stream: improve Readable legacy compat

The popular stream-shift library accesses internal Readable
state which has been modified.

Refs: googleapis/nodejs-storage#2391
Refs: mafintosh/stream-shift#10
PR-URL: nodejs#51470

lib/internal/streams/readable.js

@@ -30,6 +30,7 @@ const {
   ObjectKeys,
   ObjectSetPrototypeOf,
   Promise,
+  Proxy,
   SafeSet,
   Symbol,
   SymbolAsyncDispose,
@@ -102,6 +103,8 @@ const kErroredValue = Symbol('kErroredValue');
 const kDefaultEncodingValue = Symbol('kDefaultEncodingValue');
 const kDecoderValue = Symbol('kDecoderValue');
 const kEncodingValue = Symbol('kEncodingValue');
+const kBuffer = Symbol('kBuffer');
+const kBufferIndex = Symbol('kBufferIndex');
 
 const kEnded = 1 << 9;
 const kEndEmitted = 1 << 10;
@@ -276,8 +279,8 @@ function ReadableState(options, stream, isDuplex) {
     getHighWaterMark(this, options, 'readableHighWaterMark', isDuplex) :
     getDefaultHighWaterMark(false);
 
-  this.buffer = [];
-  this.bufferIndex = 0;
+  this[kBuffer] = [];
+  this[kBufferIndex] = 0;
   this.length = 0;
   this.pipes = [];
 
@@ -561,13 +564,13 @@ function addChunk(stream, state, chunk, addToFront) {
     // Update the buffer info.
     state.length += (state[kState] & kObjectMode) !== 0 ? 1 : chunk.length;
     if (addToFront) {
-      if (state.bufferIndex > 0) {
-        state.buffer[--state.bufferIndex] = chunk;
+      if (state[kBufferIndex] > 0) {
+        state[kBuffer][--state[kBufferIndex]] = chunk;
       } else {
-        state.buffer.unshift(chunk); // Slow path
+        state[kBuffer].unshift(chunk); // Slow path
       }
     } else {
-      state.buffer.push(chunk);
+      state[kBuffer].push(chunk);
     }
 
     if ((state[kState] & kNeedReadable) !== 0)
@@ -592,14 +595,14 @@ Readable.prototype.setEncoding = function(enc) {
 
   // Iterate over current buffer to convert already stored Buffers:
   let content = '';
-  for (const data of state.buffer.slice(state.bufferIndex)) {
+  for (const data of state[kBuffer].slice(state[kBufferIndex])) {
     content += decoder.write(data);
   }
-  state.buffer.length = 0;
-  state.bufferIndex = 0;
+  state[kBuffer].length = 0;
+  state[kBufferIndex] = 0;
 
   if (content !== '')
-    state.buffer.push(content);
+    state[kBuffer].push(content);
   state.length = content.length;
   return this;
 };
@@ -633,7 +636,7 @@ function howMuchToRead(n, state) {
   if (NumberIsNaN(n)) {
     // Only flow one buffer at a time.
     if ((state[kState] & kFlowing) !== 0 && state.length)
-      return state.buffer[state.bufferIndex].length;
+      return state[kBuffer][state[kBufferIndex]].length;
     return state.length;
   }
   if (n <= state.length)
@@ -790,7 +793,7 @@ function onEofChunk(stream, state) {
   if (decoder) {
     const chunk = decoder.end();
     if (chunk && chunk.length) {
-      state.buffer.push(chunk);
+      state[kBuffer].push(chunk);
       state.length += (state[kState] & kObjectMode) !== 0 ? 1 : chunk.length;
     }
   }
@@ -1459,7 +1462,7 @@ ObjectDefineProperties(Readable.prototype, {
     __proto__: null,
     enumerable: false,
     get: function() {
-      return this._readableState && this._readableState.buffer;
+      return this._readableState && this._readableState[kBuffer];
     },
   },
 
@@ -1541,9 +1544,15 @@ ObjectDefineProperties(Readable.prototype, {
       return this._readableState ? this._readableState.endEmitted : false;
     },
   },
-
 });
 
+function normalizeBuffer(state) {
+  if (state[kBufferIndex] > 0) {
+    state.splice(0, state[kBufferIndex]);
+    state[kBufferIndex] = 0;
+  }
+}
+
 ObjectDefineProperties(ReadableState.prototype, {
   // Legacy getter for `pipesCount`.
   pipesCount: {
@@ -1568,6 +1577,36 @@ ObjectDefineProperties(ReadableState.prototype, {
       }
     },
   },
+
+  // Legacy compat
+  buffer: {
+    __proto__: null,
+    get() {
+      return new Proxy(this._readableState[kBuffer], {
+        get(target, name) {
+          if (name === 'length') {
+            return target[kBuffer].length - target[kBufferIndex];
+          }
+          if (name === '0') {
+            return target[kBuffer][target[kBufferIndex]];
+          }
+          normalizeBuffer(target);
+          return target[name];
+        },
+        set(target, name, value, receiver) {
+          normalizeBuffer(target);
+          target[name] = value;
+          return true;
+        }
+      })
+    },
+  },
+  bufferIndex: {
+    __proto__: null,
+    get() {
+      return 0;
+    }
+  }
 });
 
 // Exposed for testing purposes only.
@@ -1582,10 +1621,10 @@ function fromList(n, state) {
   if (state.length === 0)
     return null;
 
-  let idx = state.bufferIndex;
+  let idx = state[kBufferIndex];
   let ret;
 
-  const buf = state.buffer;
+  const buf = state[kBuffer];
   const len = buf.length;
 
   if ((state[kState] & kObjectMode) !== 0) {
@@ -1665,13 +1704,13 @@ function fromList(n, state) {
   }
 
   if (idx === len) {
-    state.buffer.length = 0;
-    state.bufferIndex = 0;
+    state[kBuffer].length = 0;
+    state[kBufferIndex] = 0;
   } else if (idx > 1024) {
-    state.buffer.splice(0, idx);
-    state.bufferIndex = 0;
+    state[kBuffer].splice(0, idx);
+    state[kBufferIndex] = 0;
   } else {
-    state.bufferIndex = idx;
+    state[kBufferIndex] = idx;
   }
 
   return ret;