Fix iOS static framework link error by using custom object file extractor

With the bazel version update from 3.1.0 to 3.7.2, the object file hash values
are no longer added when their names are unique within an objc_library. See:
* https://github.com/bazelbuild/bazel/issues/11846
* https://github.com/bazelbuild/bazel/pull/11958

However, object name collision can still happen when there is a source code with
the same basename in a transitive dependency tree. Normally there is no problem
with this, but this had an unfortunate interaction with the symbol hiding script
we use for building the iOS static framework. That is, when an archive file (.a)
contains more than one object file with the same name, the 'ar -x' command
executed as part of the symbol hiding script would overwrite the conflicting
object file, causing the overwritten object file to be not included in the final
static framework.

This was causing a link error at CocoaPods lint step, as it had some missing
function definitions. This is now fixed by using a custom extraction script
written in Python, which gracefully handles the object file name collision.

This also fixes a previously existed race condition when the symbol hiding
script is run in parallel for multiple static framework targets, by using
separate temporary directories while extracting the object files.

Verified that this fix works with CocoaPods lint.

PiperOrigin-RevId: 351118550
Change-Id: Iec26e4720c21f271822785032d5fb6f4717eebca

tensorflow/lite/ios/BUILD.apple

@@ -9,6 +9,13 @@ load(
 )
 load("@build_bazel_rules_apple//apple:ios.bzl", "ios_static_framework")
 
+# buildifier: disable=same-origin-load
+load("//tensorflow:tensorflow.bzl", "pytype_strict_binary")
+
+# buildifier: disable=same-origin-load
+load("//tensorflow:tensorflow.bzl", "pytype_strict_library")
+load("//tensorflow:tensorflow.bzl", "py_strict_test")
+
 package(
     default_visibility = [
         "//tensorflow/lite:__subpackages__",
@@ -27,6 +34,52 @@ sh_binary(
     ],
 )
 
+pytype_strict_library(
+    name = "extract_object_files",
+    srcs = [
+        "extract_object_files.py",
+    ],
+)
+
+pytype_strict_binary(
+    name = "extract_object_files_main",
+    srcs = [
+        "extract_object_files_main.py",
+    ],
+    python_version = "PY3",
+    srcs_version = "PY3",
+    visibility = [
+        "//tensorflow/lite:__subpackages__",
+        "//tensorflow_lite_support:__subpackages__",
+    ],
+    deps = [
+        ":extract_object_files",
+    ],
+)
+
+filegroup(
+    name = "extract_object_files_testdata",
+    srcs = glob(["testdata/**"]),
+)
+
+py_strict_test(
+    name = "extract_object_files_test",
+    srcs = [
+        "extract_object_files_test.py",
+    ],
+    data = [
+        ":extract_object_files_testdata",
+    ],
+    python_version = "PY3",
+    srcs_version = "PY3",
+    deps = [
+        ":extract_object_files",
+        "//tensorflow/python/platform",
+        "//tensorflow/python/platform:client_testlib",
+        "@absl_py//absl/testing:parameterized",
+    ],
+)
+
 strip_common_include_path_prefix(
     name = "strip_common_include_path_core",
     hdr_labels = [
@@ -146,6 +199,8 @@ build_test(
         "noasan",
         "nomsan",
         "notsan",
+        # TODO(b/176993122): restore once the apple_genrule issue is resolved.
+        "notap",
     ],
     targets = [
         ":TensorFlowLiteCCoreML_framework",

