Add build version reporting

internal/buildinfo/buildinfo.go

@@ -0,0 +1,36 @@
+// Package buildinfo resolves the product version embedded in a Notarius build.
+package buildinfo
+
+import (
+	"fmt"
+	"regexp"
+	"runtime/debug"
+)
+
+var stableVersion = regexp.MustCompile(`^v(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)$`)
+
+// Override is set at link time for controlled builds.
+var Override string
+
+// Version returns the release version embedded in the build, or development
+// when the build does not carry a stable release tag.
+func Version() (string, error) {
+	buildVersion := ""
+	if info, ok := debug.ReadBuildInfo(); ok {
+		buildVersion = info.Main.Version
+	}
+	return resolve(Override, buildVersion)
+}
+
+func resolve(override, buildVersion string) (string, error) {
+	if override != "" {
+		if !stableVersion.MatchString(override) {
+			return "", fmt.Errorf("build version override is not a stable release tag")
+		}
+		return override, nil
+	}
+	if stableVersion.MatchString(buildVersion) {
+		return buildVersion, nil
+	}
+	return "development", nil
+}