Merge bitcoin/bitcoin#26631: test: add coverage for dust mempool policy (`-dustrelayfee` setting)

d6fc1d6a3393c571a1691a6bda60433216643616 test: add coverage for dust mempool policy (`-dustrelayfee` setting) (Sebastian Falbesoner)
8a5dbe28793d2e2ad85242f2191a6e1956b980dc test: add `CScript` method for checking for witness program (Sebastian Falbesoner)

Pull request description:

  This PR adds missing test coverage for the `-dustrelayfee` setting, which specifies the fee-rate used to define dust. Output scripts for all common types that are treated as standard by default (P2PK, P2(W)PKH, P2(W)SH, P2TR, bare multisig, null data, unknown witness versions v2+) are created and then checked for dust-mempool-policy each via the `testmempoolaccept` RPC: a tx with an output's nValue equal to the dust threshold should be accepted, one with an nValue of just one 1 satoshi below that should be rejected with reason `dust`. This is repeatedly done for a fixed (but obviously somewhat arbitrary) list of different `-dustrelayfee` settings on a single node, including the default and zero (i.e. no dust limit) settings.

  Note that the first commit introduces a necessary `CScript` helper method `IsWitnessProgram` (using PascalCase in Python is likely controversial; in this case the style for the already existing method `GetSigOpCount` was followed, which also refers to a method in the core `CScript` class).

  Some historical information about dust, contributed by pablomartin4btc:
  "The concept of dust was first introduced in https://github.com/bitcoin/bitcoin/pull/2577. This [commit](https://github.com/bitcoin/bitcoin/pull/9380/commits/eb30d1a5b215c6dd3763d7f7948f2dd8cb61f6bf) from https://github.com/bitcoin/bitcoin/pull/9380 introduced the -dustrelayfee option. Previous to that PR, the dust feerate was whatever -minrelaytxfee was set to."

ACKs for top commit:
  LarryRuane:
    ACK d6fc1d6a3393c571a1691a6bda60433216643616
  glozow:
    ACK d6fc1d6a3393c571a1691a6bda60433216643616
  kouloumos:
    ACK d6fc1d6a3393c571a1691a6bda60433216643616

Tree-SHA512: 35ea2b2497dfb466395af5665bb217f7250aa7cab9dc43539a5658ab69a454e3623ff58fce7489fcc1105b37f8cb4840a93cec658c5df1de611732bc6439ccad

1 files changed, 7 insertions, 0 deletions

diff --git a/test/functional/test_framework/script.py b/test/functional/test_framework/script.py
index f345bf02db..443cae86a1 100644
--- a/test/functional/test_framework/script.py
+++ b/test/functional/test_framework/script.py
@@ -597,6 +597,13 @@ class CScript(bytes):
             lastOpcode = opcode
         return n
 
+    def IsWitnessProgram(self):
+        """A witness program is any valid CScript that consists of a 1-byte
+           push opcode followed by a data push between 2 and 40 bytes."""
+        return ((4 <= len(self) <= 42) and
+                (self[0] == OP_0 or (OP_1 <= self[0] <= OP_16)) and
+                (self[1] + 2 == len(self)))
+
 
 SIGHASH_DEFAULT = 0 # Taproot-only default, semantics same as SIGHASH_ALL
 SIGHASH_ALL = 1