Keep snapshot PETSc I/O paths compact

src/underworld3/checkpoint/disk_snapshot.py

@@ -40,6 +40,7 @@
 import json
 import os
 import warnings
+from contextlib import contextmanager
 from typing import Any, Optional
 
 import numpy as np
@@ -247,8 +248,8 @@ def read_snapshot_metadata(path: str) -> dict:
 #
 #   /path/to/run.snap.h5              wrapper (metadata, h5py-readable)
 #   /path/to/run.snap.bulk/           companion directory (one per snapshot)
-#       {mesh_safe}.mesh.00000.h5     mesh DM dump (PETSc HDF5)
-#       {mesh_safe}.{var_clean}.00000.h5  per-variable section + vec (PETSc HDF5)
+#       mesh_0000.mesh.00000.h5       mesh DM dump (PETSc HDF5)
+#       mesh_0000.{var_clean}.00000.h5  per-variable section + vec (PETSc HDF5)
 #       ... one set per (mesh, var) ...
 #
 # The bulk-dir path is derived from the wrapper path by convention, so a
@@ -262,6 +263,23 @@ def _bulk_dir_for(wrapper_path: str) -> str:
     return base + ".bulk"
 
 
+@contextmanager
+def _short_io_path(path: str):
+    """Expose ``path`` to native I/O as a basename from its parent directory.
+
+    Some parallel PETSc/HDF5 stacks fail on valid absolute paths well below
+    ``PATH_MAX``. Snapshot artifacts retain their normal locations, while the
+    native reader or writer receives only the final path component.
+    """
+    absolute_path = os.path.abspath(path)
+    previous_directory = os.getcwd()
+    os.chdir(os.path.dirname(absolute_path))
+    try:
+        yield os.path.basename(absolute_path)
+    finally:
+        os.chdir(previous_directory)
+
+
 def _sanitise(name: str) -> str:
     """Sanitise a mesh / variable name for use as a filename component.
 
@@ -306,8 +324,13 @@ def write_snapshot(model, path: str) -> str:
     # bulk directory. write_checkpoint is collective (PETSc HDF5
     # viewer), so all ranks must participate.
     mesh_records: list[dict] = []
-    for mesh in list(model._meshes.values()):
-        mesh_safe = _sanitise(mesh.name)
+    for mesh_index, mesh in enumerate(list(model._meshes.values())):
+        # Loaded meshes commonly use their complete source path as ``name``.
+        # Embedding that path in every PETSc-HDF5 bulk filename can exceed
+        # practical MPI-I/O pathname limits even when the wrapper path itself
+        # is valid. The wrapper preserves the original name for exact restore
+        # matching, so bulk files only need a compact deterministic identifier.
+        mesh_safe = f"mesh_{mesh_index:04d}"
         mesh_vars = list(mesh.vars.values())
         # Filter to allocated variables — same skip rule as the in-memory
         # path: lazy-allocated vars with _gvec == None have no data.
@@ -319,14 +342,15 @@ def write_snapshot(model, path: str) -> str:
         # (consumed by the reload path below). Suppress the FutureWarning for
         # this internal call rather than spam every snapshot. (Migrating the
         # snapshot to write_timestep is tracked in #252.)
-        with warnings.catch_warnings():
-            warnings.simplefilter("ignore", FutureWarning)
-            mesh.write_checkpoint(
-                mesh_safe,
-                outputPath=bulk_dir,
-                meshVars=mesh_vars,
-                index=0,
-            )
+        with _short_io_path(bulk_dir) as short_bulk_dir:
+            with warnings.catch_warnings():
+                warnings.simplefilter("ignore", FutureWarning)
+                mesh.write_checkpoint(
+                    mesh_safe,
+                    outputPath=short_bulk_dir,
+                    meshVars=mesh_vars,
+                    index=0,
+                )
 
         mesh_records.append({
             "name": mesh.name,
@@ -496,10 +520,13 @@ def read_snapshot(model, path: str) -> None:
                         f"snapshot variable {var_name!r} not registered on "
                         f"mesh {mesh_name!r}"
                     )
-                var.read_checkpoint(
-                    os.path.join(bulk_dir, external_file),
-                    data_name=var_name,
-                )
+                with _short_io_path(
+                    os.path.join(bulk_dir, external_file)
+                ) as short_external_file:
+                    var.read_checkpoint(
+                        short_external_file,
+                        data_name=var_name,
+                    )
 
