Store generated struct info rather than auto-generating it in the sysroot. NFC (#19013)

I noticed while working `gen_sig_info.py` in #18985 that is was simpler
to just keep a copy of the generated file checked into source control.
We can use a unit test check that its up-to-date on each commit.  In
fact we already had a unit test that was doing this and keeping a copy
of the expected output in the test directory.

This simplifies the emcc linker code since we can always assume the
presence of the struct info file.

Also allows us to remove the `varies=False` special case in cache logic.

Author: sbc100

embuilder.py

@@ -24,7 +24,6 @@
 from tools import ports
 from tools.settings import settings
 from tools.system_libs import USE_NINJA
-import emscripten
 
 
 # Minimal subset of targets used by CI systems to build enough to useful
@@ -61,7 +60,6 @@
     'libsockets',
     'libstubs',
     'libstubs-debug',
-    'struct_info',
     'libstandalonewasm',
     'crt1',
     'crt1_proxy_main',
@@ -150,11 +148,6 @@ def build_port(port_name):
 def get_system_tasks():
   system_libraries = system_libs.Library.get_all_variations()
   system_tasks = list(system_libraries.keys())
-  # This is needed to build the generated_struct_info.json file.
-  # It is not a system library, but it needs to be built before
-  # running with FROZEN_CACHE.
-  system_tasks += ['struct_info']
-
   return system_libraries, system_tasks
 
 
@@ -257,11 +250,6 @@ def main():
         cache.erase_file('sysroot_install.stamp')
       if do_build:
         system_libs.ensure_sysroot()
-    elif what == 'struct_info':
-      if do_clear:
-        emscripten.clear_struct_info()
-      if do_build:
-        emscripten.generate_struct_info()
     elif what in PORTS:
       if do_clear:
         clear_port(what)

emcc.py

@@ -514,7 +514,6 @@ def generate_js_symbols():
   # TODO(sbc): Find a way to optimize this.  Potentially we could add a super-set
   # mode of the js compiler that would generate a list of all possible symbols
   # that could be checked in.
-  emscripten.generate_struct_info()
   _, forwarded_data = emscripten.compile_javascript(symbols_only=True)
   # When running in symbols_only mode compiler.js outputs a flat list of C symbols.
   return json.loads(forwarded_data)

emscripten.py

@@ -20,12 +20,10 @@
 import shutil
 
 from tools import building
-from tools import cache
 from tools import diagnostics
 from tools import js_manipulation
 from tools import shared
 from tools import utils
-from tools import gen_struct_info
 from tools import webassembly
 from tools import extract_metadata
 from tools.utils import exit_with_error, path_from_root
@@ -920,26 +918,5 @@ def normalize_line_endings(text):
   return text
 
 
-def clear_struct_info():
-  output_name = cache.get_lib_name('struct_info.json', varies=False)
-  cache.erase_file(output_name)
-
-
-def generate_struct_info():
-  # If we are running in BOOTSTRAPPING_STRUCT_INFO we don't populate STRUCT_INFO
-  # otherwise that would lead to infinite recursion.
-  if settings.BOOTSTRAPPING_STRUCT_INFO:
-    return
-
-  @ToolchainProfiler.profile()
-  def generate_struct_info(out):
-    gen_struct_info.main(['-q', '-o', out])
-
-  output_name = cache.get_lib_name('struct_info.json', varies=False)
-  settings.STRUCT_INFO = cache.get(output_name, generate_struct_info)
-
-
 def run(in_wasm, out_wasm, outfile_js, memfile):
-  generate_struct_info()
-
   emscript(in_wasm, out_wasm, outfile_js, memfile)

site/source/docs/contributing/developers_guide.rst

@@ -151,10 +151,19 @@ one of them, with the others kept fixed). Doing this will require rebuilding
 locally, which was not needed in the main bisection described in this
 section.
 
+Working with C structs and defines
+==================================
+
+If you change the layout of C structs or modify C defines that are used in
+JavaScript library files you may need to modify ``tools/struct_info.json``.  Any
+time that file is modified or a struct layout is changed you will need to run
+``./tools/gen_struct_info.py`` to re-generate the information used by
+JavaScript.
+
+The ``test_gen_struct_info`` test will fail if you forget to do this.
+
 See also
 ========
 
 - :ref:`Debugging`
 - :ref:`Building-Projects`
-
-