refactor(media): 资源分发 - 统一资源访问策略与云存储重定向

backend/internal/handler/resource_delivery.go

@@ -0,0 +1,88 @@
+package handler
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+
+	"infinite-canvas/backend/internal/assets"
+	"infinite-canvas/backend/internal/service"
+
+	"github.com/gin-gonic/gin"
+)
+
+func resourceAccessOptions(c *gin.Context) service.ResourceAccessOptions {
+	purpose := assets.AccessPurpose(strings.TrimSpace(c.Query("purpose")))
+	if purpose == "" && c.Query("download") == "1" {
+		purpose = assets.PurposeDownload
+	}
+	variant := assets.ResourceVariant(strings.TrimSpace(c.Query("variant")))
+	return service.ResourceAccessOptions{
+		Purpose:      purpose,
+		Variant:      variant,
+		DescribeOnly: c.Query("access") == "1",
+	}
+}
+
+// serveResourceDelivery is the only HTTP byte/redirect executor. Authorization and
+// delivery selection happen in app; this function only applies the already-resolved
+// contract to an HTTP response.
+func serveResourceDelivery(c *gin.Context, delivery *service.ResourceDelivery, cacheControl string, disposition string) {
+	if delivery == nil || delivery.Access == nil {
+		fail(c, http.StatusServiceUnavailable, fmt.Errorf("资源分发结果无效"))
+		return
+	}
+	access := delivery.Access
+	if access.URL == "" && delivery.Stream == nil {
+		fail(c, http.StatusServiceUnavailable, fmt.Errorf("资源分发结果无效"))
+		return
+	}
+	if c.Query("access") == "1" {
+		c.Header("Cache-Control", "private, no-store")
+		c.Header("Referrer-Policy", "no-referrer")
+		ok(c, gin.H{"access": access})
+		return
+	}
+	c.Header("Referrer-Policy", "no-referrer")
+	c.Header("X-Content-Type-Options", "nosniff")
+	c.Header("Accept-Ranges", "bytes")
+	if access.Delivery == assets.DeliveryCDN || access.Delivery == assets.DeliveryOrigin {
+		c.Header("Cache-Control", "private, no-store")
+		// Do not let Gin generate its default HTML redirect body. A media request
+		// should transfer only the redirect metadata; the actual bytes must be
+		// fetched from the CDN/origin selected by the access contract.
+		c.Header("Location", access.URL)
+		c.Status(http.StatusTemporaryRedirect)
+		return
+	}
+	stream := delivery.Stream
+	if stream == nil || stream.Body == nil {
+		fail(c, http.StatusServiceUnavailable, fmt.Errorf("资源流不可用"))
+		return
+	}
+	defer stream.Body.Close()
+	if cacheControl != "" {
+		c.Header("Cache-Control", cacheControl)
+	}
+	if disposition != "" {
+		c.Header("Content-Disposition", disposition)
+	}
+	resource := stream.Resource
+	mimeType := resource.MimeType
+	if mimeType == "" {
+		mimeType = "application/octet-stream"
+	}
+	if stream.ContentRange != "" {
+		c.Header("Content-Range", stream.ContentRange)
+	}
+	if stream.AcceptRanges != "" {
+		c.Header("Accept-Ranges", stream.AcceptRanges)
+	}
+	if seeker, ok := stream.Body.(io.ReadSeeker); ok {
+		c.Header("Content-Type", mimeType)
+		http.ServeContent(c.Writer, c.Request, resource.ID, resource.UpdatedAt, seeker)
+		return
+	}
+	c.DataFromReader(stream.StatusCode, stream.ContentLength, mimeType, stream.Body, nil)
+}