file parsing: check for CWD existence

changes.d/5694.fix.md

@@ -0,0 +1,3 @@
+Don't fail config file parsing if current working directory does not exist.
+(Note however this may not be enough to prevent file parsing commands failing
+elsewhere in the Python library).

cylc/flow/parsec/fileparse.py

@@ -401,7 +401,14 @@ def read_and_proc(
     fpath = _get_fpath_for_source(fpath, opts)
     fdir = os.path.dirname(fpath)
 
-    odir = os.getcwd()
+    try:
+        original_cwd = os.getcwd()
+    except FileNotFoundError:
+        # User's current working directory does not actually exist, so we won't
+        # be able to change back to it later. (Note this might not be enough to
+        # prevent file parsing commands failing due to missing cwd elsewhere in
+        # the Python library).
+        original_cwd = None
 
     # Move to the file location to give the template processor easy access to
     # other files in the workflow directory (whether source or installed).
@@ -500,7 +507,9 @@ def read_and_proc(
     if do_contin:
         flines = _concatenate(flines)
 
-    os.chdir(odir)
+    # If the user's original working directory exists, change back to it.
+    if original_cwd is not None:
+        os.chdir(original_cwd)
 
     # return rstripped lines
     return [fl.rstrip() for fl in flines]

tests/unit/parsec/test_fileparse.py

@@ -736,3 +736,21 @@ def test_get_fpath_for_source(tmp_path):
     opts.against_source = True
     assert _get_fpath_for_source(
         rundir / 'flow.cylc', opts) == str(srcdir / 'flow.cylc')
+
+def test_user_has_no_cwd(tmp_path):
+    """Test we can parse a config file even if cwd does not exist."""
+    cwd = tmp_path/"cwd"
+    os.mkdir(cwd)
+    os.chdir(cwd)
+    os.rmdir(cwd)
+    # (I am now located in a non-existent directory. Outrageous!)
+    with NamedTemporaryFile() as tf:
+        fpath = tf.name
+        tf.write(('''
+        [scheduling]
+            [[graph]]
+                R1 = "foo"
+        ''').encode())
+        tf.flush()
+        # Should not raise FileNotFoundError from os.getcwd():
+        parse(fpath=fpath, output_fname="")