Add bitcoin-tx JSON tests

[jnewbery]

This PR adds new test cases to test bitcoin-tx JSON output, as suggested by @sipa in #8817 .

[laanwj]

Awesome, thanks!
utACK 54e5d7c

[maflcko]

utACK 54e5d7c

[paveljanik]

ups, merged in the middle of my testing :-)

postmerge ACK 54e5d7c

[droark]

Question: Was txcreate2.json supposed to be empty? I'm assuming some test data was supposed to be in it. :)

[jnewbery]

Interesting. The txcreate2 test is attempting to create a transaction with a single pay-to-script output for zero satoshi. When outputting as json, bitcoin-tx returns the empty string. When outputting as hex, it returns 01000000000100000000000000000000000000, ie:

version |number in|number out|out1 amount     |out1 script length|nlocktime
01000000 00        01         0000000000000000 00                 00000000

I think that's garbage and bitcoin-tx should reject any attempt to add an output with zero satoshi. I'll open a PR to fix that.

Interestingly, bitcoin-tx does output a json object if I try to create a zero-value transaction output to a non-empty script:

$ bitcoin-tx -json -create outscript=0:"OP_DROP"
{
    "txid": "f0851b68202f736b792649cfc960259c2374badcb644ab20cac726b5f72f61c9",
    "version": 1,
    "locktime": 0,
    "vin": [
    ],
    "vout": [
        {
            "value": 0.00,
            "n": 0,
            "scriptPubKey": {
                "asm": "OP_DROP",
                "hex": "75",
                "type": "nonstandard"
            }
        }
    ],
    "hex": "0100000000010000000000000000017500000000"
}

txcreate2 is also the only test case that tests creating a pay-to-script, so we should add a new test case that tests adding a valid pay-to-script output.

[sipa]

Zero-value outputs are perfectly valid according to consensus rules.

[jnewbery]

ah, ok. Thanks. Then the fix is in bitcoin-tx to properly output zero-value, empty-script outputs.

[laanwj]

Good to see that we're finding bugs in bitcoin-tx by adding tests :)

[jnewbery]

I've mischaracterised the problem here. bitcoin-tx is outputting the transaction correctly in hex and json. However, when I try to pipe that same hex transaction back into bitcoin-tx, it fails with error: invalid transaction encoding.

[sipa]

Ah, that seems to be due to the segwit ambiguity for 0-input transactions. IIRC, DecodeTransaction in core_io.h has a boolean parameter for attempting to decode partial transactions. It should be set to true in calls from bitcoin-tx.

[jnewbery]

Thanks @sipa . New PR here: #8837 to make bitcoin-tx set the flag correctly and parse partial transactions.

@droark - I've added #8836 to fix the fact that bitcoin-util-test.py would always succeed if the output comparison file was empty. That PR also fixes txcreate2.json so it's no longer empty.

[laanwj]

This was backported in #8832, removing tag