add a wasm bazel toolchain (#603)

bazel/emscripten_toolchain/link_wrapper.py

@@ -0,0 +1,146 @@
+#!/usr/bin/env python
+"""wrapper around emcc link step.
+
+This wrapper currently serves the following purposes.
+
+1. Ensures we always link to file with .js extension. The upstream default
+   it to link to an llvm bitcode file which is never (AFAICT) want to do that.
+
+2. When building with --config=wasm the final output is multiple files, usually
+   at least one .js and one .wasm file. Since the cc_binary link step only
+   allows a single output, we must tar up the outputs into a single file.
+
+3. Add quotes around arguments that need them in the response file to work
+   around a bazel quirk.
+"""
+
+from __future__ import print_function
+
+import os
+import subprocess
+import sys
+
+# Only argument should be @path/to/parameter/file
+assert sys.argv[1][0] == '@'
+param_filename = sys.argv[1][1:]
+param_file_args = [l.strip() for l in open(param_filename, 'r').readlines()]
+
+output_index = param_file_args.index('-o') + 1
+orig_output = js_output = param_file_args[output_index]
+outdir = os.path.dirname(orig_output)
+
+# google3-only(TODO(b/139440956): Default to False once the bug is fixed)
+replace_response_file = any(' ' in a for a in param_file_args)
+
+if not os.path.splitext(orig_output)[1]:
+  js_output = orig_output + '.js'
+  param_file_args[output_index] = js_output
+  replace_response_file = True
+
+# Re-write response file if needed.
+if replace_response_file:
+  new_param_filename = param_filename + '.modified'
+  with open(new_param_filename, 'w') as f:
+    for param in param_file_args:
+      if ' ' in param:
+        f.write('"%s"' % param)
+      else:
+        f.write(param)
+      f.write('\n')
+  sys.argv[1] = '@' + new_param_filename
+
+emcc_py = os.path.join(os.environ['EMSCRIPTEN'], 'emcc.py')
+rtn = subprocess.call(['python3', emcc_py] + sys.argv[1:])
+if rtn != 0:
+  sys.exit(1)
+
+js_name = os.path.basename(js_output)
+base_name = os.path.splitext(js_name)[0]
+
+files = []
+extensions = [
+    '.js',
+    '.wasm',
+    '.wasm.map',
+    '.js.mem',
+    '.fetch.js',
+    '.worker.js',
+    '.data',
+    '.js.symbols',
+    '.wasm.debug.wasm'
+]
+
+for ext in extensions:
+  filename = base_name + ext
+  if os.path.exists(os.path.join(outdir, filename)):
+    files.append(filename)
+
+wasm_base = os.path.join(outdir, base_name + '.wasm')
+if os.path.exists(wasm_base + '.debug.wasm') and os.path.exists(wasm_base):
+  # If we have a .wasm.debug.wasm file and a .wasm file, we need to rewrite the
+  # section in the .wasm file that refers to it. The path that's in there
+  # is the blaze output path; we want it to be just the filename.
+
+  llvm_objcopy = os.path.join(
+      os.environ['EMSCRIPTEN'], 'llvm-bin/llvm-objcopy')
+  # First, check to make sure the .wasm file has the header that needs to be
+  # rewritten.
+  rtn = subprocess.call([
+      llvm_objcopy,
+      '--dump-section=external_debug_info=/dev/null',
+      wasm_base], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+  if rtn == 0:
+    # If llvm-objcopy did not return an error, the external_debug_info section
+    # must exist, so we're good to continue.
+
+    # Next we need to convert length of the filename to LEB128.
+    # Start by converting the length of the filename to a bit string.
+    bit_string = '{0:b}'.format(len(base_name + '.wasm.debug.wasm'))
+
+    # Pad the bit string with 0s so that its length is a multiple of 7.
+    while len(bit_string) % 7 != 0:
+      bit_string = '0' + bit_string
+
+    # Break up our bit string into chunks of 7.
+    # We do this backwards because the final format is little-endian.
+    final_bytes = bytearray()
+    for i in reversed(range(0, len(bit_string), 7)):
+      binary_part = bit_string[i:i + 7]
+      if i != 0:
+        # Every chunk except the last one needs to be prepended with '1'.
+        # The length of each chunk is 7, so that one has an implicit '0'.
+        binary_part = '1' + binary_part
+      final_bytes.append(int(binary_part, 2))
+    # Finally, add the actual filename.
+    final_bytes.extend(base_name + '.wasm.debug.wasm')
+
+    # Write our length + filename bytes to a temp file.
+    with open('debugsection.tmp', 'wb+') as f:
+      f.write(final_bytes)
+      f.close()
+
+    # First delete the old section.
+    subprocess.check_call([
+        llvm_objcopy,
+        wasm_base,
+        '--remove-section=external_debug_info'])
+    # Rewrite section with the new size and filename from the temp file.
+    subprocess.check_call([
+        llvm_objcopy,
+        wasm_base,
+        '--add-section=external_debug_info=debugsection.tmp'])
+
+# If we have more than one output file then create tarball
+if len(files) > 1:
+  cmd = ['tar', 'cf', 'tmp.tar'] + files
+  subprocess.check_call(cmd, cwd=outdir)
+  os.rename(os.path.join(outdir, 'tmp.tar'), orig_output)
+elif len(files) == 1:
+  # Otherwise, if only have a single output than move it to the expected name
+  if files[0] != os.path.basename(orig_output):
+    os.rename(os.path.join(outdir, files[0]), orig_output)
+else:
+  print('emcc.py did not appear to output any known files!')
+  sys.exit(1)
+
+sys.exit(0)