Let a requirement be optional, binding only a provider that already runs (hq issue 166, ADR 0330)

Making every soft dependency a provision made it hard: a mesh without influxdb could not assign
grafana, and one with influxdb in its catalogue would install it for grafana. A requirement may now
be written {"name": ..., "optional": true}. It resolves as a hard one when a provider is assigned or
reachable, and is otherwise absent: never refused, nothing installed for it. Its binding, credential
and only-when-bound files are left out, its ${bound:...|default:...} placeholders take their
defaults, the module check refuses one that could render empty, and plan and node list it unbound.

internal/catalogue/bound_into_files.go

@@ -26,21 +26,10 @@ import (
 // serves — whose keys are agreed by the requirement's name, not by this file.
 
 // bound is where a module says a value from one of its bindings belongs:
-// ${bound:<provision>.<key>}.
-var bound = regexp.MustCompile(`\$\{bound:([a-z0-9][a-z0-9.-]*[a-z0-9]):([a-z0-9][a-z0-9_-]*)\}`)
-
-// boundUsed are the (provision, key) pairs a file's content asks for, first appearance first.
-func boundUsed(content string) [][2]string {
-	var used [][2]string
-	seen := map[string]bool{}
-	for _, m := range bound.FindAllStringSubmatch(content, -1) {
-		if key := m[1] + ":" + m[2]; !seen[key] {
-			seen[key] = true
-			used = append(used, [2]string{m[1], m[2]})
-		}
-	}
-	return used
-}
+// ${bound:<provision>:<key>}, and for an optional requirement the value it takes while absent,
+// ${bound:<provision>:<key>|default:<value>} (novox/hq ADR 0330 §4). A default is never empty: an
+// absent requirement is never rendered as an empty string.
+var bound = regexp.MustCompile(`\$\{bound:([a-z0-9][a-z0-9.-]*[a-z0-9]):([a-z0-9][a-z0-9_-]*)(?:\|default:([^}\n]+))?\}`)
 
 // knownFor is everything a module may name from one of its bindings.
 //
@@ -125,7 +114,11 @@ func plainly(value any) string {
 // serve, is refused. Left as it was, the literal `${bound:x:y}` would be written into a
 // configuration file and read as a value — a connection to a host called `${bound:x:y}`, failing
 // somewhere that names neither the module nor the mesh.
-func boundInto(resource map[string]any, known map[string]map[string]string, module string) error {
+//
+// **An optional requirement left absent takes the default its placeholder names** (novox/hq ADR
+// 0330 §4), and one that names none is refused rather than rendered empty. The module check refuses
+// that already; this is the same rule where the file is made, for a manifest registered before it.
+func boundInto(resource map[string]any, known map[string]map[string]string, module string, absent map[string]bool) error {
 	if fmt.Sprint(resource["type"]) != "file" {
 		return nil
 	}
@@ -133,24 +126,43 @@ func boundInto(resource map[string]any, known map[string]map[string]string, modu
 	if !ok {
 		return nil
 	}
-	for _, pair := range boundUsed(content) {
-		provision, key := pair[0], pair[1]
+	var refused error
+	filled := bound.ReplaceAllStringFunc(content, func(token string) string {
+		if refused != nil {
+			return token
+		}
+		m := bound.FindStringSubmatch(token)
+		provision, key, fallback := m[1], m[2], m[3]
 		values, has := known[provision]
 		if !has {
-			return fmt.Errorf(
+			if absent[provision] {
+				if fallback != "" {
+					return fallback
+				}
+				refused = fmt.Errorf(
+					"%s has a file that says ${bound:%s:%s}, and %q is optional and absent here: it names no "+
+						"value for then (${bound:%s:%s|default:<value>}), so it would be written empty "+
+						"(novox/hq ADR 0330 §4)", module, provision, key, provision, provision, key)
+				return token
+			}
+			refused = fmt.Errorf(
 				"%s has a file that says ${bound:%s:%s}, and %s does not require %q. It may name %s",
 				module, provision, key, module, provision, orNothing(namesOfBindings(known)))
+			return token
 		}
 		value, said := values[key]
 		if !said {
-			return fmt.Errorf(
+			refused = fmt.Errorf(
 				"%s asks its %s binding for %q, and what answers it says %s",
 				module, provision, key, orNothing(namesOfKeys(values)))
+			return token
 		}
-		resource["content"] = strings.ReplaceAll(
-			content, fmt.Sprintf("${bound:%s:%s}", provision, key), value)
-		content = resource["content"].(string)
+		return value
+	})
+	if refused != nil {
+		return refused
 	}
+	resource["content"] = filled
 	return nil
 }