ucp

internal/ucp/agent.go

@@ -0,0 +1,151 @@
+package ucp
+
+import (
+	"context"
+	"net/http"
+	"strings"
+)
+
+// ---------------------------------------------------------------------------
+// UCP-Agent header parsing (RFC 8941 Dictionary Structured Field)
+// ---------------------------------------------------------------------------
+
+// ucpAgentProfileKey is the context key for the UCP-Agent profile URI.
+type ucpAgentProfileKey struct{}
+
+// UCPAgentProfileFromContext returns the UCP-Agent profile URI stored in the
+// request context, or empty string if none was advertised.
+func UCPAgentProfileFromContext(ctx context.Context) string {
+	v, _ := ctx.Value(ucpAgentProfileKey{}).(string)
+	return v
+}
+
+// parseUCPAgentProfile parses a single "profile" member value from an RFC 8941
+// Dictionary Structured Field header. It only handles the string-value form:
+//
+//	UCP-Agent: profile="https://agent.example/profiles/shopping-agent.json"
+//
+// Returns the profile URI (without surrounding quotes) or empty string.
+func parseUCPAgentProfile(header string) string {
+	if header == "" {
+		return ""
+	}
+
+	// Walk the dictionary entries looking for "profile".
+	// RFC 8941 §3.1.2: Dict = ((im-key | key) value) *(SP "," ((im-key | key) value))
+	i := 0
+	bs := []byte(header)
+	n := len(bs)
+
+	for i < n {
+		// Skip whitespace.
+		for i < n && (bs[i] == ' ' || bs[i] == '\t') {
+			i++
+		}
+		if i >= n {
+			break
+		}
+
+		// Read key (im-key or key). Lowercase letters, digits, "_", "-", "*".
+		start := i
+		for i < n && isKeyChar(bs[i]) {
+			i++
+		}
+		if i == start {
+			// No key found; skip to next comma or end.
+			for i < n && bs[i] != ',' {
+				i++
+			}
+			i++ // skip comma
+			continue
+		}
+		key := string(bs[start:i])
+
+		// Check for "=" separator. RFC 8941 allows bare names (boolean true)
+		// when "=? " follows — but we only care about key=value entries.
+		if i < n && bs[i] == '=' {
+			i++ // skip '='
+
+			// Read the value. For our case it's a string (starts with '"').
+			if i < n && bs[i] == '"' {
+				i++ // skip opening quote
+				valStart := i
+				for i < n && bs[i] != '"' {
+					if bs[i] == '\\' {
+						i++ // skip escape
+					}
+					i++
+				}
+				val := string(bs[valStart:i])
+				if i < n {
+					i++ // skip closing quote
+				}
+				if strings.EqualFold(key, "profile") {
+					return val
+				}
+			}
+		}
+
+		// Skip to next comma or end.
+		for i < n && bs[i] != ',' {
+			i++
+		}
+		i++ // skip comma
+	}
+
+	return ""
+}
+
+// isKeyChar reports whether b is valid in an RFC 8941 key (lc-ltr, DIGIT, "_", "-", "*").
+func isKeyChar(b byte) bool {
+	return (b >= 'a' && b <= 'z') || (b >= '0' && b <= '9') || b == '_' || b == '-' || b == '*'
+}
+
+// WithUCPAgent is HTTP middleware that reads the UCP-Agent header from incoming
+// requests, parses it per RFC 8941, and stores the profile URI in the request
+// context so downstream handlers can inspect the calling platform's identity.
+//
+// Usage:
+//
+//	mux.Handle("/ucp/v1/carts", ucp.WithUCPAgent(ucp.CartHandler(pool)))
+func WithUCPAgent(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if profile := parseUCPAgentProfile(r.Header.Get("UCP-Agent")); profile != "" {
+			ctx := context.WithValue(r.Context(), ucpAgentProfileKey{}, profile)
+			r = r.WithContext(ctx)
+		}
+		next.ServeHTTP(w, r)
+	})
+}
+
+// DefaultUCPAgentProfile is the profile URI sent on outgoing requests from
+// this commerce platform. Override via SetDefaultUCPAgentProfile or the
+// UCP_AGENT_PROFILE environment variable at init time.
+var DefaultUCPAgentProfile = "https://cart.k6n.net/.well-known/ucp"
+
+// SetDefaultUCPAgentProfile overrides the default profile URI used for outgoing
+// UCP-Agent headers.
+func SetDefaultUCPAgentProfile(profile string) {
+	if profile != "" {
+		DefaultUCPAgentProfile = profile
+	}
+}
+
+// UCPAgentHeaderValue returns the RFC 8941 Dictionary value for the UCP-Agent
+// header, encoding the given profile URI.
+func UCPAgentHeaderValue(profile string) string {
+	if profile == "" {
+		profile = DefaultUCPAgentProfile
+	}
+	return `profile="` + profile + `"`
+}
+
+// WithUCPAgentOnRequest adds the UCP-Agent header to an outgoing HTTP request.
+// It sets the header to the default profile unless a custom profile is provided.
+func WithUCPAgentOnRequest(req *http.Request, profile ...string) {
+	p := DefaultUCPAgentProfile
+	if len(profile) > 0 && profile[0] != "" {
+		p = profile[0]
+	}
+	req.Header.Set("UCP-Agent", `profile="`+p+`"`)
+}