Cache runtime version metadata fallback

internal/runtimeinfo/performance_test.go

@@ -18,7 +18,19 @@ func BenchmarkPerformanceVersion(b *testing.B) {
 			}
 		}
 	})
+	b.Run("FallbackInitialization", func(b *testing.B) {
+		b.ReportAllocs()
+		b.ResetTimer()
+		for i := 0; i < b.N; i++ {
+			fallback := newFallbackVersion(debug.ReadBuildInfo)
+			performanceVersionSink = fallback()
+			if performanceVersionSink == "" {
+				b.Fatal("fallback returned an empty version")
+			}
+		}
+	})
 	b.Run("RealMetadataFallback", func(b *testing.B) {
+		_ = Version()
 		b.ReportAllocs()
 		b.ResetTimer()
 		for i := 0; i < b.N; i++ {

internal/runtimeinfo/version.go

@@ -2,14 +2,26 @@
 // public buildinfo facade.
 package runtimeinfo
 
-import "runtime/debug"
+import (
+	"runtime/debug"
+	"sync"
+)
 
 var injectedVersion string
 
 // Version returns the injected version or the running executable's main module
 // version, falling back to "dev" when neither provides a release identifier.
 func Version() string {
-	return version(injectedVersion, debug.ReadBuildInfo)
+	if injectedVersion != "" {
+		return injectedVersion
+	}
+	return fallbackVersion()
+}
+
+var fallbackVersion = newFallbackVersion(debug.ReadBuildInfo)
+
+func newFallbackVersion(readBuildInfo func() (*debug.BuildInfo, bool)) func() string {
+	return sync.OnceValue(func() string { return version("", readBuildInfo) })
 }
 
 func version(injected string, readBuildInfo func() (*debug.BuildInfo, bool)) string {

internal/runtimeinfo/version_test.go

@@ -2,6 +2,8 @@ package runtimeinfo
 
 import (
 	"runtime/debug"
+	"sync"
+	"sync/atomic"
 	"testing"
 )
 
@@ -55,3 +57,54 @@ func TestVersion(t *testing.T) {
 		})
 	}
 }
+
+func TestFallbackVersionReadsMetadataOnceConcurrently(t *testing.T) {
+	var reads atomic.Int32
+	fallback := newFallbackVersion(func() (*debug.BuildInfo, bool) {
+		reads.Add(1)
+		return &debug.BuildInfo{Main: debug.Module{Version: "v4.5.6"}}, true
+	})
+
+	const callers = 32
+	results := make(chan string, callers)
+	var workers sync.WaitGroup
+	workers.Add(callers)
+	for i := 0; i < callers; i++ {
+		go func() {
+			defer workers.Done()
+			results <- fallback()
+		}()
+	}
+	workers.Wait()
+	close(results)
+	for result := range results {
+		if result != "v4.5.6" {
+			t.Errorf("fallback() = %q, want stable version", result)
+		}
+	}
+	if got := reads.Load(); got != 1 {
+		t.Fatalf("metadata reader called %d times, want once", got)
+	}
+}
+
+func TestFallbackVersionCachesUnavailableAndDevelopmentMetadata(t *testing.T) {
+	for _, test := range []struct {
+		name string
+		read func() (*debug.BuildInfo, bool)
+	}{
+		{name: "unavailable", read: func() (*debug.BuildInfo, bool) { return nil, false }},
+		{name: "development", read: func() (*debug.BuildInfo, bool) {
+			return &debug.BuildInfo{Main: debug.Module{Version: "(devel)"}}, true
+		}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			fallback := newFallbackVersion(test.read)
+			if got := fallback(); got != "dev" {
+				t.Fatalf("fallback() = %q, want dev", got)
+			}
+			if got := fallback(); got != "dev" {
+				t.Fatalf("repeated fallback() = %q, want dev", got)
+			}
+		})
+	}
+}