Show target platform instead of host in cross-platform export (#911)

Co-authored-by: Jannis Leidel <jannis@leidel.info>

Author: costajohnt

conda_libmamba_solver/solver.py

@@ -209,7 +209,7 @@ def _collect_all_metadata_spinner_message(
         return (
             f"Channels:\n"
             f" - {canonical_names_dashed}\n"
-            f"Platform: {context.subdir}\n"
+            f"Platform: {next((s for s in self.subdirs if s != 'noarch'), context.subdir)}\n"
             f"Collecting package metadata ({self._repodata_fn})"
         )
 

news/911-cross-platform-export-display

@@ -0,0 +1,20 @@
+### Enhancements
+
+* <news item>
+
+### Bug fixes
+
+* Show the target platform instead of the host platform in the progress
+  message during cross-platform lockfile export. (#911)
+
+### Deprecations
+
+* <news item>
+
+### Docs
+
+* <news item>
+
+### Other
+
+* <news item>

tests/test_solver.py

@@ -657,6 +657,53 @@ def test_pytorch_gpu(specs):
         raise AssertionError("No pytorch found")
 
 
+_CROSS_PLATFORM_TARGET = "linux-64" if context.subdir != "linux-64" else "win-64"
+
+
+@pytest.mark.parametrize(
+    "subdirs,expected_platform",
+    (
+        pytest.param(
+            (_CROSS_PLATFORM_TARGET, "noarch"),
+            _CROSS_PLATFORM_TARGET,
+            id="cross-platform-target",
+        ),
+        pytest.param(
+            ("noarch",),
+            context.subdir,
+            id="noarch-only-fallback",
+        ),
+    ),
+)
+def test_cross_platform_spinner_message(
+    tmp_path: Path,
+    subdirs: tuple[str, ...],
+    expected_platform: str,
+) -> None:
+    """
+    https://github.com/conda/conda-libmamba-solver/pull/911
+
+    The ``Platform:`` line of the metadata-collection spinner message should
+    show the first non-``noarch`` entry of ``self.subdirs`` (the actual
+    target platform during cross-platform export) and fall back to
+    ``context.subdir`` when ``self.subdirs`` only contains ``noarch``.
+    """
+    from conda.models.channel import Channel
+
+    solver = Solver(
+        prefix=tmp_path,
+        channels=["conda-forge"],
+        specs_to_add=["tzdata"],
+        command="create",
+    )
+    # Assign after construction so the noarch-only case bypasses the
+    # ``next(s for s in self.subdirs if s != "noarch")`` call in ``__init__``.
+    solver.subdirs = subdirs
+    message = solver._collect_all_metadata_spinner_message(channels=[Channel("conda-forge")])
+    assert f"Platform: {expected_platform}" in message
+    assert "Platform: noarch" not in message
+
+
 def test_channel_subdir_set_correctly(tmp_env: TmpEnvFixture) -> None:
     """
     https://github.com/conda/conda-libmamba-solver/issues/662