fix(02): write fixtures exclusively and only on successful flush (WR-01)

Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/summer/parity.go

@@ -21,6 +21,7 @@ func parityProxyCommand() bonfire.Command {
 			{Name: "rules", Description: "Committed YAML capture rules"},
 			{Name: "vars", Description: "Private mode-0600 variable store outside fixtures"},
 			{Name: "fixtures", Description: "Directory for nuxt/ and mcp/ session flows"},
+			{Name: "update", Description: "Overwrite existing session fixtures (true/false)"},
 		},
 		Run: runParityProxy,
 	}
@@ -88,6 +89,7 @@ func runParityProxy(ctx context.Context, in bonfire.Input, out bonfire.Output) e
 		Rules:    rules,
 		VarsPath: varsPath,
 		Fixtures: fixtures,
+		Update:   flagBool(in, "update"),
 	})
 	if err != nil {
 		return err

tide/fixture.go

@@ -49,6 +49,15 @@ func ParseFlow(raw []byte) (Flow, error) {
 
 // SaveFlow writes a validated flow atomically, syncing before rename.
 func SaveFlow(path string, flow Flow) error {
+	return saveFlow(path, flow, false)
+}
+
+// SaveFlowExclusive writes a validated flow and fails if path already exists.
+func SaveFlowExclusive(path string, flow Flow) error {
+	return saveFlow(path, flow, true)
+}
+
+func saveFlow(path string, flow Flow, exclusive bool) error {
 	if err := validateFlow(flow); err != nil {
 		return err
 	}
@@ -62,6 +71,30 @@ func SaveFlow(path string, flow Flow) error {
 			return fmt.Errorf("tide: create fixture dir: %w", err)
 		}
 	}
+	if exclusive {
+		f, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o644)
+		if err != nil {
+			return fmt.Errorf("tide: create fixture: %w", err)
+		}
+		ok := false
+		defer func() {
+			_ = f.Close()
+			if !ok {
+				_ = os.Remove(path)
+			}
+		}()
+		if _, err := f.Write(raw); err != nil {
+			return fmt.Errorf("tide: write fixture: %w", err)
+		}
+		if err := f.Sync(); err != nil {
+			return fmt.Errorf("tide: sync fixture: %w", err)
+		}
+		if err := f.Close(); err != nil {
+			return fmt.Errorf("tide: close fixture: %w", err)
+		}
+		ok = true
+		return nil
+	}
 	tmp, err := os.CreateTemp(dir, ".tide-*.tmp")
 	if err != nil {
 		return fmt.Errorf("tide: create temp fixture: %w", err)

tide/manifest.go

@@ -336,7 +336,10 @@ func recordSeed(ctx context.Context, seed *Seed, cfg ManifestConfig) error {
 	if err != nil {
 		return err
 	}
-	return SaveFlow(dest, flow)
+	if _, err := os.Stat(dest); err == nil {
+		return SaveFlow(dest, flow)
+	}
+	return SaveFlowExclusive(dest, flow)
 }
 
 func seedAlreadyRecorded(path string) (bool, error) {
@@ -404,7 +407,13 @@ func recordRoute(ctx context.Context, route Route, cfg ManifestConfig) error {
 		if err != nil {
 			return err
 		}
-		if err := SaveFlow(dest, flow); err != nil {
+		if cfg.Update {
+			if err := SaveFlow(dest, flow); err != nil {
+				return err
+			}
+			continue
+		}
+		if err := SaveFlowExclusive(dest, flow); err != nil {
 			return err
 		}
 	}

tide/proxy.go

@@ -48,6 +48,7 @@ type ProxyConfig struct {
 	VarsPath  string
 	Fixtures  string
 	MaxBody   int64
+	Update    bool
 }
 
 // Proxy is a fixed-upstream recording reverse proxy.
@@ -282,7 +283,7 @@ func (p *Proxy) recordStep(state *captureState, resp *http.Response, respBody []
 		return err
 	}
 	buf.steps = append(buf.steps, step)
-	return p.writeSessionLocked(buf)
+	return nil
 }
 
 func (p *Proxy) sessionLocked(name string) (*sessionBuf, error) {
@@ -290,8 +291,10 @@ func (p *Proxy) sessionLocked(name string) (*sessionBuf, error) {
 		return buf, nil
 	}
 	dest := p.sessionPath(name)
-	if _, err := os.Stat(dest); err == nil {
-		return nil, fmt.Errorf("tide: duplicate session %q", name)
+	if !p.cfg.Update {
+		if _, err := os.Stat(dest); err == nil {
+			return nil, fmt.Errorf("tide: duplicate session %q", name)
+		}
 	}
 	buf := &sessionBuf{name: name}
 	p.sessions[name] = buf
@@ -310,7 +313,11 @@ func (p *Proxy) writeSessionLocked(buf *sessionBuf) error {
 		Name:    buf.name,
 		Steps:   append([]Step(nil), buf.steps...),
 	}
-	return SaveFlow(p.sessionPath(buf.name), flow)
+	dest := p.sessionPath(buf.name)
+	if p.cfg.Update {
+		return SaveFlow(dest, flow)
+	}
+	return SaveFlowExclusive(dest, flow)
 }
 
 func (p *Proxy) sessionPath(name string) string {
@@ -323,9 +330,7 @@ func (p *Proxy) failSession(name string, err error) {
 	if _, ok := p.failed[name]; !ok {
 		p.failed[name] = err
 	}
-	if buf := p.sessions[name]; buf != nil && len(buf.steps) == 0 {
-		_ = os.Remove(p.sessionPath(name))
-	}
+	_ = os.Remove(p.sessionPath(name))
 }
 
 func validateSessionName(name string) error {

tide/proxy_test.go

@@ -305,6 +305,40 @@ func TestProxyDuplicateSessionName(t *testing.T) {
 	}
 }
 
+func TestProxyFailedSessionLeavesNoFixture(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		if r.URL.Path == "/ok" {
+			_, _ = w.Write([]byte(`{"ok":true}`))
+			return
+		}
+		_, _ = w.Write([]byte(`{"token":"` + testJWT + `"}`))
+	}))
+	t.Cleanup(upstream.Close)
+	fixtures := t.TempDir()
+	proxy := newTestProxy(t, upstream.URL, fixtures, testRulesYAML())
+	srv := httptest.NewServer(proxy.Handler())
+	t.Cleanup(srv.Close)
+
+	doProxy(t, srv.URL, "partial", "/ok", "")
+	req, err := http.NewRequest(http.MethodGet, srv.URL+"/leak", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	req.Header.Set(SessionHeader, "partial")
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	resp.Body.Close()
+	if err := proxy.Flush(); err == nil {
+		t.Fatal("failed session flush must surface the capture error")
+	}
+	if _, err := os.Stat(filepath.Join(fixtures, "nuxt", "partial.yaml")); !os.IsNotExist(err) {
+		t.Fatalf("partial session fixture committed: %v", err)
+	}
+}
+
 func newTestProxy(t *testing.T, upstream, fixtures, rulesYAML string) *Proxy {
 	t.Helper()
 	p, err := NewProxy(ProxyConfig{