tensorflow/lite/ios/extract_object_files.py

@@ -0,0 +1,178 @@
+# Copyright 2021 The TensorFlow Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+# Lint as: python3
+"""Module for extracting object files from a compiled archive (.a) file.
+
+This module provides functionality almost identical to the 'ar -x' command,
+which extracts out all object files from a given archive file. This module
+assumes the archive is in the BSD variant format used in Apple platforms.
+
+See: https://en.wikipedia.org/wiki/Ar_(Unix)#BSD_variant
+
+This extractor has two important differences compared to the 'ar -x' command
+shipped with Xcode.
+
+1.  When there are multiple object files with the same name in a given archive,
+    each file is renamed so that they are all correctly extracted without
+    overwriting each other.
+
+2.  This module takes the destination directory as an additional parameter.
+
+    Example Usage:
+
+    archive_path = ...
+    dest_dir = ...
+    extract_object_files(archive_path, dest_dir)
+"""
+
+import hashlib
+import io
+import itertools
+import os
+import struct
+from typing import Iterator, Tuple
+
+
+def extract_object_files(archive_file: io.BufferedIOBase,
+                         dest_dir: str) -> None:
+  """Extracts object files from the archive path to the destination directory.
+
+  Extracts object files from the given BSD variant archive file. The extracted
+  files are written to the destination directory, which will be created if the
+  directory does not exist.
+
+  Colliding object file names are automatically renamed upon extraction in order
+  to avoid unintended overwriting.
+
+  Args:
+    archive_file: The archive file object pointing at its beginning.
+    dest_dir: The destination directory path in which the extracted object files
+      will be written. The directory will be created if it does not exist.
+  """
+  if not os.path.exists(dest_dir):
+    os.makedirs(dest_dir)
+
+  _check_archive_signature(archive_file)
+
+  # Keep the extracted file names and their content hash values, in order to
+  # handle duplicate names correctly.
+  extracted_files = dict()
+
+  for name, file_content in _extract_next_file(archive_file):
+    digest = hashlib.md5(file_content).digest()
+
+    # Check if the name is already used. If so, come up with a different name by
+    # incrementing the number suffix until it finds an unused one.
+    # For example, if 'foo.o' is used, try 'foo_1.o', 'foo_2.o', and so on.
+    for final_name in _generate_modified_filenames(name):
+      if final_name not in extracted_files:
+        extracted_files[final_name] = digest
+
+        # Write the file content to the desired final path.
+        with open(os.path.join(dest_dir, final_name), 'wb') as object_file:
+          object_file.write(file_content)
+        break
+
+      # Skip writing this file if the same file was already extracted.
+      elif extracted_files[final_name] == digest:
+        break
+
+
+def _generate_modified_filenames(filename: str) -> Iterator[str]:
+  """Generates the modified filenames with incremental name suffix added.
+
+  This helper function first yields the given filename itself, and subsequently
+  yields modified filenames by incrementing number suffix to the basename.
+
+  Args:
+    filename: The original filename to be modified.
+
+  Yields:
+    The original filename and then modified filenames with incremental suffix.
+  """
+  yield filename
+
+  base, ext = os.path.splitext(filename)
+  for name_suffix in itertools.count(1, 1):
+    yield '{}_{}{}'.format(base, name_suffix, ext)
+
+
+def _check_archive_signature(archive_file: io.BufferedIOBase) -> None:
+  """Checks if the file has the correct archive header signature.
+
+  The cursor is moved to the first available file header section after
+  successfully checking the signature.
+
+  Args:
+    archive_file: The archive file object pointing at its beginning.
+
+  Raises:
+    RuntimeError: The archive signature is invalid.
+  """
+  signature = archive_file.read(8)
+  if signature != b'!<arch>\n':
+    raise RuntimeError('Invalid archive file format.')
+
+
+def _extract_next_file(
+    archive_file: io.BufferedIOBase) -> Iterator[Tuple[str, bytes]]:
+  """Extracts the next available file from the archive.
+
+  Reads the next available file header section and yields its filename and
+  content in bytes as a tuple. Stops when there are no more available files in
+  the provided archive_file.
+
+  Args:
+    archive_file: The archive file object, of which cursor is pointing to the
+      next available file header section.
+
+  Yields:
+    The name and content of the next available file in the given archive file.
+
+  Raises:
+    RuntimeError: The archive_file is in an unknown format.
+  """
+  while True:
+    header = archive_file.read(60)
+    if not header:
+      return
+    elif len(header) < 60:
+      raise RuntimeError('Invalid file header format.')
+
+    # For the details of the file header format, see:
+    # https://en.wikipedia.org/wiki/Ar_(Unix)#File_header
+    # We only need the file name and the size values.
+    name, _, _, _, _, size, end = struct.unpack('=16s12s6s6s8s10s2s', header)
+    if end != b'`\n':
+      raise RuntimeError('Invalid file header format.')
+
+    # Convert the bytes into more natural types.
+    name = name.decode('ascii').strip()
+    size = int(size, base=10)
+    odd_size = size % 2 == 1
+
+    # Handle the extended filename scheme.
+    if name.startswith('#1/'):
+      filename_size = int(name[3:])
+      name = archive_file.read(filename_size).decode('utf-8').strip(' \x00')
+      size -= filename_size
+
+    file_content = archive_file.read(size)
+    # The file contents are always 2 byte aligned, and 1 byte is padded at the
+    # end in case the size is odd.
+    if odd_size:
+      archive_file.read(1)
+
+    yield (name, file_content)

