solana Simulate Transaction doesn't return Token accounts with JSON encoding

Problem

Take this example simulateTransaction RPC call on devnet:

{
    "id": 1,
    "jsonrpc": "2.0",
    "method": "simulateTransaction",
    "params": [
        "AQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAAQHGThNI65OhkvOGV+DlO4hQdc0U6joPE4az8wnRhU/zZe0aDLGa0cy16lBWIf9ij+BU6eobQjtSBAP3ZRNOzSRTLxPOmj5LXZb8EVEBvpSsLG9P9kYCYqlowhFVOefwFCYAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAADpKDlVCWX/1NZKyq9G1F33MY5bT1fJDEh9YGJdgpuDewVKU1qZKSEGTSTocWDaOHx8NbXdvJK7geQfqEBBBUSNBt324ddloZPZy+FGzut5rBy0he1fWzeROoz1hX7/AKmOYFcfFz0Vu4XMA4jr8WltA1R3YRYSEipAl9FD4qy+FgMFAAY0RkhkVEwDAgAADAIAAAAAAAAAAAAAAAYEAgQBAAoMoIYBAAAAAAAG",
        {
            "commitment": "confirmed",
            "encoding": "base64",
            "sigVerify": false,
            "replaceRecentBlockhash": true,
            "accounts": {
                "addresses": [
                    "D9EYGf9etBGpPLHwiKMpxzn8gcHZapP6uvakAiCkdapK",
                    "Dg5iatJR92aPAZ6E9kBCrdb2DDeaPoQdexjC48MrvNGf"
                ],
                "encoding": "jsonParsed"
            }
        }
    ]
}

The two addresses are token accounts. This should return jsonParsed accounts, but it doesn't:

            "accounts": [
                {
                    "data": [
                        "6Sg5VQll/9TWSsqvRtRd9zGOW09XyQxIfWBiXYKbg3vbLzrNfN5YZjAai4m3oURtvycuDlQ8wTPq3YIlZUPQ9Egxxy0AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
                        "base64"
                    ],
                    "executable": false,
                    "lamports": 2039280,
                    "owner": "TokenkegQfeZyiNwAJbNbGKPFXCWuBvf9Ss623VQ5DA",
                    "rentEpoch": 18446744073709551615,
                    "space": 165
                },
                {
                    "data": [
                        "6Sg5VQll/9TWSsqvRtRd9zGOW09XyQxIfWBiXYKbg3sZOE0jrk6GS84ZX4OU7iFB1zRTqOg8ThrPzCdGFT/Nl2Ba9AUAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",
                        "base64"
                    ],
                    "executable": false,
                    "lamports": 2039280,
                    "owner": "TokenkegQfeZyiNwAJbNbGKPFXCWuBvf9Ss623VQ5DA",
                    "rentEpoch": 18446744073709551615,
                    "space": 165
                }
            ],

You can see both accounts are owned by the token program. A getAccountInfo call on either account with jsonParsed encoding works correctly:

            "data": {
                "parsed": {
                    "info": {
                        "isNative": false,
                        "mint": "Gh9ZwEmdLJ8DscKNTkTqPbNwLNNBjuSzaG9Vp2KGtKJr",
                        "owner": "Fkc4FN7PPhyGsAcHPW3dBBJ4BvtYkDr2rBFBgFpvy3nB",
                        "state": "initialized",
                        "tokenAmount": {
                            "amount": "767929000",
                            "decimals": 6,
                            "uiAmount": 767.929,
                            "uiAmountString": "767.929"
                        }
                    },
                    "type": "account"
                },
                "program": "spl-token",
                "space": 165
            },

Proposed Solution

The bug is caused by not passing AccountAdditionalData on this code path.

In encode_account we pass additional_data: None to UiAccount::encode (4th arg): https://github.com/solana-labs/solana/blob/30fa449a33af7fd59a2528c64f466df4df58f5a8/rpc/src/rpc.rs#L2320-L2322

This gets passed through to parse_account_data, which passes mint_decimals: None to parse_token: https://github.com/solana-labs/solana/blob/30fa449a33af7fd59a2528c64f466df4df58f5a8/account-decoder/src/parse_account_data.rs#L111-L113

This then errors when it's unable to read mint decimals: https://github.com/solana-labs/solana/blob/30fa449a33af7fd59a2528c64f466df4df58f5a8/account-decoder/src/parse_token.rs#L65-L69

And we fall back to returning base64 instead: https://github.com/solana-labs/solana/blob/30fa449a33af7fd59a2528c64f466df4df58f5a8/account-decoder/src/lib.rs#L142-L153

I haven't looked at how we do this for eg getAccountInfo, but assuming most of this code is shared we must be getting the mint decimals in AccountAdditionalData and passing that through. We need to do the same for simulateTransaction to enable returning JSON parsed token accounts.