Want Map object output

[richardkiene]

Currently mdb_v8 shows the following when inspecting a core dump containing a Map object:

<unknown JavaScript object type "JSMap">

Ideally mdb_v8 will output the contents of the Map object so that it's keys and values are available for debugging.

I encountered this issue using mdb_v8 version 1.1.3 and a core dump from NodeJS v4.6.0.

[arekinath]

FWIW you can dump out the guts of a Map like this today:

    "cache": fd411eb9: {
        "admin_ips": a0a22965: <unknown JavaScript object type "JSMap">,
        "owners": a0a22975: <unknown JavaScript object type "JSMap">,
        "vms": a0a22985: <unknown JavaScript object type "JSMap">,
    },
> a0a22975::v8print
a0a22975 JSMap {
    a0a22975 JSCollection {
        a0a22975 JSObject {
            a0a22975 JSReceiver {
                a0a22975 HeapObject < Object  {
                    a0a22974 map = 99409db9 (Map)
                }
            }
            a0a22978 properties = ad108099 (FixedArray)
            a0a2297c elements = ad108099 (FixedArray)
        }
        a0a22980 table = 93860a6d (FixedArray)
    }
}

Take that value of the table field:

> 93860a6d::v8array | ::jsprint
8: 4
a: 5
0: 0
fffffffe: 2147483647
8: 4
6: 3
fffffffe: 2147483647
938f6221: "8a70155f-752f-eb91-a755-f4edbee5c99f"
93849041: {
    "vms": 9385fa89: <unknown JavaScript object type "JSMap">,
}
fffffffe: 2147483647
938f69b1: "a11af3cb-1549-c860-e29e-c3d4e1e6f4b2"
93849051: {
    "vms": 9385fa99: <unknown JavaScript object type "JSMap">,
}
fffffffe: 2147483647
938f78f1: "50f60f7d-8c7d-ec4e-f4ba-978ca56ee281"
93849061: {
    "vms": 9385faa9: <unknown JavaScript object type "JSMap">,
}
0: 0
938f7ac9: "08339468-4f60-6ba2-cfe2-b2ba71bfd7c4"
93849071: {
    "vms": 9385fab9: <unknown JavaScript object type "JSMap">,
}
4: 2
938f815d: "930896af-bf8c-48d4-885c-6573a94b1853"
93849081: {
    "vms": 9385fac9: <unknown JavaScript object type "JSMap">,
}
2: 1
ad1080dd: undefined
ad1080dd: undefined
ad1080dd: undefined
ad1080dd: undefined                   
ad1080dd: undefined
ad1080dd: undefined
ad1080dd: undefined
ad1080dd: undefined
ad1080dd: undefined

The format of the Map table data in the V8Array is found in v8/src/collection.js and v8/src/macros.py. The first three array indices (0,1,2) are ORDERED_HASH_TABLE_BUCKET_COUNT, ORDERED_HASH_TABLE_ELEMENT_COUNT and ORDERED_HASH_TABLE_DELETED_COUNT. From 3 onwards are the actual contents of the Map -- first the primary values in each hash bucket, then followed by a complex scheme of "chains" for subsequent values in each bucket. I haven't completely untangled how to read the chains yet, but you can see the first value in each bucket easily (and if ELEMENT_COUNT < BUCKET_COUNT then you're done anyway)

It does usually look like the contents of the chains are in there at indices as plain pointers, so using ::jsprint on the entire V8Array should get you all the values, and it looks like the keys are always in the index before each value.