fix: guard cross-platform export spinner against noarch-only subdirs

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: {next(s for s in self.subdirs if s != 'noarch')}\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")
 
 
+def test_cross_platform_spinner_shows_target_platform(tmp_path: Path) -> None:
+    """
+    https://github.com/conda/conda-libmamba-solver/pull/911
+
+    When ``subdirs`` is set to a non-host platform, the ``Platform:`` line of
+    the metadata-collection spinner message should show the target platform,
+    not the host platform, and must never show ``noarch``.
+    """
+    from conda.models.channel import Channel
+
+    target = "linux-64" if context.subdir != "linux-64" else "win-64"
+    solver = Solver(
+        prefix=tmp_path,
+        channels=["conda-forge"],
+        subdirs=(target, "noarch"),
+        specs_to_add=["tzdata"],
+        command="create",
+    )
+    message = solver._collect_all_metadata_spinner_message(
+        channels=[Channel("conda-forge")]
+    )
+    assert f"Platform: {target}" in message
+    assert "Platform: noarch" not in message
+
+
+def test_cross_platform_spinner_falls_back_when_only_noarch(tmp_path: Path) -> None:
+    """
+    https://github.com/conda/conda-libmamba-solver/pull/911
+
+    If ``self.subdirs`` only contains ``noarch`` the spinner message must not
+    raise ``StopIteration``; it should fall back to ``context.subdir``.
+    """
+    from conda.models.channel import Channel
+
+    solver = Solver(
+        prefix=tmp_path,
+        channels=["conda-forge"],
+        specs_to_add=["tzdata"],
+        command="create",
+    )
+    solver.subdirs = ("noarch",)
+    message = solver._collect_all_metadata_spinner_message(
+        channels=[Channel("conda-forge")]
+    )
+    assert f"Platform: {context.subdir}" in message
+
+
 def test_channel_subdir_set_correctly(tmp_env: TmpEnvFixture) -> None:
     """
     https://github.com/conda/conda-libmamba-solver/issues/662