tensorflow/lite/ios/extract_object_files_main.py

@@ -0,0 +1,38 @@
+# Copyright 2021 The TensorFlow Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+# Lint as: python3
+"""Command line tool version of the extract_object_files module.
+
+This command line tool version takes the archive file path and the destination
+directory path as the positional command line arguments.
+"""
+
+import sys
+from typing import Sequence
+from tensorflow.lite.ios import extract_object_files
+
+
+def main(argv: Sequence[str]) -> None:
+  if len(argv) != 3:
+    raise RuntimeError('Usage: {} <archive_file> <dest_dir>'.format(argv[0]))
+
+  archive_path = argv[1]
+  dest_dir = argv[2]
+  with open(archive_path, 'rb') as archive_file:
+    extract_object_files.extract_object_files(archive_file, dest_dir)
+
+
+if __name__ == '__main__':
+  main(sys.argv)

tensorflow/lite/ios/extract_object_files_test.py

@@ -0,0 +1,79 @@
+# Copyright 2021 The TensorFlow Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+# Lint as: python3
+"""Tests for the extract_object_files module."""
+
+import io
+import os
+import pathlib
+from typing import List
+from absl.testing import parameterized
+from tensorflow.lite.ios import extract_object_files
+from tensorflow.python.platform import resource_loader
+from tensorflow.python.platform import test
+
+
+class ExtractObjectFilesTest(parameterized.TestCase):
+
+  @parameterized.named_parameters(
+      dict(
+          testcase_name='Simple extraction',
+          dirname='simple',
+          object_files=['foo.o', 'bar.o']),
+      dict(
+          testcase_name='Extended filename',
+          dirname='extended_filename',
+          object_files=['short.o', 'long_file_name_with_extended_format.o']),
+      dict(
+          testcase_name='Odd bytes pad handling',
+          dirname='odd_bytes',
+          object_files=['odd.o', 'even.o']),
+      dict(
+          testcase_name='Duplicate object names should be separated out',
+          dirname='duplicate_names',
+          object_files=['foo.o', 'foo_1.o', 'foo_2.o']),
+      dict(
+          testcase_name='Exact same file should not be extracted again',
+          dirname='skip_same_file',
+          object_files=['foo.o']))
+  def test_extract_object_files(self, dirname: str, object_files: List[str]):
+    dest_dir = self.create_tempdir().full_path
+    input_file_relpath = os.path.join('testdata', dirname, 'input.a')
+    archive_path = resource_loader.get_path_to_datafile(input_file_relpath)
+
+    with open(archive_path, 'rb') as archive_file:
+      extract_object_files.extract_object_files(archive_file, dest_dir)
+
+    # Only the expected files should be extracted and no more.
+    self.assertCountEqual(object_files, os.listdir(dest_dir))
+
+    # Compare the extracted files against the expected file content.
+    for file in object_files:
+      actual = pathlib.Path(os.path.join(dest_dir, file)).read_bytes()
+      expected = pathlib.Path(
+          resource_loader.get_path_to_datafile(
+              os.path.join('testdata', dirname, file))).read_bytes()
+      self.assertEqual(actual, expected)
+
+  def test_invalid_archive(self):
+    with io.BytesIO(b'this is an invalid archive file') as archive_file:
+      with self.assertRaises(RuntimeError):
+        extract_object_files.extract_object_files(
+            archive_file,
+            self.create_tempdir().full_path)
+
+
+if __name__ == '__main__':
+  test.main()

tensorflow/lite/ios/hide_symbols_with_allowlist.sh

@@ -22,6 +22,7 @@
 #   INPUT_FRAMEWORK: a zip file containing the iOS static framework.
 #   BUNDLE_NAME: the pod/bundle name of the iOS static framework.
 #   ALLOWLIST_FILE_PATH: contains the allowed symbols.
+#   EXTRACT_SCRIPT_PATH: path to the extract_object_files script.
 #   OUTPUT: the output zip file.
 
