don't unconditionally write record_replay_driver.{h,cc} - only write …

…them if they've changed. drops local mac arm build (on no local change, with no driver change) from 45s to 15s

build.js

@@ -29,11 +29,6 @@ spawnChecked("git", ["config", "--global", "--add", "safe.directory", __dirname]
   stdio: "inherit",
 });
 
-if (currentPlatform() == "macOS") {
-  // Make sure the main executable gets rebuilt with the new build ID.
-  spawnChecked("touch", [`${__dirname}/chrome/app/chrome_exe_main_mac.cc`]);
-}
-
 const archSuffix = buildArm ? "-arm" : "";
 
 if (!REPLAY_LOCAL_DRIVER_DIR) {
@@ -97,7 +92,7 @@ const buildSuffix =
     : process.env["LOCAL_DEVELOPER_BUILD_EXTENSION"] || "";
 const buildId = `${computeBuildId(driverDate, driverRevision)}${buildSuffix}`;
 
-fs.writeFileSync(
+writeFileSyncIfChanged(
   `${__dirname}/base/record_replay_driver.cc`,
   `
 namespace recordreplay {
@@ -108,7 +103,7 @@ namespace recordreplay {
 `
 );
 
-fs.writeFileSync(
+const drived_h_changed = writeFileSyncIfChanged(
   `${__dirname}/base/record_replay_driver.h`,
   `
 #ifndef BASE_RECORD_REPLAY_DRIVER_H_
@@ -120,6 +115,11 @@ fs.writeFileSync(
 `
 );
 
+if (currentPlatform() == "macOS" && drived_h_changed) {
+  // Make sure the main executable gets rebuilt with the new build ID.
+  spawnChecked("touch", [`${__dirname}/chrome/app/chrome_exe_main_mac.cc`]);
+}
+
 // ensure that build configuration is written with correct paths
 const gn = currentPlatform() == "windows" ? "gn.bat" : "gn";
 spawnChecked(gn, ["gen", outdir], { stdio: "inherit" });
@@ -152,6 +152,24 @@ spawnChecked(
 
 console.log(`Build finished.`);
 
+function writeFileSyncIfChanged(filename, newContents) {
+  let changed = false;
+  try {
+    const oldContents = fs.readFileSync(filename, "utf8");
+    changed = oldContents != newContents;
+  } catch (e) {
+    changed = true;
+  }
+
+  if (!changed) {
+    console.log(`Skipping ${filename} because it hasn't changed.`);
+  } else {
+    fs.writeFileSync(filename, newContents);
+  }
+
+  return changed;
+}
+
 function spawnChecked(cmd, args, options) {
   const prettyCmd = [cmd].concat(args).join(" ");
   console.error("$ " + prettyCmd);