test: make venv_tree transition to linux (#1308)

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

### Test plan

- Covered by existing test cases
- Manual testing; run on macos

Author: jbedard

py/private/py_venv/py_venv.bzl

@@ -117,7 +117,6 @@ def _common_providers(ctx, shared, executable = None):
             bin_python = shared.venv.bin_python,
             imports = shared.imports_depset,
             transitive_sources = shared.srcs_depset,
-            all_files = depset(direct = shared.venv.all_files),
         ),
         # `bazel coverage` finds this by walking the consumer's `venv` attr.
         coverage_common.instrumented_files_info(

py/private/py_venv/tests/BUILD.bazel

@@ -37,6 +37,15 @@ genrule(
     tools = [":venv"],
 )
 
+# venv_tree pins the snapshot here so every host regenerates identical bytes.
+platform(
+    name = "snapshot_platform",
+    constraint_values = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+)
+
 venv_tree(
     name = "venv_tree_snap",
     testonly = True,

py/private/py_venv/tests/venv_tree.bzl

@@ -4,15 +4,30 @@ Produces a deterministic text file listing every symlink (with target)
 and text file (with content) inside a ``py_venv`` target's output tree.
 Used via ``write_source_files`` so changes to the venv assembly surface
 as a reviewable diff.
+
+The venv is transitioned to a fixed platform so interpreter repo names
+and pyvenv.cfg contents are identical on every host.
 """
 
 load("//py/private/py_venv:defs.bzl", "VirtualenvInfo")
 
+def _snapshot_platform_transition_impl(_settings, _attr):
+    return {
+        "//command_line_option:platforms": "//py/private/py_venv/tests:snapshot_platform",
+    }
+
+_snapshot_platform_transition = transition(
+    implementation = _snapshot_platform_transition_impl,
+    inputs = [],
+    outputs = ["//command_line_option:platforms"],
+)
+
 def _venv_tree_impl(ctx):
     output = ctx.actions.declare_file(ctx.label.name + ".snap")
-    bin_python = ctx.attr.venv[VirtualenvInfo].bin_python
+    venv = ctx.attr.venv[0]
+    bin_python = venv[VirtualenvInfo].bin_python
     ctx.actions.run_shell(
-        inputs = ctx.attr.venv[VirtualenvInfo].all_files,
+        inputs = venv[DefaultInfo].default_runfiles.files,
         outputs = [output],
         arguments = [output.path, bin_python.path],
         command = r"""
@@ -56,7 +71,11 @@ venv_tree = rule(
         "venv": attr.label(
             providers = [VirtualenvInfo],
             mandatory = True,
+            cfg = _snapshot_platform_transition,
             doc = "A `py_venv` target whose venv tree to snapshot.",
         ),
+        "_allowlist_function_transition": attr.label(
+            default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
+        ),
     },
 )

py/private/py_venv/types.bzl

@@ -18,6 +18,5 @@ binary's launcher exec's the venv's `bin_python`.
         "bin_python": "File — the venv's bin/python symlink. Callers needing a launcher target point here.",
         "imports": "depset[str] — rlocation-root-relative import paths covered by this venv. Mirrors `PyInfo.imports` of the venv's dep closure.",
         "transitive_sources": "depset[File] — first-party Python sources carried by this venv (its own `srcs` plus those of any `deps` that emit `PyInfo`). Surfaced by py_binary as `PyInfo.transitive_sources` so downstream consumers see the same source closure they'd see if srcs/deps lived on the binary directly.",
-        "all_files": "depset[File] — every file and symlink declared by `assemble_venv`, for snapshot/test consumers that need the full venv output set.",
     },
 )