Make prompt debug creation concurrency safe

internal/promptdebug/debug_writer_test.go

@@ -1,10 +1,12 @@
 package promptdebug
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 	"runtime"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -91,6 +93,73 @@ func TestPromptDebugWriterAtomicallyReplacesArtifacts(t *testing.T) {
 	}
 }
 
+func TestPromptDebugWriterCreatesSharedMissingAncestorsConcurrently(t *testing.T) {
+	root := filepath.Join(t.TempDir(), "debug")
+	writer, err := NewPromptDebugWriter(root)
+	if err != nil {
+		t.Fatalf("NewPromptDebugWriter() error = %v", err)
+	}
+
+	const writerCount = 8
+	start := make(chan struct{})
+	type writeResult struct {
+		directory string
+		err       error
+	}
+	results := make(chan writeResult, writerCount)
+	var writers sync.WaitGroup
+	for index := 0; index < writerCount; index++ {
+		writers.Add(1)
+		go func(index int) {
+			defer writers.Done()
+			<-start
+			directory, err := writer.WritePreparation(PromptDebugRef{
+				ReportID: report.Daily, ValidDate: "2026-05-29", RunID: fmt.Sprintf("run-%02d", index),
+			}, promptDebugPreparationFixture(), nil)
+			results <- writeResult{directory: directory, err: err}
+		}(index)
+	}
+	close(start)
+
+	finished := make(chan struct{})
+	go func() {
+		writers.Wait()
+		close(finished)
+	}()
+	select {
+	case <-finished:
+	case <-time.After(5 * time.Second):
+		t.Fatal("concurrent prompt debug writes did not finish")
+	}
+	close(results)
+
+	directories := map[string]struct{}{}
+	for result := range results {
+		if result.err != nil {
+			t.Fatalf("WritePreparation() error = %v", result.err)
+		}
+		if _, duplicate := directories[result.directory]; duplicate {
+			t.Fatalf("duplicate debug directory %q", result.directory)
+		}
+		directories[result.directory] = struct{}{}
+		if _, err := os.Stat(filepath.Join(result.directory, "preparation.json")); err != nil {
+			t.Fatalf("preparation artifact %q: %v", result.directory, err)
+		}
+	}
+	if len(directories) != writerCount {
+		t.Fatalf("debug directories = %#v", directories)
+	}
+	if runtime.GOOS != "windows" {
+		assertPromptDebugMode(t, root, debugDirectoryMode)
+		assertPromptDebugMode(t, filepath.Join(root, "daily"), debugDirectoryMode)
+		assertPromptDebugMode(t, filepath.Join(root, "daily", "2026-05-29"), debugDirectoryMode)
+		for directory := range directories {
+			assertPromptDebugMode(t, directory, debugDirectoryMode)
+			assertPromptDebugMode(t, filepath.Join(directory, "preparation.json"), debugFileMode)
+		}
+	}
+}
+
 func TestPromptDebugWriterDisabledDoesNotAccessFilesystem(t *testing.T) {
 	writer, err := NewPromptDebugWriter("")
 	if err != nil {