internal/blueprints: fix mountpoint size keyword

Due to an oversight, the toml and json tags for the `MinSize`
field had different keywords. This commit fixes this by creating
a `minsize` toml tag and ensuring backwards compatability by
checking the old `size` tag.

If both `minsize` & `size` are set in the toml block, the
custom unmarshal function validates the input for inconsistencies.

internal/blueprint/filesystem_customizations.go

@@ -9,7 +9,12 @@ import (
 
 type FilesystemCustomization struct {
 	Mountpoint string `json:"mountpoint,omitempty" toml:"mountpoint,omitempty"`
-	MinSize    uint64 `json:"minsize,omitempty" toml:"size,omitempty"`
+	MinSize    uint64 `json:"minsize,omitempty" toml:"minsize,omitempty"`
+
+	// Note: The TOML `size` tag has been deprecated in favor of `minsize`.
+	// we check for it in the TOML unmarshaler and use it as `minsize`.
+	// However due to the TOML marshaler implementation, we can omit adding
+	// a field for this tag and get the benifit of not having to export it.
 }
 
 func (fsc *FilesystemCustomization) UnmarshalTOML(data interface{}) error {
@@ -22,19 +27,60 @@ func (fsc *FilesystemCustomization) UnmarshalTOML(data interface{}) error {
 		return fmt.Errorf("TOML unmarshal: mountpoint must be string, got %v of type %T", d["mountpoint"], d["mountpoint"])
 	}
 
+	var size uint64
+	var minsize uint64
+
+	// `size` is an alias for `minsize. We check for the `size` keyword
+	// for backwards compatibility. We don't export a `Size` field as
+	// we would like to discourage its use.
 	switch d["size"].(type) {
 	case int64:
-		fsc.MinSize = uint64(d["size"].(int64))
+		size = uint64(d["size"].(int64))
 	case string:
-		size, err := common.DataSizeToUint64(d["size"].(string))
+		s, err := common.DataSizeToUint64(d["size"].(string))
 		if err != nil {
 			return fmt.Errorf("TOML unmarshal: size is not valid filesystem size (%w)", err)
 		}
-		fsc.MinSize = size
+		size = s
+	case nil:
+		size = 0
 	default:
 		return fmt.Errorf("TOML unmarshal: size must be integer or string, got %v of type %T", d["size"], d["size"])
 	}
 
+	switch d["minsize"].(type) {
+	case int64:
+		minsize = uint64(d["minsize"].(int64))
+	case string:
+		s, err := common.DataSizeToUint64(d["minsize"].(string))
+		if err != nil {
+			return fmt.Errorf("TOML unmarshal: minsize is not valid filesystem size (%w)", err)
+		}
+		minsize = s
+	case nil:
+		minsize = 0
+	default:
+		return fmt.Errorf("TOML unmarshal: minsize must be integer or string, got %v of type %T", d["minsize"], d["minsize"])
+	}
+
+	if size == 0 && minsize == 0 {
+		return fmt.Errorf("TOML unmarshal: minsize must be greater than 0, got %v", minsize)
+	}
+
+	if size > 0 && minsize == 0 {
+		fsc.MinSize = size
+		return nil
+	}
+
+	if size == 0 && minsize > 0 {
+		fsc.MinSize = minsize
+		return nil
+	}
+
+	if size > 0 && minsize > 0 {
+		return fmt.Errorf("TOML unmarshal: size and minsize cannot both be set (size is an alias for minsize)")
+	}
+
 	return nil
 }