Fix config validation edge cases

internal/core/diagnostics/run_dir.go

@@ -12,7 +12,14 @@ import (
 	"gitea.maximumdirect.net/eric/notarius/internal/framework/contracts"
 )
 
-const defaultWorkDir = "/tmp/notarius"
+const (
+	defaultWorkDir                = "/tmp/notarius"
+	maxRunDirectoryCreateAttempts = 16
+)
+
+var utcNow = func() time.Time {
+	return time.Now().UTC()
+}
 
 // RunDirectory represents a per-run diagnostics directory.
 type RunDirectory struct {
@@ -77,18 +84,27 @@ func NewRunDirectory(workDir string, retention RetentionMode) (*RunDirectory, er
 		return nil, fmt.Errorf("create diagnostics work directory %q: %w", workDir, err)
 	}
 
-	createdAt := time.Now().UTC()
-	runID := fmt.Sprintf("run-%d", createdAt.UnixNano())
-	runPath := filepath.Join(workDir, runID)
-	if err := os.Mkdir(runPath, 0o755); err != nil {
-		return nil, fmt.Errorf("create diagnostics run directory %q: %w", runPath, err)
+	var lastRunPath string
+	for attempt := 0; attempt < maxRunDirectoryCreateAttempts; attempt++ {
+		createdAt := utcNow()
+		runID := fmt.Sprintf("run-%d", createdAt.UnixNano())
+		runPath := filepath.Join(workDir, runID)
+		lastRunPath = runPath
+		if err := os.Mkdir(runPath, 0o755); err != nil {
+			if os.IsExist(err) {
+				continue
+			}
+			return nil, fmt.Errorf("create diagnostics run directory %q: %w", runPath, err)
+		}
+
+		return &RunDirectory{
+			path:      runPath,
+			retention: retention,
+			createdAt: createdAt,
+		}, nil
 	}
 
-	return &RunDirectory{
-		path:      runPath,
-		retention: retention,
-		createdAt: createdAt,
-	}, nil
+	return nil, fmt.Errorf("create diagnostics run directory %q: exhausted unique run ID attempts", lastRunPath)
 }
 
 func (r *RunDirectory) Path() string {