yield libc virtual package only if libc_version is truthy (conda#12293)

* yield libc virtual package only if libc_version is truthy

* test glibc override

conda/plugins/virtual_packages/linux.py

@@ -31,4 +31,5 @@ def conda_virtual_packages():
         # Default to glibc when using CONDA_SUBDIR var
         libc_family = "glibc"
     libc_version = os.getenv(f"CONDA_OVERRIDE_{libc_family.upper()}", libc_version)
-    yield CondaVirtualPackage(libc_family, libc_version, None)
+    if libc_version:
+        yield CondaVirtualPackage(libc_family, libc_version, None)

news/12267-conda-override-glibc

@@ -0,0 +1,19 @@
+### Enhancements
+
+* <news item>
+
+### Bug fixes
+
+* Don't export `__glibc` virtual package when `CONDA_OVERRIDE_GLIBC=""` (#12267)
+
+### Deprecations
+
+* <news item>
+
+### Docs
+
+* <news item>
+
+### Other
+
+* <news item>

tests/plugins/test_virtual_packages.py

@@ -121,3 +121,24 @@ def test_subdir_override():
             assert not any(
                 (p.name in platform_virtual_packages and p.name != expected) for p in virtual
             )
+
+
+def test_glibc_override():
+    """
+    Conda should not produce a libc virtual package when CONDA_OVERRIDE_GLIBC=""
+    """
+    for version in "", "1.0":
+        with env_vars(
+            {"CONDA_SUBDIR": "linux-64", "CONDA_OVERRIDE_GLIBC": version},
+            stack_callback=conda_tests_ctxt_mgmt_def_pol,
+        ):
+            packages = conda.core.index.get_reduced_index(
+                context.default_prefix,
+                context.default_channels,
+                context.subdirs,
+                (),
+                context.repodata_fns[0],
+            )
+            virtual = [p for p in packages if p.channel.name == "@"]
+            libc_exported = any("libc" in p.name for p in virtual)
+            assert libc_exported == bool(version)