feat: add statfile remote storage (#8443)

* feat: add statfile; add error for remote storage misses

* feat: statfile implementations for storage providers

* test: add unit tests for StatFile method across providers

Add comprehensive unit tests for the StatFile implementation covering:
- S3: interface compliance and error constant accessibility
- Azure: interface compliance, error constants, and field population
- GCS: interface compliance, error constants, error detection, and field population

Also fix variable shadowing issue in S3 and Azure StatFile implementations where
named return parameters were being shadowed by local variable declarations.

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix: address StatFile review feedback

- Use errors.New for ErrRemoteObjectNotFound sentinel
- Fix S3 HeadObject 404 detection to use awserr.Error code check
- Remove hollow field-population tests that tested nothing
- Remove redundant stdlib error detection tests
- Trim verbose doc comment on ErrRemoteObjectNotFound

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix: address second round of StatFile review feedback

- Rename interface assertion tests to TestXxxRemoteStorageClientImplementsInterface
- Delegate readFileRemoteEntry to StatFile in all three providers
- Revert S3 404 detection to RequestFailure.StatusCode() check
- Fix double-slash in GCS error message format string
- Add storage type prefix to S3 error message for consistency

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix: comments

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

weed/remote_storage/s3/s3_storage_client.go

@@ -3,9 +3,11 @@ package s3
 import (
 	"fmt"
 	"io"
+	"net/http"
 	"reflect"
 
 	"github.com/aws/aws-sdk-go/aws"
+	"github.com/aws/aws-sdk-go/aws/awserr"
 	"github.com/aws/aws-sdk-go/aws/credentials"
 	"github.com/aws/aws-sdk-go/aws/request"
 	"github.com/aws/aws-sdk-go/aws/session"
@@ -119,6 +121,33 @@ func (s *s3RemoteStorageClient) Traverse(remote *remote_pb.RemoteStorageLocation
 	}
 	return
 }
+
+func (s *s3RemoteStorageClient) StatFile(loc *remote_pb.RemoteStorageLocation) (remoteEntry *filer_pb.RemoteEntry, err error) {
+	resp, err := s.conn.HeadObject(&s3.HeadObjectInput{
+		Bucket: aws.String(loc.Bucket),
+		Key:    aws.String(loc.Path[1:]),
+	})
+	if err != nil {
+		if reqErr, ok := err.(awserr.RequestFailure); ok && reqErr.StatusCode() == http.StatusNotFound {
+			return nil, remote_storage.ErrRemoteObjectNotFound
+		}
+		return nil, fmt.Errorf("stat s3 %s%s: %w", loc.Bucket, loc.Path, err)
+	}
+	remoteEntry = &filer_pb.RemoteEntry{
+		StorageName: s.conf.Name,
+	}
+	if resp.ContentLength != nil {
+		remoteEntry.RemoteSize = *resp.ContentLength
+	}
+	if resp.LastModified != nil {
+		remoteEntry.RemoteMtime = resp.LastModified.Unix()
+	}
+	if resp.ETag != nil {
+		remoteEntry.RemoteETag = *resp.ETag
+	}
+	return remoteEntry, nil
+}
+
 func (s *s3RemoteStorageClient) ReadFile(loc *remote_pb.RemoteStorageLocation, offset int64, size int64) (data []byte, err error) {
 	downloader := s3manager.NewDownloaderWithClient(s.conn, func(u *s3manager.Downloader) {
 		u.PartSize = int64(4 * 1024 * 1024)
@@ -208,21 +237,7 @@ func toTagging(attributes map[string][]byte) *s3.Tagging {
 }
 
 func (s *s3RemoteStorageClient) readFileRemoteEntry(loc *remote_pb.RemoteStorageLocation) (*filer_pb.RemoteEntry, error) {
-	resp, err := s.conn.HeadObject(&s3.HeadObjectInput{
-		Bucket: aws.String(loc.Bucket),
-		Key:    aws.String(loc.Path[1:]),
-	})
-	if err != nil {
-		return nil, err
-	}
-
-	return &filer_pb.RemoteEntry{
-		RemoteMtime: resp.LastModified.Unix(),
-		RemoteSize:  *resp.ContentLength,
-		RemoteETag:  *resp.ETag,
-		StorageName: s.conf.Name,
-	}, nil
-
+	return s.StatFile(loc)
 }
 
 func (s *s3RemoteStorageClient) UpdateFileMetadata(loc *remote_pb.RemoteStorageLocation, oldEntry *filer_pb.Entry, newEntry *filer_pb.Entry) (err error) {

weed/remote_storage/s3/s3_storage_client_test.go

@@ -6,6 +6,7 @@ import (
 	"github.com/aws/aws-sdk-go/aws/credentials"
 	awss3 "github.com/aws/aws-sdk-go/service/s3"
 	"github.com/seaweedfs/seaweedfs/weed/pb/remote_pb"
+	"github.com/seaweedfs/seaweedfs/weed/remote_storage"
 	"github.com/stretchr/testify/require"
 )
 
@@ -55,3 +56,12 @@ func TestS3MakeUsesStaticCredentialsWhenKeysAreProvided(t *testing.T) {
 	require.Equal(t, conf.S3AccessKey, credValue.AccessKeyID)
 	require.Equal(t, conf.S3SecretKey, credValue.SecretAccessKey)
 }
+
+func TestS3RemoteStorageClientImplementsInterface(t *testing.T) {
+	var _ remote_storage.RemoteStorageClient = (*s3RemoteStorageClient)(nil)
+}
+
+func TestS3ErrRemoteObjectNotFoundIsAccessible(t *testing.T) {
+	require.Error(t, remote_storage.ErrRemoteObjectNotFound)
+	require.Equal(t, "remote object not found", remote_storage.ErrRemoteObjectNotFound.Error())
+}