feat: backend service

backend/internal/server/middleware/userid/userid.go

@@ -0,0 +1,70 @@
+// Package userid extracts the calling user identifier from the trusted
+// `X-User-ID` header injected by gateway and exposes it through the request
+// context.
+//
+// Backend trusts the header value because the network segment between gateway
+// and backend is the trust boundary (see `ARCHITECTURE.md` §15). The
+// middleware therefore only validates the syntactic shape (UUID) and rejects
+// malformed or absent values with the standard `400 invalid_request` envelope.
+package userid
+
+import (
+	"context"
+	"net/http"
+	"strings"
+
+	"galaxy/backend/internal/server/httperr"
+
+	"github.com/gin-gonic/gin"
+	"github.com/google/uuid"
+)
+
+// Header is the canonical case-correct header name carrying the trusted user
+// id forwarded by gateway.
+const Header = "X-User-ID"
+
+// userIDContextKey is the unexported context key used to store the parsed
+// user id. The unexported value type prevents accidental collisions with
+// keys defined in unrelated packages.
+type userIDContextKey struct{}
+
+// Middleware returns the gin middleware that requires a syntactically valid
+// `X-User-ID` header on every authenticated user request and stores the
+// parsed UUID on the request context.
+func Middleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		raw := strings.TrimSpace(c.GetHeader(Header))
+		if raw == "" {
+			httperr.Abort(c, http.StatusBadRequest, httperr.CodeInvalidRequest, Header+" header is required")
+			return
+		}
+
+		userID, err := uuid.Parse(raw)
+		if err != nil {
+			httperr.Abort(c, http.StatusBadRequest, httperr.CodeInvalidRequest, Header+" header must be a valid UUID")
+			return
+		}
+
+		c.Request = c.Request.WithContext(WithValue(c.Request.Context(), userID))
+		c.Next()
+	}
+}
+
+// FromContext returns the user id stored on ctx by Middleware. The boolean
+// reports whether a value was found.
+func FromContext(ctx context.Context) (uuid.UUID, bool) {
+	if ctx == nil {
+		return uuid.Nil, false
+	}
+	value, ok := ctx.Value(userIDContextKey{}).(uuid.UUID)
+	if !ok {
+		return uuid.Nil, false
+	}
+	return value, true
+}
+
+// WithValue stores userID on ctx under the package-private context key.
+// Exposed for tests that need to build a context outside the middleware.
+func WithValue(ctx context.Context, userID uuid.UUID) context.Context {
+	return context.WithValue(ctx, userIDContextKey{}, userID)
+}