metadata: reorganize and add consistency checks

This commit adds in IsValid() checks for the metadata structures that
let us enforce stronger invariants than those imposed by the protobuf
package. The main uses of this will be to check that metadata is valid
before writing it to the filesystem, and to check that the filesystem
contains valid metadata before returning it to the user. These functions
also will log the exact reason if the validity checks fail.

To have these checks in the metadata package, all of the various
constants have been moved to a single metadata/constants.go file. The
uses of these constants were changed accordingly.

Finally, this commit standardizes our use of errors so that they always
begin with an appropriate prefix.

Change-Id: I99008e2ee803ebe5f6236eb8d83fc83efcd22718

1 files changed, 34 insertions, 7 deletions

diff --git a/metadata/config.go b/metadata/config.go
index 47b6cce..8c7be55 100644
--- a/metadata/config.go
+++ b/metadata/config.go
@@ -19,7 +19,7 @@
  */
 
 // Package metadata contains all of the on disk structures.
-// These structures are definied in meatadata.proto. The package also
+// These structures are defined in metadata.proto. The package also
 // contains functions for manipulating these structures, specifically:
 //    * Reading and Writing the Config file to disk
 //    * Getting and Setting Policies for directories
@@ -27,21 +27,48 @@
 package metadata
 
 //go:generate protoc --go_out=. metadata.proto
-import "github.com/golang/protobuf/jsonpb"
+import (
+	"io"
+	"strings"
+
+	"github.com/golang/protobuf/jsonpb"
+)
 
 // WriteConfig outputs the Config data as nicely formatted JSON
-func WriteConfig(config *Config) (string, error) {
+func WriteConfig(config *Config, out io.Writer) error {
 	m := jsonpb.Marshaler{
-		EmitDefaults: false,
+		EmitDefaults: true,
 		EnumsAsInts:  false,
 		Indent:       "\t",
 		OrigName:     true,
 	}
-	return m.MarshalToString(config)
+	if err := m.Marshal(out, config); err != nil {
+		return err
+	}
+
+	_, err := out.Write([]byte{'\n'})
+	return err
 }
 
 // ReadConfig writes the JSON data into the config structure
-func ReadConfig(input string) (*Config, error) {
+func ReadConfig(in io.Reader) (*Config, error) {
 	config := new(Config)
-	return config, jsonpb.UnmarshalString(input, config)
+	// Allow (and ignore) unknown fields for forwards compatibility.
+	u := jsonpb.Unmarshaler{
+		AllowUnknownFields: true,
+	}
+	return config, u.Unmarshal(in, config)
+}
+
+// HasCompatibilityOption returns true if the specified string is in the list of
+// compatibility options. This assumes the compatibility options are in a comma
+// separated string.
+func (c *Config) HasCompatibilityOption(option string) bool {
+	options := strings.Split(c.Compatibility, ",")
+	for _, o := range options {
+		if o == option {
+			return true
+		}
+	}
+	return false
 }