Merge bitcoin/bitcoin#24553: contrib: fix signet miner (sighash mismatch)

12cc020 contrib: fix signet miner (sighash mismatch) (Sebastian Falbesoner)

Pull request description:

  gruve-p reported that the signet miner doesn't work anymore (see bitcoin/bitcoin#24501 (comment)), failing with the following error of the `walletprocesspsbt` RPC:

  ```
  error code: -22
  error message:
  Specified sighash value does not match value stored in PSBT
  .....
  subprocess.CalledProcessError: Command '['bitcoin-cli', '-signet', '-stdin', 'walletprocesspsbt']' returned non-zero exit status 22
  ```

  PSBT signing was changed to use SIGHASH_DEFAULT by default in #22514. The signet miner script sets the sighash type of the created PSBT to SIGHASH_ALL (3 is the per-input type PSBT_IN_SIGHASH_TYPE, following a little-endian 32 unsigned integer of the sighash type):

  https://github.com/bitcoin/bitcoin/blob/e04720ec3336e3df7fce522e3b1da972aa65ff62/contrib/signet/miner#L169-L170

  hence this leads to a sighash mismatch when the `walletprocesspsbt` RPC is called. Fix this by explicitly passing the correct sighash type. The same change was needed in one of our functional tests, see commit d399266.

  Note that instead of feeding the PSBT via `-stdin` it is directly passed as parameter, as I couldn't figure out a way to pass multiple parameters otherwise (separating by newline also didn't work).

ACKs for top commit:
  kallewoof:
    ACK 12cc020
  ajtowns:
    ACK 12cc020 ; code review only

Tree-SHA512: 8509e768e96f85e28c0ca0dc2d35874aa29623febddc46bf90472ec38f38cb3a1b5407c563fd9101d07088775d0fdb18e9137cc38955e847885b83c16591c736

Author: MarcoFalke

contrib/signet/miner

@@ -8,7 +8,7 @@ import base64
 import json
 import logging
 import math
-import os.path
+import os
 import re
 import struct
 import sys
@@ -493,10 +493,11 @@ def do_generate(args):
         logging.debug("Mining block delta=%s start=%s mine=%s", seconds_to_hms(mine_time-bestheader["time"]), mine_time, is_mine)
         mined_blocks += 1
         psbt = generate_psbt(tmpl, reward_spk, blocktime=mine_time)
-        psbt_signed = json.loads(args.bcli("-stdin", "walletprocesspsbt", input=psbt.encode('utf8')))
+        input_stream = os.linesep.join([psbt, "true", "ALL"]).encode('utf8')
+        psbt_signed = json.loads(args.bcli("-stdin", "walletprocesspsbt", input=input_stream))
         if not psbt_signed.get("complete",False):
             logging.debug("Generated PSBT: %s" % (psbt,))
-            sys.stderr.write("PSBT signing failed")
+            sys.stderr.write("PSBT signing failed\n")
             return 1
         block, signet_solution = do_decode_psbt(psbt_signed["psbt"])
         block = finish_block(block, signet_solution, args.grind_cmd)