When JS-based tracing (see below) was first implemented, the intended usecase was to enable long-running tracers that could stream results back via a subscription channel.
This method works a bit differently. (For full details, see [PR](https://github.com/ethereum/go-ethereum/pull/17914))
- It streams output to disk during the execution, to not blow up the memory usage on the node
- It uses `jsonl` as output format (to allow streaming)
- Uses a cross-client standardized output, so called 'standard json'
* Uses `op` for string-representation of opcode, instead of `op`/`opName` for numeric/string, and other simlar small differences.
* has `refund`
* Represents memory as a contiguous chunk of data, as opposed to a list of `32`-byte segments like `debug_traceTransaction`
This means that this method is only 'useful' for callers who control the node -- at least sufficiently to be able to read the artefacts from the filesystem after the fact.
The method can be used to dump a certain transaction out of a given block:
Files are created in a temp-location, with the naming standard `block_<blockhash:4>-<txindex>-<txhash:4>-<random suffix>`. Each opcode immediately streams to file, with no in-geth buffering aside from whatever buffering the os normally does.
On the server side, it also adds some more info when regenerating historical state, namely, the reexec-number if `required historical state is not avaiable` is encountered, so a user can experiment with increasing that setting. It also prints out the remaining block until it reaches target:
```
INFO [10-15|13:48:25.263] Regenerating historical state block=2385959 target=2386012 remaining=53 elapsed=3m30.990537767s
INFO [10-15|13:48:33.342] Regenerating historical state block=2386012 target=2386012 remaining=0 elapsed=3m39.070073163s
INFO [10-15|13:48:33.343] Historical state regenerated block=2386012 elapsed=3m39.070454362s nodes=10.03mB preimages=652.08kB
INFO [10-15|13:48:33.352] Wrote trace file=/tmp/block_0x14490c57-0-0xfbbd6d91-715824834
INFO [10-15|13:48:33.352] Wrote trace file=/tmp/block_0x14490c57-1-0x71076194-187462969
INFO [10-15|13:48:34.421] Wrote trace file=/tmp/block_0x14490c57-2-0x3f4263fe-056924484
```
The `options` is as follows:
```
type StdTraceConfig struct {
*vm.LogConfig
Reexec *uint64
TxHash *common.Hash
}
```
### debug_standardTraceBadBlockToFile
This method is similar to `debug_standardTraceBlockToFile`, but can be used to obtain info about a block which has been _rejected_ as invalid (for some reason).
### debug_traceTransaction
**OBS** In most scenarios, `debug.standardTraceBlockToFile` is better suited for tracing!
The `traceTransaction` debugging method will attempt to run the transaction in the exact same manner
as it was executed on the network. It will replay any transaction that may have been executed prior
to this one before it will finally attempt to execute the transaction that corresponds to the given
hash.
In addition to the hash of the transaction you may give it a secondary *optional* argument, which
specifies the options for this specific call. The possible options are:
*`disableStorage`: `BOOL`. Setting this to true will disable storage capture (default = false).
*`disableMemory`: `BOOL`. Setting this to true will disable memory capture (default = false).
*`disableStack`: `BOOL`. Setting this to true will disable stack capture (default = false).
*`tracer`: `STRING`. Setting this will enable JavaScript-based transaction tracing, described below. If set, the previous four arguments will be ignored.
*`timeout`: `STRING`. Overrides the default timeout of 5 seconds for JavaScript-based tracing calls. Valid values are described [here](https://golang.org/pkg/time/#ParseDuration).
Specifying the `tracer` option in the second argument enables JavaScript-based tracing. In this mode, `tracer` is interpreted as a JavaScript expression that is expected to evaluate to an object with (at least) three methods, named `step`, `result`, and `fault`.
`step`is a function that takes two arguments, log and db, and is called for each step of the EVM, or when an error occurs, as the specified transaction is traced.
`log` has the following fields:
-`op`: Object, an OpCode object representing the current opcode
For efficiency, the same `log` object is reused on each execution step, updated with current values; make sure to copy values you want to preserve beyond the current call. For instance, this step function will not work:
-`getBalance(address)` - returns a `big.Int` with the specified account's balance
-`getNonce(address)` - returns a Number with the specified account's nonce
-`getCode(address)` - returns a byte slice with the code for the specified account
-`getState(address, hash)` - returns the state value for the specified account and the specified hash
-`exists(address)` - returns true if the specified address exists
If the step function throws an exception or executes an illegal operation at any point, it will not be called on any further VM steps, and the error will be returned to the caller.
The second function, `result`, takes two arguments `ctx` and `db`, and is expected to return a JSON-serializable value to return to the RPC caller.
`ctx` is the context in which the transaction is executing and has the following fields:
-`type` - String, one of the two values `CALL` and `CREATE`
-`from` - Address, sender of the transaction
-`to` - Address, target of the transaction
-`input` - Buffer, input transaction data
-`gas` - Number, gas budget of the transaction
-`value` - big.Int, amount to be transferred in wei
-`block` - Number, block number
-`output` - Buffer, value returned from EVM
-`gasUsed` - Number, amount of gas used in executing the transaction (excludes txdata costs)
-`time` - String, execution runtime
The third function, `fault`, takes two arguments, `log` and `db`, just like `step` and is invoked when an error happens during the execution of an opcode which wasn't reported in `step`. The method `log.getError()` has information about the error.
Note that several values are Golang big.Int objects, not JavaScript numbers or JS bigints. As such, they have the same interface as described in the godocs. Their default serialization to JSON is as a Javascript number; to serialize large numbers accurately call `.String()` on them. For convenience, `big.NewInt(x)` is provided, and will convert a uint to a Go BigInt.
Usage example, returns the top element of the stack at each CALL opcode only:
The `debug_traceCall` method lets you run an `eth_call` within the context of the given block execution using the final state of parent block as the base. The block can be specified either by hash or by number. It takes the same input object as a `eth_call`.
It returns the same output as `debug_traceTransaction`. A tracer can be specified as a third argument, similar to `debug_traceTransaction`.
`Object` - The transaction call object
-`from`: `DATA`, 20 Bytes - (optional) The address the transaction is sent from.
-`to`: `DATA`, 20 Bytes - The address the transaction is directed to.
-`gas`: `QUANTITY` - (optional) Integer of the gas provided for the transaction execution. eth_call consumes zero gas, but this parameter may be needed by some executions.
-`gasPrice`: `QUANTITY` - (optional) Integer of the gasPrice used for each paid gas
-`value`: `QUANTITY` - (optional) Integer of the value sent with this transaction
-`data`: `DATA` - (optional) Hash of the method signature and encoded parameters. For details see Ethereum Contract ABI in the Solidity documentation
