fix(py_venv): copy the _virtualenv.py shim instead of symlinking it (#1287)

### Changes are visible to end-users: no

### Test plan

- Covered by existing test cases
- New test cases added

Author: jbedard

py/private/modify_mtree.awk

@@ -3,24 +3,17 @@
 # following and inlining the target bytes.
 #
 # Forked from @tar.bzl//tar/private:preserve_symlinks.awk and tracking
-# https://github.com/bazel-contrib/tar.bzl/pull/115. Two behavioural
-# differences remain:
+# https://github.com/bazel-contrib/tar.bzl/pull/115. One behavioural
+# difference remains:
 #
-#   1. `bazel-out/` vs `external/` strip is exclusive (`if` / `else if`)
-#      rather than two sequential `sub`s. Without this, paths matching
-#      both regexes — e.g. `bazel-out/<cfg>/bin/external/<repo>/...`,
-#      the canonical shape of a generated wheel file — get
-#      over-stripped down to `external/<repo>/...`, miss the
-#      `symlink_map` lookup, and dangle inside the OCI layer.
-#   2. Classified `bazel-out/` or `external/` targets NOT in the
-#      layer's mtree fall back to `type=file content=...` (bsdtar
-#      inlines the bytes) instead of being written as a dangling
-#      `type=link link=external/<repo>/...`. Exercised by py_venv's
-#      `_virtualenv.py` symlink into the rules_py source tree, which
-#      under bzlmod's `external/aspect_rules_py+/...` layout isn't a
-#      separate tar entry.
+#   The `bazel-out/` vs `external/` strip is exclusive (`if` / `else if`)
+#   rather than two sequential `sub`s. Without this, paths matching
+#   both regexes — e.g. `bazel-out/<cfg>/bin/external/<repo>/...`,
+#   the canonical shape of a generated wheel file — get
+#   over-stripped down to `external/<repo>/...`, miss the
+#   `symlink_map` lookup, and dangle inside the OCI layer.
 #
-# Send a follow-up PR to bazel-contrib/tar.bzl with both once #115
+# Send a follow-up PR to bazel-contrib/tar.bzl once #115
 # lands so this fork can retire.
 #
 # Invoked from `_run_tar_action` in [py_image_layer.bzl](py_image_layer.bzl)
@@ -180,16 +173,8 @@ END {
                 linked_to = make_relative_link(mapped_link, field0)
             } else if (resolved_path ~ /^bazel-out\// || resolved_path ~ /^external\//) {
                 # Classified to a Bazel-tree path but the target row
-                # isn't in this layer's mtree. Slow path falls back to
-                # `type=file content=...` so bsdtar inlines the target
-                # bytes; the hot path has no equivalent (declared
-                # symlinks whose targets aren't in the layer are a
-                # config bug) so we emit a dangling `type=link link=...`
-                # to surface the issue visibly.
-                if (original_line ~ /type=file/) {
-                    out_lines[++n] = original_line
-                    continue
-                }
+                # isn't in this layer's mtree — a config bug. Emit a
+                # dangling `type=link link=...` to surface it visibly.
                 linked_to = resolved_path
             } else {
                 # Already a relative path

py/private/py_venv/venv.bzl

@@ -928,12 +928,15 @@ def assemble_venv(
     declared.append(bin_activate)
 
     # _virtualenv.py + _virtualenv.pth — distutils shim for pip interop.
+    # Materialised as a real file, not a symlink into the rules_py source
+    # tree, so tar/OCI/rsync etc. consumers of the venv can resolve the file.
     virtualenv_shim_py_out = ctx.actions.declare_file(
         "{}/_virtualenv.py".format(site_packages_rel),
     )
-    ctx.actions.symlink(
+    ctx.actions.expand_template(
+        template = virtualenv_shim_py,
         output = virtualenv_shim_py_out,
-        target_file = virtualenv_shim_py,
+        substitutions = {},
     )
     declared.append(virtualenv_shim_py_out)
 

py/tests/py-internal-venv/test.py

@@ -15,6 +15,11 @@
 # The virtualenv module should have already been loaded at interpreter startup
 assert "_virtualenv" in sys.modules
 
+# The shim must be a real file inside the venv, not a symlink resolving into
+# the rules_py source tree — symlinked copies dangle outside Bazel (tar/OCI).
+_shim = os.path.realpath(sys.modules["_virtualenv"].__file__)
+assert _shim.endswith("site-packages/_virtualenv.py"), _shim
+
 # Note that we can't assume that a `.runfiles` tree has been created as CI may
 # use a different layout.