fix: use global singleton for Sass transpiler to prevent CI errors

Fixes #90

Changes the Sass transpiler from per-Manager lazy initialization to a
global singleton with lazy initialization. This prevents race conditions
and resource issues when Sites are reloaded during watch mode or when
multiple Manager instances are created.

Root Cause:
- Each renderers.Manager previously had its own sassTranspiler field
- When watch mode detects changes requiring a full reload (e.g., _config.yml
  changes), a new Site instance is created with a new Manager
- The old Manager's transpiler was never closed, leading to abandoned
  Dart Sass child processes
- In CI environments, this caused "connection is shut down" and
  "unexpected EOF" errors when the stdin/stdout streams were closed
  unexpectedly during process shutdown/startup overlap

Solution:
- Move to a package-level global singleton transpiler with lazy init
- Aligns with godartsass recommendation: "create one and use that for
  all the SCSS processing needed"
- The transpiler is thread-safe and stateless (include paths are passed
  to Execute()), so a single instance can safely serve all Managers

Testing:
- Added site/testdata/site1/assets/css/main.scss for regression testing
- Updated site/drop_test.go to account for new test file
- All tests pass, linting passes
- Verified SCSS compilation works correctly

Author: osteele

renderers/renderers.go

@@ -14,6 +14,16 @@ import (
 	"github.com/osteele/liquid"
 )
 
+// Global Sass transpiler singleton, shared across all Manager instances.
+// This avoids race conditions and resource leaks when Sites are reloaded during watch mode.
+// The transpiler is thread-safe and stateless (include paths are passed to Execute()),
+// so a single instance can safely serve all Managers throughout the process lifetime.
+var (
+	globalSassTranspiler     *sass.Transpiler
+	globalSassTranspilerOnce sync.Once
+	globalSassTranspilerErr  error
+)
+
 // Renderers applies transformations to a document.
 type Renderers interface {
 	ApplyLayout(string, []byte, liquid.Bindings) ([]byte, error)
@@ -24,13 +34,10 @@ type Renderers interface {
 // Manager applies a rendering transformation to a file.
 type Manager struct {
 	Options
-	cfg            config.Config
-	liquidEngine   *liquid.Engine
-	sassTempDir    string
-	sassHash       string
-	sassTranspiler *sass.Transpiler
-	sassInitOnce   sync.Once
-	sassInitErr    error
+	cfg          config.Config
+	liquidEngine *liquid.Engine
+	sassTempDir  string
+	sassHash     string
 }
 
 // Options configures a rendering manager.
@@ -175,12 +182,12 @@ func (p *Manager) makeLiquidEngine() *liquid.Engine {
 	return engine
 }
 
-// getSassTranspiler returns the SASS transpiler, initializing it if necessary.
-// This uses lazy initialization to avoid creating the transpiler at package load time,
-// which can cause "connection is shut down" errors in CI/CD environments.
+// getSassTranspiler returns the global SASS transpiler singleton, initializing it if necessary.
+// Using a global singleton avoids race conditions when Sites are reloaded during watch mode,
+// and matches the godartsass recommendation to "create one and use that for all SCSS processing."
 func (p *Manager) getSassTranspiler() (*sass.Transpiler, error) {
-	p.sassInitOnce.Do(func() {
-		p.sassTranspiler, p.sassInitErr = sass.Start(sass.Options{})
+	globalSassTranspilerOnce.Do(func() {
+		globalSassTranspiler, globalSassTranspilerErr = sass.Start(sass.Options{})
 	})
-	return p.sassTranspiler, p.sassInitErr
+	return globalSassTranspiler, globalSassTranspilerErr
 }

site/drop_test.go

@@ -38,7 +38,7 @@ func TestSite_ToLiquid_pages(t *testing.T) {
 	drop := readTestSiteDrop(t)
 	ps, ok := drop["pages"]
 	require.True(t, ok, fmt.Sprintf("pages has type %T", drop["pages"]))
-	require.Len(t, ps, 2)
+	require.Len(t, ps, 3) // includes main.scss for SCSS transpiler testing
 
 	ps, ok = drop["html_pages"]
 	require.True(t, ok, fmt.Sprintf("pages has type %T", drop["pages"]))

site/testdata/site1/assets/css/main.scss

@@ -0,0 +1,17 @@
+---
+---
+/* Test SCSS file for issue #90 regression testing */
+body {
+  background-color: #f0f0f0;
+  color: #333;
+}
+
+.container {
+  max-width: 1200px;
+  margin: 0 auto;
+
+  .header {
+    font-size: 2em;
+    font-weight: bold;
+  }
+}