Debugger stops on the wrong line if they share the source image and the same instruction

[rcjsuen]

github.com/docker/buildx v0.29.1-desktop.1 28f6246ff24e2c05095e8741e48c48dcb2d3b4bc

1. Put a breakpoint on line 4.
2. Start the debugger.
3. It stops on line 2. :(

FROM alpine:3.22 AS source
RUN echo "Hello world"
FROM alpine:3.22
RUN echo "Hello world"
COPY --from=source . .

1. setBreakpoints request and response looks good.
2. breakpoint event says the breakpoint is on line 4.
3. stackTrace response says we're stopped on line 2.

{
    "command": "setBreakpoints",
    "arguments": {
        "source": {
            "name": "Dockerfile",
            "path": "/tmp/buildx-debugging-demo/Dockerfile"
        },
        "lines": [
            4
        ],
        "breakpoints": [
            {
                "line": 4
            }
        ],
        "sourceModified": false
    },
    "type": "request",
    "seq": 3
}
{
    "seq": 4,
    "type": "response",
    "request_seq": 3,
    "success": true,
    "command": "setBreakpoints",
    "body": {
        "breakpoints": [
            {
                "id": 1,
                "verified": false,
                "line": 4,
                "endLine": 4
            }
        ]
    }
}
{
    "seq": 9,
    "type": "event",
    "event": "breakpoint",
    "body": {
        "reason": "changed",
        "breakpoint": {
            "id": 1,
            "verified": true,
            "line": 4,
            "endLine": 4
        }
    }
}
{
    "seq": 10,
    "type": "event",
    "event": "stopped",
    "body": {
        "reason": "breakpoint",
        "description": "Paused on breakpoint",
        "threadId": 1,
        "hitBreakpointIds": [
            1
        ]
    }
}
{
    "command": "threads",
    "type": "request",
    "seq": 6
}
{
    "seq": 11,
    "type": "response",
    "request_seq": 6,
    "success": true,
    "command": "threads",
    "body": {
        "threads": [
            {
                "id": 1,
                "name": "default"
            }
        ]
    }
}
{
    "command": "stackTrace",
    "arguments": {
        "threadId": 1,
        "startFrame": 0,
        "levels": 20
    },
    "type": "request",
    "seq": 7
}
{
    "seq": 12,
    "type": "response",
    "request_seq": 7,
    "success": true,
    "command": "stackTrace",
    "body": {
        "stackFrames": [
            {
                "id": 3,
                "name": "[source 2/2] RUN echo \"Hello world\"",
                "source": {
                    "name": "Dockerfile",
                    "path": "/tmp/buildx-debugging-demo/Dockerfile"
                },
                "line": 2,
                "column": 0,
                "endLine": 2
            }
        ]
    }
}

[jsternberg]

I can probably update the limitations documented here to point out this can also happen with non-source lines.

https://github.com/docker/buildx/blob/master/docs/dap.md#limitations

The issue happens whenever there's an identical digest. It's just that a digest also includes the digests of the parents and I didn't expect to see a lot of FROM a; RUN b be the exact same. In the case when this sequence is identical, we can't tell the difference during execution.