         # Phase 3a: restore state-bearer dataclasses.
         if _GROUP_PYTHON_STATE in f:
@@ -949,13 +976,17 @@ def extract_var_via_bridge(wrapper_path: str, var_name: str):
     # Rebuild a transient source mesh + variable to read DOFs into.
     # We deliberately don't register them with the live model — these
     # are throwaway and exit scope on return.
-    src_mesh = uw.discretisation.Mesh(os.path.join(bulk_dir, mesh_file_rel))
+    with _short_io_path(
+        os.path.join(bulk_dir, mesh_file_rel)
+    ) as short_mesh_file:
+        src_mesh = uw.discretisation.Mesh(short_mesh_file)
     src_var = uw.discretisation.MeshVariable(
         var_name, src_mesh, components, degree=degree, continuous=continuous,
     )
-    src_var.read_checkpoint(
-        os.path.join(bulk_dir, var_file_rel), data_name=var_name
-    )
+    with _short_io_path(
+        os.path.join(bulk_dir, var_file_rel)
+    ) as short_var_file:
+        src_var.read_checkpoint(short_var_file, data_name=var_name)
 
     coords = np.asarray(src_var.coords).copy()
     values = np.asarray(src_var.array[...]).reshape(

tests/test_0010_snapshot_disk_format.py

@@ -214,6 +214,45 @@ def test_write_snapshot_produces_wrapper_and_bulk_dir(tmp_path):
     assert any("V.00000.h5" in f for f in files)
 
 
+def test_snapshot_bulk_filenames_do_not_expand_loaded_mesh_name(tmp_path):
+    """A source pathname remains metadata, not a PETSc bulk filename."""
+    import h5py
+    import os
+
+    uw, model, mesh, T, V = _fresh_model_mesh_and_vars()
+    mesh.name = "/g/data/project/user/" + "nested_directory/" * 12 + "mesh.msh.h5"
+
+    path = str(tmp_path / "compact.snap.h5")
+    model.save_state(file=path)
+
+    bulk = str(tmp_path / "compact.snap.bulk")
+    files = sorted(os.listdir(bulk))
+    assert files
+    assert all(filename.startswith("mesh_0000.") for filename in files)
+    assert max(map(len, files)) < 64
+
+    with h5py.File(path, "r") as h5:
+        assert list(h5["meshes"]) == ["mesh_0000"]
+        assert h5["meshes"]["mesh_0000"].attrs["name"] == mesh.name
+
+
+def test_short_io_path_uses_basename_and_restores_cwd(tmp_path):
+    """Native PETSc/HDF5 calls do not receive the full user output path."""
+    import os
+    from underworld3.checkpoint.disk_snapshot import _short_io_path
+
+    original_directory = os.getcwd()
+    target_directory = tmp_path / "nested" / "snapshot.bulk"
+    target_directory.mkdir(parents=True)
+    target_file = target_directory / "mesh_0000.T.00000.h5"
+
+    with _short_io_path(str(target_file)) as short_path:
+        assert short_path == target_file.name
+        assert os.getcwd() == str(target_directory)
+
+    assert os.getcwd() == original_directory
+
+
 def test_write_snapshot_populates_wrapper_layout(tmp_path):
     """The wrapper carries the per-mesh + per-variable metadata that
     makes 'what's in this snapshot?' answerable from h5py alone."""