Cache runtime version metadata fallback

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)
+			}
+		})
+	}
+}