fs: Add safe locking semantics for format.json (#4523)

This patch also reverts previous changes which were
merged for migration to the newer disk format. We will
be bringing these changes in subsequent releases. But
we wish to add protection in this release such that
future release migrations are protected.

Revert "fs: Migration should handle bucketConfigs as regular objects. (#4482)"
This reverts commit 976870a391.

Revert "fs: Migrate object metadata to objects directory. (#4195)"
This reverts commit 76f4f20609.

cmd/format-config-v1.go

@@ -33,6 +33,15 @@ type fsFormat struct {
 	Version string `json:"version"`
 }
 
+// FS format version strings.
+const (
+	// Represents the current backend disk structure
+	// version under `.minio.sys` and actual data namespace.
+
+	// formatConfigV1.fsFormat.Version
+	fsFormatBackendV1 = "1"
+)
+
 // xlFormat - structure holding 'xl' format.
 type xlFormat struct {
 	Version string `json:"version"` // Version of 'xl' format.
@@ -42,6 +51,15 @@ type xlFormat struct {
 	JBOD []string `json:"jbod"`
 }
 
+// XL format version strings.
+const (
+	// Represents the current backend disk structure
+	// version under `.minio.sys` and actual data namespace.
+
+	// formatConfigV1.xlFormat.Version
+	xlFormatBackendV1 = "1"
+)
+
 // formatConfigV1 - structure holds format config version '1'.
 type formatConfigV1 struct {
 	Version string `json:"version"` // Version of the format config.
@@ -51,6 +69,68 @@ type formatConfigV1 struct {
 	XL     *xlFormat `json:"xl,omitempty"` // XL field holds xl format.
 }
 
+// Format json file.
+const (
+	// Format config file carries backend format specific details.
+	formatConfigFile = "format.json"
+
+	// Format config tmp file carries backend format.
+	formatConfigFileTmp = "format.json.tmp"
+)
+
+// `format.json` version value.
+const (
+	// formatConfigV1.Version represents the version string
+	// of the current structure and its fields in `format.json`.
+	formatFileV1 = "1"
+
+	// Future `format.json` structure changes should have
+	// its own version and should be subsequently listed here.
+)
+
+// Constitutes `format.json` backend name.
+const (
+	// Represents FS backend.
+	formatBackendFS = "fs"
+
+	// Represents XL backend.
+	formatBackendXL = "xl"
+)
+
+// CheckFS if the format is FS and is valid with right values
+// returns appropriate errors otherwise.
+func (f *formatConfigV1) CheckFS() error {
+	// Validate if format config version is v1.
+	if f.Version != formatFileV1 {
+		return fmt.Errorf("Unknown format file version '%s'", f.Version)
+	}
+
+	// Validate if we have the expected format.
+	if f.Format != formatBackendFS {
+		return fmt.Errorf("FS backend format required. Found '%s'", f.Format)
+	}
+
+	// Check if format is currently supported.
+	if f.FS.Version != fsFormatBackendV1 {
+		return fmt.Errorf("Unknown backend FS format version '%s'", f.FS.Version)
+	}
+
+	// Success.
+	return nil
+}
+
+// LoadFormat - loads format config v1, returns `errUnformattedDisk`
+// if reading format.json fails with io.EOF.
+func (f *formatConfigV1) LoadFormat(lk *lock.LockedFile) error {
+	_, err := f.ReadFrom(lk)
+	if errorCause(err) == io.EOF {
+		// No data on disk `format.json` still empty
+		// treat it as unformatted disk.
+		return traceError(errUnformattedDisk)
+	}
+	return err
+}
+
 func (f *formatConfigV1) WriteTo(lk *lock.LockedFile) (n int64, err error) {
 	// Serialize to prepare to write to disk.
 	var fbytes []byte
@@ -88,6 +168,21 @@ func (f *formatConfigV1) ReadFrom(lk *lock.LockedFile) (n int64, err error) {
 	return int64(len(fbytes)), nil
 }
 
+func newFSFormat() (format *formatConfigV1) {
+	return newFSFormatV1()
+}
+
+// newFSFormatV1 - initializes new formatConfigV1 with FS format info.
+func newFSFormatV1() (format *formatConfigV1) {
+	return &formatConfigV1{
+		Version: formatFileV1,
+		Format:  formatBackendFS,
+		FS: &fsFormat{
+			Version: fsFormatBackendV1,
+		},
+	}
+}
+
 /*
 
 All disks online
@@ -811,10 +906,10 @@ func loadFormatXL(bootstrapDisks []StorageAPI, readQuorum int) (disks []StorageA
 
 func checkFormatXLValue(formatXL *formatConfigV1) error {
 	// Validate format version and format type.
-	if formatXL.Version != "1" {
+	if formatXL.Version != formatFileV1 {
 		return fmt.Errorf("Unsupported version of backend format [%s] found", formatXL.Version)
 	}
-	if formatXL.Format != "xl" {
+	if formatXL.Format != formatBackendXL {
 		return fmt.Errorf("Unsupported backend format [%s] found", formatXL.Format)
 	}
 	if formatXL.XL.Version != "1" {
@@ -916,10 +1011,10 @@ func initFormatXL(storageDisks []StorageAPI) (err error) {
 		}
 		// Allocate format config.
 		formats[index] = &formatConfigV1{
-			Version: "1",
-			Format:  "xl",
+			Version: formatFileV1,
+			Format:  formatBackendXL,
 			XL: &xlFormat{
-				Version: "1",
+				Version: xlFormatBackendV1,
 				Disk:    mustGetUUID(),
 			},
 		}