Use fsync in reskindex to ensure file is written to disk

This should (hopefully) resolve occasional errors where the rename step would
fail because the temporary file did not exist. In addition, this also exits with
an error code if something goes wrong so we notice it early, rather than having
to scroll through pages of logs at release time.

scripts/reskindex.js

@@ -1,5 +1,6 @@
 #!/usr/bin/env node
 const fs = require('fs');
+const { promises: fsp } = fs;
 const path = require('path');
 const glob = require('glob');
 const util = require('util');
@@ -25,6 +26,8 @@ async function reskindex() {
     const header = args.h || args.header;
 
     const strm = fs.createWriteStream(componentIndexTmp);
+    // Wait for the open event to ensure the file descriptor is set
+    await new Promise(resolve => strm.once("open", resolve));
 
     if (header) {
        strm.write(fs.readFileSync(header));
@@ -53,14 +56,9 @@ async function reskindex() {
 
     strm.write("export {components};\n");
     // Ensure the file has been fully written to disk before proceeding
+    await util.promisify(fs.fsync)(strm.fd);
     await util.promisify(strm.end);
-    fs.rename(componentIndexTmp, componentIndex, function(err) {
-        if (err) {
-            console.error("Error moving new index into place: " + err);
-        } else {
-            console.log('Reskindex: completed');
-        }
-    });
+    await fsp.rename(componentIndexTmp, componentIndex);
 }
 
 // Expects both arrays of file names to be sorted
@@ -77,9 +75,17 @@ function filesHaveChanged(files, prevFiles) {
     return false;
 }
 
+// Wrapper since await at the top level is not well supported yet
+function run() {
+    (async function() {
+        await reskindex();
+        console.log("Reskindex completed");
+    })();
+}
+
 // -w indicates watch mode where any FS events will trigger reskindex
 if (!args.w) {
-    reskindex();
+    run();
     return;
 }
 
@@ -87,5 +93,5 @@ let watchDebouncer = null;
 chokidar.watch(path.join(componentsDir, componentJsGlob)).on('all', (event, path) => {
     if (path === componentIndex) return;
     if (watchDebouncer) clearTimeout(watchDebouncer);
-    watchDebouncer = setTimeout(reskindex, 1000);
+    watchDebouncer = setTimeout(run, 1000);
 });