refactor: defer zip manifest building to execution phase to improve analysis phase performance (#3381)

When py_binary/py_test were being built, they were flattening the
runfiles
depsets at analysis time in order to create the zip file mapping
manifest for
their implicit zipapp outputs. This flattening was necessary because
they had
to filter out the original main executable from the runfiles that didn't
belong
in the zipapp. This flattening is expensive for large builds, in some
cases
adding over 400 seconds of time and significant memory overhead.

To fix, have the zip file manifest use the `runfiles_with_exe` object,
which is
the runfiles, but pre-filtered for the files zip building doesn't want.
This
then allows passing the depsets directly to `Args.add_all` and using
map_each
to transform them.

Additionally, pass `runfiles.empty_filenames` using a lambda. Accessing
that
attribute implicitly flattens the runfiles.

Finally, because the original profiles indicated `str.format()` was a
non-trivial
amount of time (46 seconds / 15% of build time), switch to using `+`
instead.

This is a more incremental alternative to #3380 which achieves _most_ of
the
same optimization with only Starlark changes, as opposed to introducing
an
external script written in C++.

[Profile of a large
build](https://github.com/user-attachments/assets/e90ae699-a04d-44df-b53c-1156aa890af5),
which shows a Starlark CPU profile. It shows an overall build
time of 305 seconds. 46 seconds (15%) are spent in `map_zip_runfiles`,
half of which
is in `str.startswith()` and the other half in `str.format()`.

---------

Co-authored-by: Richard Levasseur <[email protected]>

Author: tobyh-canva

CHANGELOG.md

@@ -73,6 +73,8 @@ END_UNRELEASED_TEMPLATE
   ([#3085](https://github.com/bazel-contrib/rules_python/issues/3085)).
 * (toolchains) local toolchains now tell the `sys.abiflags` value of the
   underlying runtime.
+* (performance) 90% reduction in py_binary/py_test analysis phase cost.
+  ([#3381](https://github.com/bazel-contrib/rules_python/pull/3381)).
 
 {#v0-0-0-added}
 ### Added

python/private/py_executable.bzl

@@ -380,9 +380,8 @@ def _create_executable(
     _create_zip_file(
         ctx,
         output = zip_file,
-        original_nonzip_executable = executable,
         zip_main = zip_main,
-        runfiles = runfiles_details.default_runfiles.merge(extra_runfiles),
+        runfiles = runfiles_details.runfiles_without_exe.merge(extra_runfiles),
     )
 
     extra_files_to_build = []
@@ -803,7 +802,7 @@ def _create_windows_exe_launcher(
         use_default_shell_env = True,
     )
 
-def _create_zip_file(ctx, *, output, original_nonzip_executable, zip_main, runfiles):
+def _create_zip_file(ctx, *, output, zip_main, runfiles):
     """Create a Python zipapp (zip with __main__.py entry point)."""
     workspace_name = ctx.workspace_name
     legacy_external_runfiles = _py_builtins.get_legacy_external_runfiles(ctx)
@@ -819,17 +818,27 @@ def _create_zip_file(ctx, *, output, original_nonzip_executable, zip_main, runfi
             _get_zip_runfiles_path("__init__.py", workspace_name, legacy_external_runfiles),
         ),
     )
-    for path in runfiles.empty_filenames.to_list():
-        manifest.add("{}=".format(_get_zip_runfiles_path(path, workspace_name, legacy_external_runfiles)))
+
+    def map_zip_empty_filenames(list_paths_cb):
+        return [
+            _get_zip_runfiles_path(path, workspace_name, legacy_external_runfiles) + "="
+            for path in list_paths_cb().to_list()
+        ]
+
+    manifest.add_all(
+        # NOTE: Accessing runfiles.empty_filenames implicitly flattens the runfiles.
+        # Smuggle a lambda in via a list to defer that flattening.
+        [lambda: runfiles.empty_filenames],
+        map_each = map_zip_empty_filenames,
+        allow_closure = True,
+    )
 
     def map_zip_runfiles(file):
-        if file != original_nonzip_executable and file != output:
-            return "{}={}".format(
-                _get_zip_runfiles_path(file.short_path, workspace_name, legacy_external_runfiles),
-                file.path,
-            )
-        else:
-            return None
+        return (
+            # NOTE: Use "+" for performance
+            _get_zip_runfiles_path(file.short_path, workspace_name, legacy_external_runfiles) +
+            "=" + file.path
+        )
 
     manifest.add_all(runfiles.files, map_each = map_zip_runfiles, allow_closure = True)
 
@@ -850,37 +859,37 @@ def _create_zip_file(ctx, *, output, original_nonzip_executable, zip_main, runfi
         ))
         inputs.append(zip_repo_mapping_manifest)
 
-    for artifact in runfiles.files.to_list():
-        # Don't include the original executable because it isn't used by the
-        # zip file, so no need to build it for the action.
-        # Don't include the zipfile itself because it's an output.
-        if artifact != original_nonzip_executable and artifact != output:
-            inputs.append(artifact)
-
     zip_cli_args = ctx.actions.args()
     zip_cli_args.add("cC")
     zip_cli_args.add(output)
 
     ctx.actions.run(
         executable = ctx.executable._zipper,
         arguments = [zip_cli_args, manifest],
-        inputs = depset(inputs),
+        inputs = depset(inputs, transitive = [runfiles.files]),
         outputs = [output],
         use_default_shell_env = True,
         mnemonic = "PythonZipper",
         progress_message = "Building Python zip: %{label}",
     )
 
 def _get_zip_runfiles_path(path, workspace_name, legacy_external_runfiles):
+    maybe_workspace = ""
     if legacy_external_runfiles and path.startswith(_EXTERNAL_PATH_PREFIX):
-        zip_runfiles_path = paths.relativize(path, _EXTERNAL_PATH_PREFIX)
+        zip_runfiles_path = path.removeprefix(_EXTERNAL_PATH_PREFIX)
     else:
         # NOTE: External runfiles (artifacts in other repos) will have a leading
         # path component of "../" so that they refer outside the main workspace
-        # directory and into the runfiles root. By normalizing, we simplify e.g.
+        # directory and into the runfiles root. So we simplify it, e.g.
         # "workspace/../foo/bar" to simply "foo/bar".
-        zip_runfiles_path = paths.normalize("{}/{}".format(workspace_name, path))
-    return "{}/{}".format(_ZIP_RUNFILES_DIRECTORY_NAME, zip_runfiles_path)
+        if path.startswith("../"):
+            zip_runfiles_path = path[3:]
+        else:
+            zip_runfiles_path = path
+            maybe_workspace = workspace_name + "/"
+
+    # NOTE: Use "+" for performance
+    return _ZIP_RUNFILES_DIRECTORY_NAME + "/" + maybe_workspace + zip_runfiles_path
 
 def _create_executable_zip_file(
         ctx,