fs: remove custom Buffer pool for streams

The performance benefit of using a custom pool are negligable.
Furthermore, it causes problems with Workers and transferrable.
Rather than further adding complexity for compat with Workers,
just remove the pooling logic.

Refs: nodejs#33880 (comment)
Fixes: nodejs#31733

lib/internal/fs/streams.js

@@ -26,29 +26,8 @@ const { toPathIfFileURL } = require('internal/url');
 const kIoDone = Symbol('kIoDone');
 const kIsPerformingIO = Symbol('kIsPerformingIO');
 
-const kMinPoolSpace = 128;
 const kFs = Symbol('kFs');
 
-let pool;
-// It can happen that we expect to read a large chunk of data, and reserve
-// a large chunk of the pool accordingly, but the read() call only filled
-// a portion of it. If a concurrently executing read() then uses the same pool,
-// the "reserved" portion cannot be used, so we allow it to be re-used as a
-// new pool later.
-const poolFragments = [];
-
-function allocNewPool(poolSize) {
-  if (poolFragments.length > 0)
-    pool = poolFragments.pop();
-  else
-    pool = Buffer.allocUnsafe(poolSize);
-  pool.used = 0;
-}
-
-function roundUpToMultipleOf8(n) {
-  return (n + 7) & ~7;  // Align to 8 byte boundary.
-}
-
 function _construct(callback) {
   const stream = this;
   if (typeof stream.fd === 'number') {
@@ -188,70 +167,51 @@ ReadStream.prototype.open = openReadFs;
 ReadStream.prototype._construct = _construct;
 
 ReadStream.prototype._read = function(n) {
-  if (!pool || pool.length - pool.used < kMinPoolSpace) {
-    // Discard the old pool.
-    allocNewPool(this.readableHighWaterMark);
-  }
+  n = this.pos !== undefined ?
+    MathMin(this.end - this.pos + 1, n) :
+    MathMin(this.end - this.bytesRead + 1, n);
 
-  // Grab another reference to the pool in the case that while we're
-  // in the thread pool another read() finishes up the pool, and
-  // allocates a new one.
-  const thisPool = pool;
-  let toRead = MathMin(pool.length - pool.used, n);
-  const start = pool.used;
-
-  if (this.pos !== undefined)
-    toRead = MathMin(this.end - this.pos + 1, toRead);
-  else
-    toRead = MathMin(this.end - this.bytesRead + 1, toRead);
+  if (n <= 0) {
+    this.push(null);
+    return;
+  }
 
-  // Already read everything we were supposed to read!
-  // treat as EOF.
-  if (toRead <= 0)
-    return this.push(null);
+  const buf = Buffer.allocUnsafeSlow(n);
 
-  // the actual read.
   this[kIsPerformingIO] = true;
   this[kFs]
-    .read(this.fd, pool, pool.used, toRead, this.pos, (er, bytesRead) => {
+    .read(this.fd, buf, 0, n, this.pos, (er, bytesRead, buf) => {
       this[kIsPerformingIO] = false;
+
       // Tell ._destroy() that it's safe to close the fd now.
-      if (this.destroyed) return this.emit(kIoDone, er);
+      if (this.destroyed) {
+        this.emit(kIoDone, er);
+        return;
+      }
 
       if (er) {
         errorOrDestroy(this, er);
-      } else {
-        let b = null;
-        // Now that we know how much data we have actually read, re-wind the
-        // 'used' field if we can, and otherwise allow the remainder of our
-        // reservation to be used as a new pool later.
-        if (start + toRead === thisPool.used && thisPool === pool) {
-          const newUsed = thisPool.used + bytesRead - toRead;
-          thisPool.used = roundUpToMultipleOf8(newUsed);
-        } else {
-          // Round down to the next lowest multiple of 8 to ensure the new pool
-          // fragment start and end positions are aligned to an 8 byte boundary.
-          const alignedEnd = (start + toRead) & ~7;
-          const alignedStart = roundUpToMultipleOf8(start + bytesRead);
-          if (alignedEnd - alignedStart >= kMinPoolSpace) {
-            poolFragments.push(thisPool.slice(alignedStart, alignedEnd));
-          }
-        }
-
-        if (bytesRead > 0) {
-          this.bytesRead += bytesRead;
-          b = thisPool.slice(start, start + bytesRead);
+      } else if (bytesRead > 0) {
+        this.bytesRead += bytesRead;
+
+        if (bytesRead !== buf.length) {
+          // Slow path. Shrink to fit.
+          // Copy instead of slice so that we don't retain
+          // large backing buffer for small reads.
+          const dst = Buffer.allocUnsafeSlow(bytesRead);
+          buf.copy(dst, 0, 0, bytesRead);
+          buf = dst;
         }
 
-        this.push(b);
+        this.push(buf);
+      } else {
+        this.push(null);
       }
     });
 
-  // Move the pool positions, and internal position for reading.
-  if (this.pos !== undefined)
-    this.pos += toRead;
-
-  pool.used = roundUpToMultipleOf8(pool.used + toRead);
+  if (this.pos !== undefined) {
+    this.pos += n;
+  }
 };
 
 ReadStream.prototype._destroy = function(err, cb) {

test/known_issues/test-crypto-authenticated-stream.js → test/parallel/test-crypto-authenticated-stream.js

@@ -121,7 +121,6 @@ function test(config) {
 
 tmpdir.refresh();
 
-// OK
 test({
   cipher: 'aes-128-ccm',
   aad: Buffer.alloc(1),
@@ -131,7 +130,6 @@ test({
   plaintextLength: 32768,
 });
 
-// Fails the fstream test.
 test({
   cipher: 'aes-128-ccm',
   aad: Buffer.alloc(1),