main: use VERSION file as fallback for version reporting (#614)

Currently, if Alloy was built without setting build information, it
reports its version as "v0.0.0".

This commit adds using the VERSION file as fallback, so if the version
information wasn't present (such as when ussing `go run .`), the
reported version is taken from the VERSION file at the root of the
repository.

The version reported is then appended with `-devel` to indicate that it
is not the final release.

For example, as the VERSION file currently contains `v1.1.0`, then the
fallback reported version will be `v1.1.0-devel`.

fallback_version.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"bufio"
+	"bytes"
+	_ "embed"
+	"strings"
+
+	"github.com/grafana/alloy/internal/build"
+)
+
+//go:embed VERSION
+var fallbackVersionText []byte
+
+// fallbackVersion returns a version string to use for when the version isn't
+// explicitly set at build time. The version string will always have -devel
+// appended to it.
+func fallbackVersion() string {
+	return fallbackVersionFromText(fallbackVersionText)
+}
+
+func fallbackVersionFromText(text []byte) string {
+	// Find the first line in fallbackVersionText which isn't a blank line or a
+	// line starting with #.
+	scanner := bufio.NewScanner(bytes.NewReader(text))
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if len(line) == 0 || strings.HasPrefix(line, "#") {
+			continue
+		}
+
+		return line + "-devel"
+	}
+
+	// We shouldn't hit this case since we always control the contents of the
+	// VERSION file, but just in case we'll return the existing version.
+	return build.Version
+}

fallback_version_test.go

@@ -0,0 +1,20 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func Test_fallbackVersionFromText(t *testing.T) {
+	in := `# This is a comment 
+# This is another comment 
+
+v1.2.3
+
+This line is ignored!`
+	expect := "v1.2.3-devel"
+
+	actual := fallbackVersionFromText([]byte(in))
+	require.Equal(t, expect, actual)
+}

main.go

@@ -18,6 +18,12 @@ import (
 )
 
 func init() {
+	// If the build version wasn't set by the build process, we'll set it based
+	// on the version string in VERSION.
+	if build.Version == "" || build.Version == "v0.0.0" {
+		build.Version = fallbackVersion()
+	}
+
 	prometheus.MustRegister(build.NewCollector("alloy"))
 }