perf(gatsby-plugin-mdx): Stop clobbering the same file over and over again (#27974)

* perf(gatsby-plugin-mdx): Stop clobbering the same file over and over again

* Undo the await part, can be separate PR

Co-authored-by: gatsbybot <[email protected]>

Author: pvdz

packages/gatsby-plugin-mdx/gatsby/on-create-node.js

@@ -157,6 +157,8 @@ async function onCreateNodeLessBabel(
   })
 }
 
+const writeCache = new Set()
+
 async function cacheScope({
   cache,
   scopeImports,
@@ -169,6 +171,16 @@ async function cacheScope({
 
 export default { ${scopeIdentifiers.join(`, `)} }`
 
+  // Multiple files sharing the same imports/exports will lead to the same file writes.
+  // Prevent writing the same content to the same file over and over again (reduces io pressure).
+  // This also prevents an expensive babel step whose outcome is based on this same value
+  if (writeCache.has(scopeFileContent)) {
+    return
+  }
+
+  // Make sure other calls see this value being processed during async time
+  writeCache.add(scopeFileContent)
+
   // if parent node is a file, convert relative imports to be
   // relative to new .cache location
   if (parentNode.internal.type === `File`) {