feat: add configurable default ETag for files without metadata

When versitygw serves files that were placed on the filesystem outside
of the gateway, those files have no stored etag attribute and are
served with an empty ETag. This causes problems for S3 clients that
rely on ETags being non empty.

The new --default-etag flag (VGW_DEFAULT_ETAG) lets operators specify
a fallback ETag value returned for any object that lacks a stored etag
attribute, making pre-existing filesystem data behave more predictably
under S3 workloads without requiring a metadata migration.

cmd/versitygw/posix.go

@@ -34,6 +34,7 @@ var (
 	forceNoTmpFile       bool
 	forceNoCopyFileRange bool
 	actionsConcurrency   int
+	defaultEtag          string
 )
 
 func posixCommand() *cli.Command {
@@ -115,6 +116,12 @@ will be translated into the file /mnt/fs/gwroot/mybucket/a/b/c/myobject`,
 				EnvVars:     []string{"VGW_DISABLE_COPY_FILE_RANGE"},
 				Destination: &forceNoCopyFileRange,
 			},
+			&cli.StringFlag{
+				Name:        "default-etag",
+				Usage:       "default ETag value returned for objects that do not have a stored etag attribute (e.g. files placed on the filesystem outside of versitygw)",
+				EnvVars:     []string{"VGW_DEFAULT_ETAG"},
+				Destination: &defaultEtag,
+			},
 		},
 	}
 }
@@ -149,6 +156,7 @@ func runPosix(ctx *cli.Context) error {
 		ValidateBucketNames:  disableStrictBucketNames,
 		Concurrency:          actionsConcurrency,
 		CopyObjectThreshold:  copyObjectThreshold,
+		DefaultEtag:          defaultEtag,
 	}
 
 	var ms meta.MetadataStorer

cmd/versitygw/scoutfs.go

@@ -106,6 +106,12 @@ move interfaces as well as support for tiered filesystems.`,
 				Value:       5000,
 				Destination: &actionsConcurrency,
 			},
+			&cli.StringFlag{
+				Name:        "default-etag",
+				Usage:       "default ETag value returned for objects that do not have a stored etag attribute (e.g. files placed on the filesystem outside of versitygw)",
+				EnvVars:     []string{"VGW_DEFAULT_ETAG"},
+				Destination: &defaultEtag,
+			},
 		},
 	}
 }
@@ -135,6 +141,7 @@ func runScoutfs(ctx *cli.Context) error {
 	opts.SetProjectID = setProjectID
 	opts.Concurrency = actionsConcurrency
 	opts.CopyObjectThreshold = copyObjectThreshold
+	opts.DefaultEtag = defaultEtag
 
 	be, err := scoutfs.New(ctx.Args().Get(0), opts)
 	if err != nil {