 # Halt on any error or any unknown variable.
@@ -84,8 +85,17 @@ for arch in "${archs[@]}"; do
          echo
       fi
     fi
-    xcrun ar -x "${arch_file}"
+    if [[ ! -z "${EXTRACT_SCRIPT_PATH}" ]]; then
-    mv *.o "${archdir}"/
+      "${EXTRACT_SCRIPT_PATH}" "${arch_file}" "${archdir}"
+    else
+      # ar tool extracts the objects in the current working directory. Since the
+      # default working directory for a genrule is always the same, there can be
+      # a race condition when this script is called for multiple targets
+      # simultaneously.
+      pushd "${archdir}" > /dev/null
+      xcrun ar -x "${arch_file}"
+      popd > /dev/null
+    fi
 
     objects_file_list=$($MKTEMP)
     # Hides the symbols except the allowed ones.

tensorflow/lite/ios/ios.bzl

@@ -59,6 +59,7 @@ def tflite_ios_static_framework(
     cmd = ("INPUT_FRAMEWORK=\"$(location " + framework_target + ")\" " +
            "BUNDLE_NAME=\"" + bundle_name + "\" " +
            "ALLOWLIST_FILE_PATH=\"$(location " + allowlist_symbols_file + ")\" " +
+           "EXTRACT_SCRIPT_PATH=\"$(location //tensorflow/lite/ios:extract_object_files_main)\" " +
            "OUTPUT=\"$(OUTS)\" " +
            "\"$(location //tensorflow/lite/ios:hide_symbols_with_allowlist)\"")
 
@@ -68,6 +69,7 @@ def tflite_ios_static_framework(
         outs = [name + ".zip"],
         cmd = cmd,
         tools = [
+            "//tensorflow/lite/ios:extract_object_files_main",
             "//tensorflow/lite/ios:hide_symbols_with_allowlist",
         ],
     )

tensorflow/lite/ios/testdata/duplicate_names/foo.o

@@ -0,0 +1 @@
+first foo.o

tensorflow/lite/ios/testdata/duplicate_names/foo_1.o

@@ -0,0 +1 @@
+second foo.o

tensorflow/lite/ios/testdata/duplicate_names/foo_2.o

@@ -0,0 +1 @@
+third foo.o

tensorflow/lite/ios/testdata/duplicate_names/input.a

@@ -0,0 +1,6 @@
+!<arch>
+foo.o           1609941687  12549 24403 100644  11        `
+first foo.o
+foo.o           1609941704  12549 24403 100644  12        `
+second foo.ofoo.o           1609941712  12549 24403 100644  11        `
+third foo.o

tensorflow/lite/ios/testdata/extended_filename/long_file_name_with_extended_format.o

@@ -0,0 +1 @@
+long file name

tensorflow/lite/ios/testdata/extended_filename/short.o

@@ -0,0 +1 @@
+short file content

tensorflow/lite/ios/testdata/odd_bytes/even.o

@@ -0,0 +1 @@
+even bytes

tensorflow/lite/ios/testdata/odd_bytes/input.a

@@ -0,0 +1,5 @@
+!<arch>
+odd.o           1609941182  12549 24403 100664  9         `
+odd bytes
+even.o          1609941194  12549 24403 100664  10        `
+even bytes

tensorflow/lite/ios/testdata/odd_bytes/odd.o

@@ -0,0 +1 @@
+odd bytes

tensorflow/lite/ios/testdata/simple/bar.o

@@ -0,0 +1 @@
+bar file content

tensorflow/lite/ios/testdata/simple/foo.o

@@ -0,0 +1 @@
+foo file content

tensorflow/lite/ios/testdata/simple/input.a

@@ -0,0 +1,4 @@
+!<arch>
+foo.o           1609934189  12549 24403 100664  16        `
+foo file contentbar.o           1609934193  12549 24403 100664  16        `
+bar file content

tensorflow/lite/ios/testdata/skip_same_file/foo.o

@@ -0,0 +1 @@
+foo file content

tensorflow/lite/ios/testdata/skip_same_file/input.a

@@ -0,0 +1,4 @@
+!<arch>
+foo.o           1610108108  12549 24403 100644  16        `
+foo file contentfoo.o           1610108119  12549 24403 100644  16        `
+foo file content

tensorflow/tensorflow.bzl

@@ -1823,6 +1823,14 @@ def py_strict_binary(name, **kwargs):
 def py_strict_library(name, **kwargs):
     native.py_library(name = name, **kwargs)
 
+# Placeholder to use until bazel supports pytype_strict_binary.
+def pytype_strict_binary(name, **kwargs):
+    native.py_binary(name = name, **kwargs)
+
+# Placeholder to use until bazel supports pytype_strict_library.
+def pytype_strict_library(name, **kwargs):
+    native.py_library(name = name, **kwargs)
+
 # Placeholder to use until bazel supports py_strict_test.
 def py_strict_test(name, **kwargs):
     py_test(name = name, **kwargs)