Merge bitcoin/bitcoin#28545: test: Avoid test failure on Linux root without cap-add LINUX_IMMUTABLE

fa40b3e test: Avoid test failure on Linux root without cap-add LINUX_IMMUTABLE (MarcoFalke)

Pull request description:

  This turns a test failure on Linux when running the test as `root`, but without the `LINUX_IMMUTABLE` capability, into an early return, with a suggestion to turn on `LINUX_IMMUTABLE` next time (if possible).

ACKs for top commit:
  pinheadmz:
    utACK fa40b3e
  jonatack:
    ACK fa40b3e

Tree-SHA512: d986ff8aeae5f8267c21a23d5be16f7c5a4d4d3be045a6999d8b39c7b8672cfe915dedde762cc9965cdc4970940bffc4b0d1412833d8036d4425450eb6181f67

Author: fanquake

test/functional/feature_reindex_readonly.py

@@ -6,6 +6,7 @@
 - Start a node, generate blocks, then restart with -reindex after setting blk files to read-only
 """
 
+import os
 import platform
 import stat
 import subprocess
@@ -45,6 +46,11 @@ def reindex_readonly(self):
                     self.log.warning(f"stdout: {e.stdout}")
                 if e.stderr:
                     self.log.warning(f"stderr: {e.stderr}")
+                if os.getuid() == 0:
+                    self.log.warning("Return early on Linux under root, because chattr failed.")
+                    self.log.warning("This should only happen due to missing capabilities in a container.")
+                    self.log.warning("Make sure to --cap-add LINUX_IMMUTABLE if you want to run this test.")
+                    return
 
         self.log.debug("Attempt to restart and reindex the node with the unwritable block file")
         with self.nodes[0].assert_debug_log(expected_msgs=['FlushStateToDisk', 'failed to open file'], unexpected_msgs=[]):