cmd/evm: t8n support native tracers

[jsvisa]

t8n is a useful tool to test state transition. Currently, we use t8n to analyze some exploit transactions, such as SaddleFinance Exploiter, a simple workflow as below:

1. save tx's prestate into alloc.json;
2. save tx's block metadata into env.json;
3. save tx's rawtrasaction into txs.json;
4. now apply the tx: ./evm t8n --trace ...;
5. from the outputted alloc and traces to find some useful patterns.

But currently t8n only supports JSONLogger traces, we want to extend it to support all traces under eth/tracers

[holiman]

Interesting, I'll look into this.
Related: are you using the prestateTracer to dump the prestate, or something else?

[jsvisa]

Interesting, I'll look into this.

Related: are you using the prestateTracer to dump the prestate, or something else?

sorry for the late reply. Yes, I first use prestateTracer to dump the states(via debug_traceTransaction RPC)

[rjl493456442]

A few nitpicks, otherwise lgtm.

[holiman]

Unfortunately, we cannot "unify" jsonl vs json. The --trace option configures the json-logger which emits jsonl.

jsonl - is json-lines, consisting of multiple lines of output, where each separate line is a standalone json object.
json - is json, consisting of one single json object, over arbitrary many lines.

So, when using --trace option, we need to still output jsonl files. And all other tracers emit one result at the end of execution, into json format. These two formats are incompatible. We could rename the output to, e.g. .output, but I don't know if that's an improvement? IMO let's keep them separate.

[jsvisa]

@holiman Thanks, you are right. jsonl is json lines, while json is not.

[holiman]

When you changed this to instantiate the tracer earlier, you made it so that every transaction uses the same tracer instance. I don't think that's how the lifecycle of a tracer works - all other tracer-usage instantiates a new tracer for each transaction. (correct me if I'm wrong, @s1na )

Please revert this change so the tracer is instantiated in getTracer

[jsvisa]

ok, let's instantiate inside the getTracer, one tracer for each tx

[holiman]

Suggested change

	Usage: "Output full trace logs to a file named: trace-<txIndex>-<txHash>.json or trace-<txIndex>-<txHash>.jsonl",
	Usage: "Output json opcode traces to files (trace-<txIndex>-<txHash>.jsonl),

IMO the --trace configures the json tracer. For the other ones, it's implied. Why would you want to run a calltracer if it does not produce any output?

I'll push a fix