Publish viewer downloads on WASM without hard links

Author: FrancescAlted

src/blosc2/b2view/ordinary_file.py

@@ -8,6 +8,7 @@
 from urllib.parse import urlsplit
 from urllib.request import url2pathname
 
+import blosc2
 from blosc2.core import find_url_separator, fsspec_filesystem, is_fsspec_url, parse_container_url
 
 MAX_READ_BYTES = 16 << 20
@@ -71,6 +72,19 @@ def ordinary_filesystem(source, storage_options=None):
     return fsspec_filesystem(source, options)
 
 
+def _publish_download(temporary, destination, overwrite):
+    if overwrite:
+        os.replace(temporary, destination)
+    elif blosc2.IS_WASM:
+        # Emscripten has no hard links. Its single-threaded virtual FS cannot
+        # interleave a writer between this check and rename (no callbacks/awaits).
+        if os.path.lexists(destination):
+            raise FileExistsError(destination)
+        os.rename(temporary, destination)
+    else:
+        os.link(temporary, destination)
+
+
 class OrdinaryFile:
     """A direct regular file with bounded reads and independent transfer aliases.
 
@@ -186,10 +200,7 @@ def download(self, destination, *, overwrite=False, progress=None, cancel=None):
             self._check_open()
             if cancel is not None and cancel():
                 raise InterruptedError("File operation cancelled")
-            if overwrite:
-                os.replace(temporary, destination)
-            else:
-                os.link(temporary, destination)
+            _publish_download(temporary, destination, overwrite)
             return str(destination)
         finally:
             Path(temporary).unlink(missing_ok=True)

tests/b2view/test_directories.py

@@ -74,6 +74,7 @@ def test_hdf5_mount_in_directory(tmp_path):
 
 
 def test_chained_fsspec_directory(tmp_path):
+    pytest.importorskip("fsspec")
     import zipfile
 
     archive = tmp_path / "files.zip"

tests/b2view/test_ordinary_files.py

@@ -150,6 +150,7 @@ def test_empty_unknown_pdf_and_missing_files(tmp_path):
 
 
 def test_chained_fsspec_file_and_storage_options(tmp_path, monkeypatch):
+    pytest.importorskip("fsspec")
     import zipfile
 
     from blosc2.b2view import ordinary_file
@@ -177,7 +178,7 @@ def filesystem(source, options):
         assert all(options == {"custom": "value"} for options in seen)
 
 
-def test_publication_race_and_late_cancellation(tmp_path, monkeypatch):
+def test_late_cancellation(tmp_path):
     source = tmp_path / "source.txt"
     source.write_bytes(b"hello")
     file = OrdinaryFile(source)
@@ -192,6 +193,14 @@ def progress(done, total):
         file.download(destination, progress=progress, cancel=lambda: cancelled)
     assert not destination.exists()
     assert not list(tmp_path.glob(".b2view-download-*"))
+
+
+@pytest.mark.skipif(blosc2.IS_WASM, reason="emscripten has no hard links")
+def test_publication_race(tmp_path, monkeypatch):
+    source = tmp_path / "source.txt"
+    source.write_bytes(b"hello")
+    file = OrdinaryFile(source)
+    destination = tmp_path / "destination.txt"
     original = __import__("os").link
 
     def race(temporary, target):
@@ -205,6 +214,30 @@ def race(temporary, target):
     assert not list(tmp_path.glob(".b2view-download-*"))
 
 
+@pytest.mark.parametrize("competing_writer", [False, True])
+def test_wasm_download_publication(tmp_path, monkeypatch, competing_writer):
+    monkeypatch.setattr(blosc2, "IS_WASM", True)
+    source = tmp_path / "source.txt"
+    source.write_bytes(b"hello")
+    destination = tmp_path / "destination.txt"
+    file = OrdinaryFile(source)
+
+    def progress(done, total):
+        if competing_writer:
+            destination.write_bytes(b"existing")
+
+    if competing_writer:
+        with pytest.raises(FileExistsError):
+            file.download(destination, progress=progress)
+        assert destination.read_bytes() == b"existing"
+    else:
+        file.download(destination, progress=progress)
+        assert destination.read_bytes() == b"hello"
+        with pytest.raises(FileExistsError):
+            file.download(destination)
+    assert not list(tmp_path.glob(".b2view-download-*"))
+
+
 @pytest.mark.parametrize(
     "source",
     [