feat: validate and append custom per-service Caddy configs to generated Caddyfile

internal/machine/caddyconfig/template.go

@@ -0,0 +1,77 @@
+package caddyconfig
+
+import (
+	"fmt"
+	"net"
+	"strconv"
+	"strings"
+)
+
+// templateContext holds the data available to Caddyfile templates.
+type templateContext struct {
+	// Name is the current service name.
+	Name string
+	// Upstreams maps service names to their container IPs.
+	Upstreams map[string][]string
+}
+
+// upstreamsTemplateFn returns a template function that generates a space separated string of upstreams for the service.
+// It optionally accepts a service name and a port number: {{upstreams [service-name] [port]}}.
+func upstreamsTemplateFn(tmplCtx templateContext) func(args ...any) (string, error) {
+	return func(args ...any) (string, error) {
+		var serviceName string
+		var port int
+
+		// Parse arguments.
+		switch len(args) {
+		case 0:
+			// Current service, default port.
+			serviceName = tmplCtx.Name
+		case 1:
+			// Either port (int) for current service or service name (string).
+			switch arg := args[0].(type) {
+			case int:
+				serviceName = tmplCtx.Name
+				port = arg
+			case string:
+				serviceName = arg
+				port = 0
+			default:
+				return "", fmt.Errorf("upstreams function: invalid argument type: %T", arg)
+			}
+		case 2:
+			// Service name and port.
+			name, ok := args[0].(string)
+			if !ok {
+				return "", fmt.Errorf("upstreams function: first argument must be service name (string)")
+			}
+			serviceName = name
+
+			p, ok := args[1].(int)
+			if !ok {
+				return "", fmt.Errorf("upstreams function: second argument must be port (int)")
+			}
+			port = p
+		default:
+			return "", fmt.Errorf("upstreams function: too many arguments; expected 0-2, got %d", len(args))
+		}
+
+		ips, ok := tmplCtx.Upstreams[serviceName]
+		if !ok || len(ips) == 0 {
+			// No upstreams available.
+			return "", nil
+		}
+
+		// Build the space separated upstreams string.
+		var upstreams []string
+		for _, ip := range ips {
+			if port > 0 {
+				upstreams = append(upstreams, net.JoinHostPort(ip, strconv.Itoa(port)))
+			} else {
+				upstreams = append(upstreams, ip)
+			}
+		}
+
+		return strings.Join(upstreams, " "), nil
+	}
+}