Disjoint handling in transpiler fails to rejoin barriers

[jakelishman]

Environment

• Qiskit Terra version: main @ ce02f06
• Python version: 3.10
• Operating system: macOS

What is happening?

The preset passmangers for transpile are failing to correctly reconstruct barriers temporarily inserted during the compiler routing, which is leaking out invalid UUID objects inside the label field of those barriers.

How can we reproduce the issue?

Given a set up of:

from qiskit import transpile, QuantumCircuit
from qiskit.transpiler import CouplingMap

cm = CouplingMap([
    (0, 1),
    (1, 2),
    #
    (3, 4),
])
qc = QuantumCircuit(cm.size(), cm.size())
qc.cx(0, 1)
qc.cx(1, 2)
qc.cx(2, 0)
#
qc.cx(3, 4)
qc.measure(qc.qubits, qc.clbits)

out = transpile(qc, coupling_map=cm, seed_transpiler=0)

out contains Barrier instructions whose label attributes are not strings (they're UUID objects) and shouldn't exist at all, because the input circuit doesn't have any:

for i, ins in enumerate(out.data):
    if ins.operation.name == "barrier":
        print(i, repr(ins.operation.label))
        # Coerce to string so I can draw the circuit after.
        ins.operation.label = str(ins.operation.label)

8 UUID('31558989-c21a-4e5c-b014-01639295df7b')
10 UUID('31558989-c21a-4e5c-b014-01639295df7b')
12 UUID('31558989-c21a-4e5c-b014-01639295df7b')
14 UUID('31558989-c21a-4e5c-b014-01639295df7b')
15 UUID('31558989-c21a-4e5c-b014-01639295df7b')

I can't draw the circuit because the visualiser will explode, as will QPY (how this issue was detected). If I coerce those particular labels just to be string instances, the circuit looks like this:

                                                    ┌───┐                          ┌───┐                  31558989-c21a-4e5c-b014-01639295df7b ┌─┐
q_2 -> 0 ───────────────────────X───────────────────┤ H ├───────■──────────────────┤ H ├───────────────────────────────────░───────────────────┤M├───
                                │                   └───┘       │                  └───┘                                   ░                   └╥┘
                                │                   ┌───┐     ┌─┴─┐                ┌───┐                  31558989-c21a-4e5c-b014-01639295df7b  ║ ┌─┐
q_0 -> 1 ──■────────────────────X───────────────────┤ H ├──■──┤ X ├────────────────┤ H ├───────────────────────────────────░────────────────────╫─┤M├
           │                                        └───┘  │  └───┘                └───┘                                   ░                    ║ └╥┘
         ┌─┴─┐                ┌───┐                      ┌─┴─┐┌───┐ 31558989-c21a-4e5c-b014-01639295df7b                  ┌─┐                   ║  ║
q_1 -> 2 ┤ X ├────────────────┤ H ├──────────────────────┤ X ├┤ H ├──────────────────░────────────────────────────────────┤M├───────────────────╫──╫─
         └───┘                └───┘                      └───┘└───┘                  ░                                    └╥┘                   ║  ║
               31558989-c21a-4e5c-b014-01639295df7b  ┌─┐                                                                   ║                    ║  ║
q_3 -> 3 ──■────────────────────░────────────────────┤M├───────────────────────────────────────────────────────────────────╫────────────────────╫──╫─
           │                    ░                    └╥┘                                                                   ║                    ║  ║
         ┌─┴─┐ 31558989-c21a-4e5c-b014-01639295df7b   ║   ┌─┐                                                              ║                    ║  ║
q_4 -> 4 ┤ X ├──────────────────░─────────────────────╫───┤M├──────────────────────────────────────────────────────────────╫────────────────────╫──╫─
         └───┘                  ░                     ║   └╥┘                                                              ║                    ║  ║
    c: 5/═════════════════════════════════════════════╩════╩═══════════════════════════════════════════════════════════════╩════════════════════╩══╩═
                                                      3    4                                                               1                    0  2

so it's clear the temporary barrier we insert before measures isn't being recombined and then deleted.

What should happen?

• The temporary barrier should have been recombined after the disjoint handling
• The temporary barrier should have been removed before the circuit exited
• We probably shouldn't use a literal UUID in the Barrier.label field since that's supposed to be constrained to be a string (everything except Barrier seems to enforce that, too).

Any suggestions?

No response

[mtreinish]

So the root of the issue here is a shared state issue. When SabreLayout (or other layout passes too) calls qiskit.transpiler.passes.layout.disjoint_utils.run_pass_over_connected_components it passes the input dag directly and when there is >1 connected component the first thing as part of it does is split the barriers on the dag it receives in place to enable separating the circuit into it's connected components. This is modifying the input dag which when there is more > 1 connected component gets returned as is because we need to run routing separately. This is causing the barrier to leak out with the uuid label and that barrier isn't getting removed (from #10323) because the label changed.

There are two paths to fixing this, the first is just making a deepcopy of the dag so that there isn't a shared state when we split the barriers which will preserve the original input dag. The other option is to run disjoint_utils.combine_barriers(retain_uuid=False) before returning the input dag when there is more than one connected component to reverse the barrier splitting (this will also involve fixing the uuid piece to work with existing labels).

[jakelishman]

My feeling is that running the barrier combiner with a limit to ensure it considers only the barriers split by the previous step is the cleaner approach, and the more reliable from a performance standpoint.