Readable stream locks proposal expanded

readable-stream-locks-1.md

abstract operation: ReadFromReadableStream(stream) contains most of what we have now.

ReadableStreamLock class

Has a [[stream]] internal slot.

Present as an intrinsic named %ReadableStreamLock%.

new ReadableStreamLock(stream)

• Brand-check that stream is a real ReadableStream.
• If stream@[[locked]] is true, throw TypeError
• Set this@[[stream]] to stream
• Set stream@[[locked]] to true

ReadableStreamLock.prototype.read()

• If this@[[stream]] is undefined, throw TypeError
• Return ReadFromReadableStream(this.[[stream]])

ReadableStreamLock.prototype.release()

• If this@[[stream]] is undefined, return.
• Set this@[[stream]]@[[locked]] to false.
• Set this@[[stream]] to undefined.

ReadableStream class modifications

new ReadableStream(...)

• Set this@[[locked]] to false.

ReadableStream.prototype.read()

• If this@[[locked]] is true, throw a TypeError.
• Return ReadFromReadableStream(this).

ReadableStream.prototype.pipeTo(...)

• Add var lock = new %ReadableStreamLock%(this)
• Use lock.read() throughout instead of this.read()
• One each branch that denotes a "finish" of the piping, i.e. each place that we fulfill or reject the return-value promise, call lock.release().
• May need some try { ... } finally { ... }s.

[optionally add] get ReadableStream.prototype.isLocked

• Return this@[[locked]].

readable-stream-locks-2.md

Static %ReadableStreamLocks% "weak set" exists.

ReadableStreamLock class

Has a [[stream]] internal slot.

Present as an intrinsic named %ReadableStreamLock%.

new ReadableStreamLock(stream)

• If %ReadableStreamLocks% has stream (the stream is already locked), throw TypeError
• Set this@[[stream]] to stream
• Add this@[[stream]] to %ReadableStreamLocks%

ReadableStreamLock.isLocked(stream)

• Return true if %ReadableStreamLocks% has stream; false otherwise

ReadableStreamLock.prototype.read()

• If this@[[stream]] is undefined, throw TypeError
• Remove this@[[stream]] from %ReadableStreamLocks%
• Execute the equivalent of:

try {
  return this@[[stream]].read();
} finally {
  // Add this@[[stream]] back to %ReadableStreamLocks%
}

ReadableStreamLock.prototype.release()

• If this@[[stream]] is undefined, return.
• Remove this@[[stream]] from %ReadableStreamLocks%.
• Set this@[[stream]] to undefined.

ReadableStream class modifications

ReadableStream.prototype.read()

• If %ReadableStreamLocks% has this, throw a TypeError.
• ... rest of the existing algorithm goes here.

ReadableStream.prototype.pipeTo(...)

• Add var lock = new %ReadableStreamLock%(this)
• Use lock.read() throughout instead of this.read()
• One each branch that denotes a "finish" of the piping, i.e. each place that we fulfill or reject the return-value promise, call lock.release().
• May need some try { ... } finally { ... }s.