuesDir is where the values are left: the account's runtime directory, gone at logout.
+func valuesDir() string {
+	return filepath.Join(runUserDir, strconv.Itoa(os.Getuid()), "i3status-rust", "values")
+}
+
+// keep writes one change of a value to its file: the value's `text`, or no file once it is deleted.
+func keep(dir string, w Wanted, c stdio.StateChange) error {
+	path := filepath.Join(dir, w.file())
+	if c.Op == "delete" || c.Op == "purge" {
+		if err := os.Remove(path); err != nil && !os.IsNotExist(err) {
+			return err
+		}
+		return nil
+	}
+	var v struct {
+		Text string `json:"text"`
+	}
+	if err := json.Unmarshal(c.Value, &v); err != nil {
+		return fmt.Errorf("%s's value for %s has no text: %w", w.State, w.Key, err)
+	}
+	if err := os.MkdirAll(dir, 0o700); err != nil {
+		return err
+	}
+	tmp := path + ".new"
+	if err := os.WriteFile(tmp, []byte(v.Text+"\n"), 0o600); err != nil {
+		return err
+	}
+	return os.Rename(tmp, path)
+}
+
+// Watcher is a function that watches one key of one state; the SDK's in the bundle, a fake in a test.
+type Watcher func(state, key string, handle func(stdio.StateChange) error) error
+
+// followValues watches every value the bars name, starting the ones not yet watched each time it is
+// called; one the runtime refuses — a read not granted yet — is tried again the next time.
+func followValues(files []string, dir string, watching map[Wanted]bool, watch Watcher) []string {
+	var said []string
+	for _, w := range wantedIn(files) {
+		if watching[w] {
+			continue
+		}
+		w := w
+		err := watch(w.State, w.Key, func(c stdio.StateChange) error { return keep(dir, w, c) })
+		if err != nil {
+			said = append(said, fmt.Sprintf("cannot watch %s %s: %v", w.State, w.Key, err))
+			continue
+		}
+		watching[w] = true
+	}
+	return said
+}
+
+// FollowValues runs followValues every half minute for as long as the bundle runs, so a block a push
+// adds is followed without a restart.
+func FollowValues() {
+	watching := map[Wanted]bool{}
+	watch := func(state, key string, handle func(stdio.StateChange) error) error {
+		return stdio.State(state).Watch(key, handle)
+	}
+	for {
+		files, _ := filepath.Glob(filepath.Join(configDir(), "*.toml"))
+		for _, s := range followValues(files, valuesDir(), watching, watch) {
+			fmt.Fprintln(os.Stderr, "i3status-rust:", s)
+		}
+		time.Sleep(30 * time.Second)
+	}
+}