[Cronet] Build static libcronet.a for iOS with complete dependencies.

components/cronet/tools/hide_symbols.py

Index: components/cronet/tools/hide_symbols.py
diff --git a/components/cronet/tools/hide_symbols.py b/components/cronet/tools/hide_symbols.py
new file mode 100755
index 0000000000000000000000000000000000000000..7f41e48b38e982fa847df625f54287ecf801ca5b
--- /dev/null
+++ b/components/cronet/tools/hide_symbols.py
@@ -0,0 +1,103 @@
+#!/usr/bin/env python
+
+# Copyright 2017 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Create a static library which exposes only symbols which are explicitly marked
+# as visible e.g., by __attribute__((visibility("default"))).
+#
+# See BUILD.gn in this directory for usage example.
+#
+# This way, we can reduce risk of symbol conflict when linking it into apps
+# by exposing internal symbols, especially in third-party libraries.
+
+import optparse
+import os
+import subprocess
+
+
+# Mapping from GN's target_cpu attribute to ld's -arch parameter.
+# Taken from the definition of config("compiler") in:
+#   //build/config/mac/BUILD.gn
+GN_CPU_TO_LD_ARCH = {
+  'x64': 'x86_64',
+  'x86': 'i386',
+  'armv7': 'armv7',
+  'arm': 'armv7',
+  'arm64': 'arm64',
+}
+
+
+def main():
+  parser = optparse.OptionParser()
+  parser.add_option(
+      '--input_libs',
+      help='Comma-separated paths to input .a files which contain symbols '
+           'which must be always linked.')
+  parser.add_option(
+      '--deps_lib',
+      help='The path to a complete static library (.a file) which contains all '
+           'dependencies of --input_libs. .o files in this library are also '
+           'added to the output library, but only if they are referred from '
+           '--input_libs.')
+  parser.add_option(
+      '--output_obj',
+      help='Outputs the generated .o file here. This is an intermediate file.')
+  parser.add_option(
+      '--output_lib',
+      help='Outputs the generated .a file here.')
+  parser.add_option(
+      '--current_cpu',
+      help='The current processor architecture in the format of the target_cpu '
+           'attribute in GN.')
+  (options, args) = parser.parse_args()
+  assert not args
+
+  # ld -r concatenates multiple .o files and .a files into a single .o file,
+  # while "hiding" symbols not marked as visible.
+  command = [
+    'xcrun', 'ld',
+    '-arch', GN_CPU_TO_LD_ARCH[options.current_cpu],
+    '-r',
+  ]
+  for input_lib in options.input_libs.split(','):
+    # By default, ld only pulls .o files out of a static library if needed to
+    # resolve some symbol reference. We apply -force_load option to input_lib
+    # (but not to deps_lib) to force pulling all .o files.
+    command += ['-force_load', input_lib]
+  command += [
+    options.deps_lib,
+    '-o', options.output_obj
+  ]
+  try:
+    subprocess.check_call(command)
+  except subprocess.CalledProcessError:
+    # Work around LD failure for x86 Debug buiilds when it fails with error:
+    # ld: scattered reloc r_address too large for architecture i386
+    if options.current_cpu == "x86":
+      # Copy input lib into output lib without dependencies.
+      # TODO(mef): Consider including dependencies without hiding them.
+      command = [
+        'xcrun', 'lipo',
+        '-arch', 'i386', options.input_libs,

[kapishnikov, 2017/05/22 17:15:57]

Should "options.deps_lib" be also included?

[mef, 2017/05/22 20:53:53]

Maybe. I'm getting cryptic error though:
fatal error:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/lipo:
obj/components/cronet/ios/libcronet_static.a and
obj/components/cronet/ios/libcronet_deps_complete.a have the same architectures
(i386) and can't be in the same fat output file

[Hiroshi Ichikawa, 2017/05/22 23:42:36]

Lipo is a command to combine libraries in mulitple architectures into a single
fat binary. And it reports the error probably because it doesn't have a feature
to combine multiple libraries in the same architecture, which is the role of
"ld" command.

If the goal here is to combine input_libs and deps_lib without hiding the
symbols, you can probably use "ld" command in the similar way as above, but
without "-r" parameter, and specify output_lib instead of output_obj as an
output:

  59   command = [
  60     'xcrun', 'ld',
  61     '-arch', GN_CPU_TO_LD_ARCH[options.current_cpu],
  63   ]
  64   for input_lib in options.input_libs.split(','):
  65     # By default, ld only pulls .o files out of a static library if needed
to
  66     # resolve some symbol reference. We apply -force_load option to
input_lib
  67     # (but not to deps_lib) to force pulling all .o files.
  68     command += ['-force_load', input_lib]
  69   command += [
  70     options.deps_lib,
  71     '-o', options.output_lib
  72   ]

[mef, 2017/05/23 20:56:36]

Great point! I've used libtool to merge libcronet_static.a and
libcronet_deps_complete.a and it appears to produce the right output.
The size of combined library is ~780MB, which could cause problems further down
the line, so we may want to strip some of the symbols. 

[Hiroshi Ichikawa, 2017/05/24 05:10:23]

In the ld command above, it automatically strips unused object files in deps_lib
(but not in input_libs), by applying -force_load option to input_libs but not to
deps_lib. Not really sure what happens with your command line using libtools,
but maybe it doesn't strip any object files?

Anyway I'm fine leaving it as is for now because it may not matter. But you may
want to leave TODO for it.

+        '-output', options.output_lib,
+        '-create',
+      ]
+      subprocess.check_call(command)
+      return
+
+  if os.path.exists(options.output_lib):
+    os.remove(options.output_lib)
+
+  # Creates a .a file which contains a single .o file.
+  command = [
+    'xcrun', 'ar', '-r',
+    options.output_lib,
+    options.output_obj,
+  ]
+  subprocess.check_call(command)
+
+
+if __name__ == "__main__":
+  main()