fs: read full size if known in promises.readFile

If we have an estimate of the file size available from the previous
stat call, use that for the size of the first chunk to be read.
This increases performance by reading more data (and, most likely,
all data) at once without incurring memory overhead in most situations.

PR-URL: #37127
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: addaleax

lib/internal/fs/promises.js

@@ -315,20 +315,21 @@ async function readFileHandle(filehandle, options) {
     throw new ERR_FS_FILE_TOO_LARGE(size);
 
   const chunks = [];
-  const chunkSize = size === 0 ?
-    kReadFileMaxChunkSize :
-    MathMin(size, kReadFileMaxChunkSize);
+  let isFirstChunk = true;
+  const firstChunkSize = size === 0 ? kReadFileMaxChunkSize : size;
+  const chunkSize = MathMin(firstChunkSize, kReadFileMaxChunkSize);
   let endOfFile = false;
   do {
     if (signal?.aborted) {
       throw lazyDOMException('The operation was aborted', 'AbortError');
     }
-    const buf = Buffer.alloc(chunkSize);
+    const buf = Buffer.alloc(isFirstChunk ? firstChunkSize : chunkSize);
     const { bytesRead, buffer } =
-      await read(filehandle, buf, 0, chunkSize, -1);
+      await read(filehandle, buf, 0, buf.length, -1);
     endOfFile = bytesRead === 0;
     if (bytesRead > 0)
       ArrayPrototypePush(chunks, buffer.slice(0, bytesRead));
+    isFirstChunk = false;
   } while (!endOfFile);
 
   const result = chunks.length === 1 ? chunks[0] : Buffer.concat(chunks);