fup: factor out metadata retrieval into a separate file

We'll need this for cleanup operations as well. This should likely be factored
out again into an entirely separate package that deals with storage access.

web/fup/fuphttp/errors.go

@@ -0,0 +1,42 @@
+package fuphttp
+
+import (
+	"errors"
+	"fmt"
+
+	"gocloud.dev/gcerrors"
+)
+
+type fupError struct {
+	Code gcerrors.ErrorCode
+	msg  string
+	err  error
+}
+
+func (e *fupError) Error() string {
+	var msg string
+	if e.msg == "" {
+		msg = fmt.Sprintf("code=%v", e.Code)
+	} else {
+		msg = fmt.Sprintf("%s (code=%v)", e.msg, e.Code)
+	}
+	if e.err != nil {
+		msg = fmt.Sprintf("%s: %s", msg, e.err)
+	}
+	return msg
+}
+
+func (e *fupError) Unwrap() error {
+	return e.err
+}
+
+func errorCode(err error) gcerrors.ErrorCode {
+	if err == nil {
+		return gcerrors.OK
+	}
+	var e *fupError
+	if errors.As(err, &e) {
+		return e.Code
+	}
+	return gcerrors.Code(err)
+}

web/fup/fuphttp/fuphttp.go

@@ -11,7 +11,6 @@ import (
 	"io/fs"
 	"log"
 	"net/http"
-	"strconv"
 	"time"
 
 	"github.com/google/safehtml"
@@ -90,52 +89,25 @@ func (a *Application) rawDownload(rw http.ResponseWriter, r *http.Request) {
 	vars := mux.Vars(r)
 	filename := vars["filename"]
 
-	attrs, err := a.storageBackend.Attributes(ctx, filename)
-	switch gcerrors.Code(err) {
+	meta, err := metadata(ctx, a.storageBackend, filename)
+	switch errorCode(err) {
 	case gcerrors.NotFound:
 		a.notFound(rw, r)
 		return
 	default:
-		log.Printf("rawDownload(%q) Attributes: %v", filename, err)
+		log.Printf("rawDownload(%q) metadata: %v", filename, err)
 		a.internalError(rw, r)
 		return
 	case gcerrors.OK:
 		// OK
 	}
 
-	var exp time.Time
-	if expStr, ok := attrs.Metadata["expires-at"]; ok {
-		// Check for expiry.
-		expSec, err := strconv.ParseInt(expStr, 10, 64)
-		if err != nil {
-			log.Printf("parsing expiration %q for %q: %v", expStr, filename, err)
-			a.internalError(rw, r)
-			return
-		}
-
-		exp = time.Unix(expSec, 0)
-		if exp.Before(time.Now()) {
-			// This file has expired. Delete it from the backing storage.
-			err := a.storageBackend.Delete(ctx, filename)
-			switch gcerrors.Code(err) {
-			case gcerrors.NotFound:
-				// Something probably deleted it before we could get there.
-			case gcerrors.OK:
-				// This was fine.
-			default:
-				log.Printf("deleting expired file %q: %v", filename, err)
-			}
-			a.notFound(rw, r)
-			return
-		}
-	}
-
 	// Do things with attributes?
 	if a.redirectToBlobstore {
 		redirectExpiry := a.redirectExpiry
-		if !exp.IsZero() {
+		if !meta.ExpiresAt.IsZero() {
 			// Cap the redirect lifetime to the remaining lifetime of the object.
-			remainingLifetime := exp.Sub(time.Now())
+			remainingLifetime := meta.ExpiresAt.Sub(time.Now())
 			if remainingLifetime < redirectExpiry {
 				redirectExpiry = remainingLifetime
 			}
@@ -143,7 +115,7 @@ func (a *Application) rawDownload(rw http.ResponseWriter, r *http.Request) {
 		u, err := a.storageBackend.SignedURL(ctx, filename, &blob.SignedURLOptions{
 			Expiry: redirectExpiry,
 		})
-		switch gcerrors.Code(err) {
+		switch errorCode(err) {
 		case gcerrors.NotFound:
 			// This is unlikely to get returned.
 			a.notFound(rw, r)
@@ -159,13 +131,13 @@ func (a *Application) rawDownload(rw http.ResponseWriter, r *http.Request) {
 	}
 
 	// Perform direct serving.
-	a.rawDownloadDirect(ctx, rw, r, filename, attrs)
+	a.rawDownloadDirect(ctx, rw, r, filename, meta)
 }
 
-func (a *Application) rawDownloadDirect(ctx context.Context, rw http.ResponseWriter, r *http.Request, filename string, attrs *blob.Attributes) {
+func (a *Application) rawDownloadDirect(ctx context.Context, rw http.ResponseWriter, r *http.Request, filename string, meta *Metadata) {
 	// TODO(lukegb): Range header and conditionals?.
 	rdr, err := a.storageBackend.NewReader(ctx, filename, nil)
-	switch gcerrors.Code(err) {
+	switch errorCode(err) {
 	case gcerrors.NotFound:
 		a.notFound(rw, r)
 		return
@@ -177,6 +149,7 @@ func (a *Application) rawDownloadDirect(ctx context.Context, rw http.ResponseWri
 	}
 	defer rdr.Close()
 
+	attrs := meta.Attributes
 	if v := attrs.CacheControl; v != "" {
 		rw.Header().Set("Cache-Control", v)
 	}

web/fup/fuphttp/metadata.go

@@ -0,0 +1,65 @@
+package fuphttp
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"strconv"
+	"time"
+
+	"gocloud.dev/blob"
+	"gocloud.dev/gcerrors"
+)
+
+type Metadata struct {
+	// ExpiresAt is the expiry time of the object.
+	// May be the zero time, if this object does not expire.
+	// Stored in expires-at.
+	ExpiresAt time.Time
+
+	// Attributes is the underlying gocloud Attributes.
+	Attributes *blob.Attributes
+}
+
+// metadata retrieves the Metadata for the object.
+// Note: if the object is expired, it will delete it.
+func metadata(ctx context.Context, bucket *blob.Bucket, filename string) (*Metadata, error) {
+	attrs, err := bucket.Attributes(ctx, filename)
+	if err != nil {
+		return nil, fmt.Errorf("Attributes(%q): %w", filename, err)
+	}
+
+	m := Metadata{
+		Attributes: attrs,
+	}
+
+	var exp time.Time
+	if expStr, ok := attrs.Metadata["expires-at"]; ok {
+		// Check for expiry.
+		expSec, err := strconv.ParseInt(expStr, 10, 64)
+		if err != nil {
+			return nil, fmt.Errorf("parsing expiration %q for %q: %v", expStr, filename, err)
+		}
+
+		exp = time.Unix(expSec, 0)
+		if exp.Before(time.Now()) {
+			// This file has expired. Delete it from the backing storage.
+			err := bucket.Delete(ctx, filename)
+			switch errorCode(err) {
+			case gcerrors.NotFound:
+				// Something probably deleted it before we could get there.
+			case gcerrors.OK:
+				// This was fine.
+			default:
+				log.Printf("deleting expired file %q: %v", filename, err)
+			}
+			return nil, &fupError{
+				Code: gcerrors.NotFound,
+			}
+		}
+
+		m.ExpiresAt = exp
+	}
+
+	return &m, nil
+}