plugin/discovery: SHA256() method to get the hash of each plugin

This will be used later to verify that installed plugins match what's
available on the releases server.

plugin/discovery/meta.go

@@ -1,6 +1,10 @@
 package discovery
 
 import (
+	"crypto/sha256"
+	"io"
+	"os"
+
 	"github.com/blang/semver"
 )
 
@@ -26,3 +30,21 @@ type PluginMeta struct {
 func (m PluginMeta) VersionObj() (semver.Version, error) {
 	return semver.Make(m.Version)
 }
+
+// SHA256 returns a SHA256 hash of the content of the referenced executable
+// file, or an error if the file's contents cannot be read.
+func (m PluginMeta) SHA256() ([]byte, error) {
+	f, err := os.Open(m.Path)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	h := sha256.New()
+	_, err = io.Copy(h, f)
+	if err != nil {
+		return nil, err
+	}
+
+	return h.Sum(nil), nil
+}

plugin/discovery/meta_test.go

@@ -0,0 +1,22 @@
+package discovery
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestMetaSHA256(t *testing.T) {
+	m := PluginMeta{
+		Path: "test-fixtures/current-style-plugins/mockos_mockarch/terraform-foo-bar-V0.0.1",
+	}
+	hash, err := m.SHA256()
+	if err != nil {
+		t.Fatalf("failed: %s", err)
+	}
+
+	got := fmt.Sprintf("%x", hash)
+	want := "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855" // (hash of empty file)
+	if got != want {
+		t.Errorf("incorrect hash %s; want %s", got, want)
+	}
+}