cmd/tsconnect: prefetch main.wasm when serving

Avoids waterfalling of requests from the file (its load is triggered
from JavaScript).

Also has other cleanups to index.html, adding a <title> and moving the
<script> to being loaded sooner (but still not delaying page rendering
by using the defer attribute).

Fixes #5141
Fixes #5135

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

cmd/tsconnect/serve.go

@@ -83,10 +83,19 @@ func generateServeIndex(distFS fs.FS) ([]byte, error) {
 		return nil, fmt.Errorf("Could not parse esbuild-metadata.json: %w", err)
 	}
 	entryPointsToHashedDistPaths := make(map[string]string)
+	mainWasmPath := ""
 	for outputPath, output := range esbuildMetadata.Outputs {
 		if output.EntryPoint != "" {
 			entryPointsToHashedDistPaths[output.EntryPoint] = path.Join("dist", outputPath)
 		}
+		if path.Ext(outputPath) == ".wasm" {
+			for input := range output.Inputs {
+				if input == "src/main.wasm" {
+					mainWasmPath = path.Join("dist", outputPath)
+					break
+				}
+			}
+		}
 	}
 
 	indexBytes := rawIndexBytes
@@ -96,6 +105,10 @@ func generateServeIndex(distFS fs.FS) ([]byte, error) {
 			indexBytes = bytes.ReplaceAll(indexBytes, []byte(defaultDistPath), []byte(hashedDistPath))
 		}
 	}
+	if mainWasmPath != "" {
+		mainWasmPrefetch := fmt.Sprintf("</title>\n<link rel='preload' as='fetch' crossorigin='anonymous' href='%s'>", mainWasmPath)
+		indexBytes = bytes.ReplaceAll(indexBytes, []byte("</title>"), []byte(mainWasmPrefetch))
+	}
 
 	return indexBytes, nil
 }