test: avoid sporadic MINIMALDATA failure in feature_taproot.py (fixes #27595)

The functional test feature_taproot.py fails in some rare cases on the
execution of the `"branched_codesep"` spending script. The problem
occurs if the first data-push (having random content with a random
length in the range [0, 510]) has a length of 1 and the single byte has
value of [1...16] or [-1]; in this case, the data-push is not minimally
encoded by test framework's CScript class (i.e. doesn't use the special
op-codes OP_1...OP_16 or OP_1NEGATE) and the script interpreter throws
an SCRIPT_ERR_MINIMALDATA error:

```
test_framework.authproxy.JSONRPCException: non-mandatory-script-verify-flag (Data push larger than necessary) (-26)
```

Background:
The functional test framework's CScript class translates passed
bytes/bytearrays always to data pushes using OP_PUSHx/OP_PUSHDATA{1,2,4}
op-codes. E.g. the expression `CScript(bytes([1]))` yields
`bytes([OP_PUSH1, 1])` instead of the minimal-encoded `bytes([OP_1])`.

Fix this by adapting the random-size range to [2,...], i.e. never pass
byte-arrays below length two to be pushed.

Closes #27595.

1 files changed, 5 insertions, 1 deletions

diff --git a/test/functional/feature_taproot.py b/test/functional/feature_taproot.py
index 8be2040d91..34197a5c48 100755
--- a/test/functional/feature_taproot.py
+++ b/test/functional/feature_taproot.py
@@ -738,7 +738,11 @@ def spenders_taproot_active():
         scripts = [
             ("pk_codesep", CScript(random_checksig_style(pubs[1]) + bytes([OP_CODESEPARATOR]))),  # codesep after checksig
             ("codesep_pk", CScript(bytes([OP_CODESEPARATOR]) + random_checksig_style(pubs[1]))),  # codesep before checksig
-            ("branched_codesep", CScript([random_bytes(random.randrange(511)), OP_DROP, OP_IF, OP_CODESEPARATOR, pubs[0], OP_ELSE, OP_CODESEPARATOR, pubs[1], OP_ENDIF, OP_CHECKSIG])),  # branch dependent codesep
+            ("branched_codesep", CScript([random_bytes(random.randrange(2, 511)), OP_DROP, OP_IF, OP_CODESEPARATOR, pubs[0], OP_ELSE, OP_CODESEPARATOR, pubs[1], OP_ENDIF, OP_CHECKSIG])),  # branch dependent codesep
+            # Note that the first data push in the "branched_codesep" script has the purpose of
+            # randomizing the sighash, both by varying script size and content. In order to
+            # avoid MINIMALDATA script verification errors caused by not-minimal-encoded data
+            # pushes (e.g. `OP_PUSH1 1` instead of `OP_1`), we set a minimum data size of 2 bytes.
         ]
         random.shuffle(scripts)
         tap = taproot_construct(pubs[0], scripts)