Register circuit fails when injecting input a second time

[ttoktassynov]

Bug Description

When injecting input to the register only circuit a second time in a row, after first successful input, we receive following error of Quantum Register:

(node:87745) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:87745) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
(node:87745) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'push' of undefined
    at ClassicalRegisterNode._inputCallback (/Users/ttoktassynov/projects/nodejs_projects/node-red-contrib-quantum/nodes/quantum/classical-register/classical-register.js:45:19)
    at /Users/ttoktassynov/projects/nodejs_projects/node-red-contrib-quantum/node_modules/@node-red/runtime/lib/nodes/Node.js:203:26
    at Object.trigger (/Users/ttoktassynov/projects/nodejs_projects/node-red-contrib-quantum/node_modules/@node-red/runtime/lib/hooks.js:113:9)
    at ClassicalRegisterNode.Node._emitInput (/Users/ttoktassynov/projects/nodejs_projects/node-red-contrib-quantum/node_modules/@node-red/runtime/lib/nodes/Node.js:195:11)
    at ClassicalRegisterNode.Node.emit (/Users/ttoktassynov/projects/nodejs_projects/node-red-contrib-quantum/node_modules/@node-red/runtime/lib/nodes/Node.js:179:25)
    at ClassicalRegisterNode.Node.receive (/Users/ttoktassynov/projects/nodejs_projects/node-red-contrib-quantum/node_modules/@node-red/runtime/lib/nodes/Node.js:476:10)
    at Immediate.<anonymous> (/Users/ttoktassynov/projects/nodejs_projects/node-red-contrib-quantum/node_modules/@node-red/runtime/lib/flows/Flow.js:679:52)
    at processImmediate (internal/timers.js:461:21)
(node:87745) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2)

Most likely it has to do with state management functionanility.

Steps to Reproduce Bug

Draw this circuit:

or simply import this json

Expected Behaviour & Actual Behaviour

Additional Information

Log file can be found here.

System Information

• Node.js Version: v12.22.1

• Operating System:Darwin tma.local 19.6.0 Darwin Kernel Version 19.6.0: Sun Jul 5 00:43:10 PDT 2020; root:xnu-6153.141.1~9/RELEASE_X86_64 x86_64

[louislefevre]

Seems to be caused by the registerArr variable in quantum-register.js. On first execution, its initial value is []. On second execution, its initial value is undefined.

Also, it's worth noting that this behaviour happens in the same loop:

• First execution is successful.
• Second execution fails.
• Third execution is successful.
• Fourth execution fails.

[louislefevre]

Okay so I think I figured out the cause. This is the flow of execution:

• quantum-circuit node is loaded, initialising registers to an empty array in the global state.
• The flow is run for the first time, and at some point state.del('registers') is executed, setting registers to undefined. The flow still finishes executing successfully.
• The flow is run for the second time, BUT because quantum-circuit has not been re-loaded (therefore initialising registers to an empty array again), registers is still left as undefined from the previous run.
• Due to registers being undefined, an error occurs in Node-RED. Node-RED will automatically restart everything when this happens, which in turn causes quantum-circuit nodes to be re-loaded, thereby initialising registers to an empty array again. So the reason every other execution works is because Node-RED is automatically restarting everything behind the scenes.

A fix I've just quickly tested that seems to work is to replace all instances of state.del('registers') with state.setPersistent('registers', []). However, I'm not sure if this will have any unforeseen consequences, since registers is no longer removed completely like it was before.

[ttoktassynov]

Maybe try to implement in different branch and see if tests pass?

[louislefevre]

Maybe try to implement in different branch and see if tests pass?

The tests still pass locally, but since we don't have tests for all scenarios I'm still unsure as I don't know the reason for deleting 'registers' from the state in the first place.