From 8c7b7be72cdaf54466b22fde96b9e42dfbe2e04f Mon Sep 17 00:00:00 2001
From: Oliver Sanders <oliver.sanders@metoffice.gov.uk>
Date: Tue, 18 Feb 2025 10:35:25 +0000
Subject: [PATCH] work around NFS caching issues (#6603)

* scan: refresh NFS cache for contact file
* load_contact_file: refresh NFS cache for directories above contact file
* infer_latest_run: refresh NFS cache for the run directory
---
 changes.d/6506.fix.md       |  1 +
 cylc/flow/network/scan.py   | 17 ++++++++++++++---
 cylc/flow/workflow_files.py | 38 ++++++++++++++++++++++++++++++++++++-
 3 files changed, 52 insertions(+), 4 deletions(-)
 create mode 100644 changes.d/6506.fix.md

diff --git a/changes.d/6506.fix.md b/changes.d/6506.fix.md
new file mode 100644
index 00000000000..5017c89c994
--- /dev/null
+++ b/changes.d/6506.fix.md
@@ -0,0 +1 @@
+Work around caching behaviour observed on NFS filesystems which could cause workflows to appear to be stopped or even to not exist, when they are running.
diff --git a/cylc/flow/network/scan.py b/cylc/flow/network/scan.py
index 6de28ada517..d02572e498b 100644
--- a/cylc/flow/network/scan.py
+++ b/cylc/flow/network/scan.py
@@ -316,10 +316,21 @@ async def is_active(flow, is_active):
             False to filter for stopped and unregistered flows.
 
     """
-    contact = flow['path'] / SERVICE / CONTACT
-    _is_active = contact.exists()
+    service = flow['path'] / SERVICE
+    # NOTE: We must list the service directory contents rather than checking
+    # for the existence of the contact file directly, because listing the
+    # directory forces NFS filesystems to recompute their local cache.
+    # See https://github.com/cylc/cylc-flow/issues/6506
+    try:
+        contents = await scandir(service)
+    except FileNotFoundError:
+        _is_active = False
+    else:
+        _is_active = any(
+            path.name == WorkflowFiles.Service.CONTACT for path in contents
+        )
     if _is_active:
-        flow['contact'] = contact
+        flow['contact'] = service / CONTACT
     return _is_active == is_active
 
 
diff --git a/cylc/flow/workflow_files.py b/cylc/flow/workflow_files.py
index 347f9e1dbff..add79adbbd6 100644
--- a/cylc/flow/workflow_files.py
+++ b/cylc/flow/workflow_files.py
@@ -22,8 +22,10 @@
 
 """
 
+from contextlib import suppress
 from enum import Enum
 import errno
+from collections import deque
 import os
 from pathlib import Path
 import re
@@ -620,8 +622,29 @@ def get_workflow_srv_dir(id_):
     return os.path.join(run_d, WorkflowFiles.Service.DIRNAME)
 
 
+def refresh_nfs_cache(path: Path):
+    """Refresh NFS cache for dirs between ~/cylc-run and <path> inclusive.
+
+    On NFS filesystems, the non-existence of files/directories may become
+    cashed. To work around this, we can list the contents of these directories
+    which refreshes the NFS cache.
+
+    See: https://github.com/cylc/cylc-flow/issues/6506
+
+    Arguments:
+        path: The directory to refresh.
+
+    Raises:
+        FileNotFoundError: If any of the directories between ~/cylc-run and
+        this directory (inclsive) are not present.
+
+    """
+    cylc_run_dir = get_cylc_run_dir()
+    for subdir in reversed(path.relative_to(cylc_run_dir).parents):
+        deque((cylc_run_dir / subdir).iterdir(), maxlen=0)
+
+
 def load_contact_file(id_: str, run_dir=None) -> Dict[str, str]:
-    """Load contact file. Return data as key=value dict."""
     if not run_dir:
         path = Path(get_contact_file_path(id_))
     else:
@@ -630,6 +653,14 @@ def load_contact_file(id_: str, run_dir=None) -> Dict[str, str]:
             WorkflowFiles.Service.DIRNAME,
             WorkflowFiles.Service.CONTACT
         )
+
+    if not path.exists():
+        # work around NFS caching issues
+        try:
+            refresh_nfs_cache(path)
+        except FileNotFoundError as exc:
+            raise ServiceFileError("Couldn't load contact file") from exc
+
     try:
         with open(path) as f:
             file_content = f.read()
@@ -919,6 +950,11 @@ def infer_latest_run(
     except ValueError:
         raise ValueError(f"{path} is not in the cylc-run directory") from None
 
+    if not path.exists():
+        # work around NFS caching issues
+        with suppress(FileNotFoundError):
+            refresh_nfs_cache(path)
+
     if not path.exists():
         raise InputError(
             f'Workflow ID not found: {id_}\n(Directory not found: {path})'