Merge #15771: qa: Prevent concurrency issues reading .cookie file

90bce24576 qa: Prevent concurrency issues reading .cookie file (João Barbosa)

Pull request description:

  Hopefully fixes #15733.

ACKs for commit 90bce2:

Tree-SHA512: 4db06a63bb57e8ae56a4eab9d352a9a8d66dd7425cf491ca5f9e1ec5e60e61cb5af9eedbd32a0a9f9bdd770d767adf499eed05dd03221686eb357f6417441b61

test/functional/test_framework/util.py

@@ -323,12 +323,14 @@ def get_auth_cookie(datadir):
                 if line.startswith("rpcpassword="):
                     assert password is None  # Ensure that there is only one rpcpassword line
                     password = line.split("=")[1].strip("\n")
-    if os.path.isfile(os.path.join(datadir, "regtest", ".cookie")) and os.access(os.path.join(datadir, "regtest", ".cookie"), os.R_OK):
+    try:
         with open(os.path.join(datadir, "regtest", ".cookie"), 'r', encoding="ascii") as f:
             userpass = f.read()
             split_userpass = userpass.split(':')
             user = split_userpass[0]
             password = split_userpass[1]
+    except OSError:
+        pass
     if user is None or password is None:
         raise ValueError("No RPC credentials")
     return user, password