fix: create separte workdir per variant (#4934)

Author: nichmor

crates/pixi_build_type_conversions/src/snapshots/pixi_build_type_conversions__project_model__tests__conversions_v1_docs@workspace_variants.snap

@@ -31,6 +31,20 @@ expression: project_model
               "url": null,
               "license": null
             }
+          },
+          "python": {
+            "binary": {
+              "version": "*",
+              "build": null,
+              "buildNumber": null,
+              "fileName": null,
+              "channel": null,
+              "subdir": null,
+              "md5": null,
+              "sha256": null,
+              "url": null,
+              "license": null
+            }
           }
         },
         "buildDependencies": {},

crates/pixi_command_dispatcher/src/build/work_dir_key.rs

@@ -1,6 +1,7 @@
 //! See [`WorkDirKey`] for more information.
 
 use std::{
+    collections::BTreeMap,
     ffi::OsStr,
     hash::{Hash, Hasher},
 };
@@ -71,4 +72,13 @@ impl WorkDirKey {
             None => unique_key,
         }
     }
+
+    pub fn variant_key(variant_key: &BTreeMap<String, String>) -> String {
+        let mut hasher = Xxh3::new();
+        for (key, value) in variant_key {
+            key.hash(&mut hasher);
+            value.hash(&mut hasher);
+        }
+        URL_SAFE_NO_PAD.encode(hasher.finish().to_ne_bytes())
+    }
 }

crates/pixi_command_dispatcher/src/source_build/mod.rs

@@ -543,6 +543,18 @@ impl SourceBuildSpec {
                 })
             })?;
 
+        // Create a variant-specific work directory by including the actual variant values
+        // in the work directory key.
+        // This ensures different variants get separate work directories,
+        // preventing files like .pyc accumulation across builds (when using multiple python variants).
+        let variant_work_dir_hash = WorkDirKey::variant_key(&output.metadata.variant);
+
+        // not using set_file_name as it requires &mut self
+        let work_directory = PathBuf::from(format!(
+            "{}-{variant_work_dir_hash}",
+            work_directory.to_string_lossy()
+        ));
+
         // Determine final directories for everything.
         let directories = Directories::new(&work_directory, host_platform);
 

docs/source_files/pixi_workspaces/pixi_build/workspace_variants/pixi.toml

@@ -37,8 +37,12 @@ version = "0.1.0"
 [package.build]
 backend = { name = "pixi-build-python", version = "==0.4.0" }
 
+[package.build.config]
+noarch = false
+
 [package.host-dependencies]
 hatchling = "==1.26.3"
+python = "*"
 
 [package.run-dependencies]
 cpp_math = { path = "packages/cpp_math" }

tests/integration_python/test_build.py

@@ -0,0 +1,50 @@
+import shutil
+from pathlib import Path
+
+import pytest
+
+from .common import repo_root, verify_cli_command
+
+
+@pytest.mark.extra_slow
+def test_workspace_variants_separate_work_directories(
+    pixi: Path,
+    tmp_pixi_workspace: Path,
+) -> None:
+    """Test that building with multiple Python variants creates separate work directories.
+
+    This test verifies the fix for issue #4878 where .pyc files from different
+    Python versions would accumulate in the same work directory, causing package
+    sizes to grow progressively.
+
+    The fix ensures that each variant combination gets its own work directory by
+    including variants in the work directory key hash.
+    """
+    # Find the workspace_variants project
+    workspace_variants_project = repo_root().joinpath(
+        "docs/source_files/pixi_workspaces/pixi_build/workspace_variants"
+    )
+
+    # Remove existing .pixi folders
+    shutil.rmtree(workspace_variants_project.joinpath(".pixi"), ignore_errors=True)
+
+    # Copy to workspace
+    shutil.copytree(workspace_variants_project, tmp_pixi_workspace, dirs_exist_ok=True)
+
+    # Build the packages for all variants
+    verify_cli_command(
+        [pixi, "build", "--path", tmp_pixi_workspace, "--output-dir", str(tmp_pixi_workspace)],
+    )
+
+    # Check that work directories exist and are separate for each variant
+    work_dir = tmp_pixi_workspace / ".pixi" / "build" / "work"
+    assert work_dir.exists(), "Work directory should exist"
+
+    # Get all work directories (should be different for py311 and py312)
+    work_dirs = list(work_dir.glob("python_rich-*"))
+
+    # Should have at least 2 work directories (one per Python variant)
+    assert len(work_dirs) >= 2, (
+        f"Expected at least 2 work directories for different Python variants, "
+        f"found {len(work_dirs)}: {[d.name for d in work_dirs]}"
+    )