Merge bitcoin#28612: Test: followups to bitcoin#27823

5ab6419 test: randomized perturbing in feature_init (L0la L33tz)
64b80d5 test: simplify feature_init (Fabian Jahr)

Pull request description:

  Fixes bitcoin#28603

  Added suggested simplifications and implemented randomization

ACKs for top commit:
  theStack:
    Light ACK 5ab6419
  maflcko:
    lgtm ACK 5ab6419
  achow101:
    ACK 5ab6419

Tree-SHA512: e6f43eef7f8dd12c7fccbe437cb430dc9d383825d7ab2caa0382d061f88dec6d28522e1ec78f3f58f26d35cba93512fa21e330c48d06b1d8141a16f07050af5a

test/functional/feature_init.py

@@ -5,6 +5,7 @@
 """Stress tests related to node initialization."""
 import os
 from pathlib import Path
+from random import randint
 import shutil
 
 from test_framework.test_framework import BitcoinTestFramework, SkipTest
@@ -133,15 +134,12 @@ def check_clean_start():
 
             for target_file in target_files:
                 self.log.info(f"Perturbing file to ensure failure {target_file}")
-                with open(target_file, "rb") as tf_read:
-                    contents = tf_read.read()
-                    tweaked_contents = bytearray(contents)
+                with open(target_file, "r+b") as tf:
                     # Since the genesis block is not checked by -checkblocks, the
                     # perturbation window must be chosen such that a higher block
                     # in blk*.dat is affected.
-                    tweaked_contents[150:350] = b'1' * 200
-                with open(target_file, "wb") as tf_write:
-                    tf_write.write(bytes(tweaked_contents))
+                    tf.seek(randint (150, 15000))
+                    tf.write(b'1' * randint(20, 2000))
 
             start_expecting_error(err_fragment)