Heap snapshots aren't viewable in vscode viewer

[IanButterworth]

vscode now has a built in heapsnapshot viewer that seems more powerful than the chrome devtools one.

Unfortunately julia-generated heapsnapshot files do not open and report Error: invalid profile JSON, whilst they open in devtools.

File sizes:

• from a chrome webpage: 5 MB (renders instantly)
• from a fresh julia session: 175 MB (hangs indefinitely and reports Error: invalid profile JSON)

2023-12-30 19:27:38.621 [info] ExtensionService#_doActivateExtension ms-vscode.vscode-js-profile-table, startup: false, activationEvent: 'onCustomEditor:jsProfileVisualizer.heapsnapshot.table'
2023-12-30 19:27:38.772 [error] UnhandledPromiseRejection: 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(). The promise rejected with the reason "invalid profile JSON".

Asked upstream at microsoft/vscode#199239 (comment) with a file attached.

Both files validate in jsonlint.com

Headers:
julia's

{
    "snapshot": {
        "meta": {
            "node_fields": [
                "type",
                "name",
                "id",
                "self_size",
                "edge_count",
                "trace_node_id",
                "detachedness"
            ],
            "node_types": [
                [
                    "synthetic",
                    "jl_task_t",
                    "jl_module_t",
                    "jl_array_t",
                    "object",
                    "jl_datatype_t",
                    "String",
                    "jl_sym_t",
                    "jl_svec_t"
                ],
                "string",
                "number",
                "number",
                "number",
                "number",
                "number"
            ],
            "edge_fields": [
                "type",
                "name_or_index",
                "to_node"
            ],
            "edge_types": [
                [
                    "internal",
                    "element",
                    "hidden",
                    "property"
                ],
                "string_or_number",
                "from_node"
            ]
        },
        "node_count": 911769,
        "edge_count": 3024013
    },

Chrome's

{
    "snapshot": {
        "meta": {
            "node_fields": [
                "type",
                "name",
                "id",
                "self_size",
                "edge_count",
                "trace_node_id",
                "detachedness"
            ],
            "node_types": [
                [
                    "hidden",
                    "array",
                    "string",
                    "object",
                    "code",
                    "closure",
                    "regexp",
                    "number",
                    "native",
                    "synthetic",
                    "concatenated string",
                    "sliced string",
                    "symbol",
                    "bigint",
                    "object shape"
                ],
                "string",
                "number",
                "number",
                "number",
                "number",
                "number"
            ],
            "edge_fields": [
                "type",
                "name_or_index",
                "to_node"
            ],
            "edge_types": [
                [
                    "context",
                    "element",
                    "property",
                    "internal",
                    "hidden",
                    "shortcut",
                    "weak"
                ],
                "string_or_number",
                "node"
            ],
            "trace_function_info_fields": [
                "function_id",
                "name",
                "script_name",
                "script_id",
                "line",
                "column"
            ],
            "trace_node_fields": [
                "id",
                "function_info_index",
                "count",
                "size",
                "children"
            ],
            "sample_fields": [
                "timestamp_us",
                "last_assigned_id"
            ],
            "location_fields": [
                "object_index",
                "script_id",
                "line",
                "column"
            ]
        },
        "node_count": 58856,
        "edge_count": 245569,
        "trace_function_count": 0
    },

cc. @NHDaly

[IanButterworth]

@vilterp from your work, do you have any idea why this happens?

[vilterp]

I don't immediately know 😕 Seems like we'd have to find the source of the vscode viewer and look through it til we find what it doesn't like…

[vilterp]

Chased it over to this repo and it looks like they wrote the .heapsnapshot parser in Rust, lol: https://github.com/microsoft/vscode-v8-heap-tools/blob/main/v8-heap-parser/src/decoder.rs

It does have a test/ directory with an example heap snapshot; we could put ours alongside it and debug from there.

[IanButterworth]

Great. I think part of the problem may be their extension suppressing the actual error. If we can figure out how to use the rust decoder directly it might just give a better error?

[vilterp]

Indeed, ran their tests on our file and got

'Error("missing field `trace_function_info_fields`", line: 1, column: 434)', v8-heap-parser/src/decoder.rs:574:33

Looks like we'd need to match these fields exactly for the rust parser to work as-is: https://github.com/microsoft/vscode-v8-heap-tools/blob/c5b34396392397925ecbb4ecb904a27a2754f2c1/v8-heap-parser/src/decoder.rs#L48-L50

[IanButterworth]

Are they easy to drop in and leave empty? I think the vscode viewer has a graph viewer that might be interesting for your other work

[vilterp]

Yes, I think we should be able to drop them in and leave them empty.

What does the graph view look like? When I use it in my vscode I only see a table.

[IanButterworth]

Great. There's a screenshot here

https://code.visualstudio.com/updates/v1_85#_visualize-heap-snapshots