feat: Electron Fuses, package time feature toggles (#24241)

* feat: add new 'fuses' feature for package-time build-flag style feature control

* feat: put ENABLE_RUN_AS_NODE behind a fuse as well

* chore: address PR feedback

* build: move FUSE_EXPORT to headers

* build: use hex codes for kFuseWire char[]

* docs: add fuse wire documentation

* chore: update fuses.json info

* Apply suggestions from code review

Co-authored-by: Jeremy Rose <jeremya@chromium.org>

* chore: add link to fuse schema

* Update shell/app/electron_library_main.mm

Co-authored-by: Jeremy Rose <jeremya@chromium.org>

Co-authored-by: Jeremy Rose <jeremya@chromium.org>

build/fuses/build.py

@@ -0,0 +1,101 @@
+#!/usr/bin/env python3
+
+import json
+import os
+import sys
+
+dir_path = os.path.dirname(os.path.realpath(__file__))
+
+SENTINEL = "dL7pKGdnNz796PbbjQWNKmHXBZaB9tsX"
+
+TEMPLATE_H = """
+#ifndef ELECTRON_FUSES_H_
+#define ELECTRON_FUSES_H_
+
+#if defined(WIN32)
+#define FUSE_EXPORT __declspec(dllexport)
+#else
+#define FUSE_EXPORT __attribute__((visibility("default")))
+#endif
+
+namespace electron {
+
+namespace fuses {
+
+extern const volatile char kFuseWire[];
+
+{getters}
+
+}  // namespace fuses
+
+}  // namespace electron
+
+#endif  // ELECTRON_FUSES_H_
+"""
+
+TEMPLATE_CC = """
+#include "electron/fuses.h"
+
+namespace electron {
+
+namespace fuses {
+
+const volatile char kFuseWire[] = { /* sentinel */ {sentinel}, /* fuse_version */ {fuse_version}, /* fuse_wire_length */ {fuse_wire_length}, /* fuse_wire */ {initial_config}};
+
+{getters}
+
+}
+
+}
+"""
+
+with open(os.path.join(dir_path, "fuses.json"), 'r') as f:
+  fuse_defaults = json.load(f)
+
+fuse_version = fuse_defaults['_version']
+del fuse_defaults['_version']
+del fuse_defaults['_schema']
+del fuse_defaults['_comment']
+
+if fuse_version >= pow(2, 8):
+  raise Exception("Fuse version can not exceed one byte in size")
+
+fuses = fuse_defaults.keys()
+
+initial_config = ""
+getters_h = ""
+getters_cc = ""
+index = len(SENTINEL) + 1
+for fuse in fuses:
+  index += 1
+  initial_config += fuse_defaults[fuse]
+  name = ''.join(word.title() for word in fuse.split('_'))
+  getters_h += "FUSE_EXPORT bool Is{name}Enabled();\n".replace("{name}", name)
+  getters_cc += """
+bool Is{name}Enabled() {
+  return kFuseWire[{index}] == '1';
+}
+""".replace("{name}", name).replace("{index}", str(index))
+
+def c_hex(n):
+  s = hex(n)[2:]
+  return "0x" + s.rjust(2, '0')
+
+def hex_arr(s):
+  arr = []
+  for char in s:
+    arr.append(c_hex(ord(char)))
+  return ",".join(arr)
+
+header = TEMPLATE_H.replace("{getters}", getters_h.strip())
+impl = TEMPLATE_CC.replace("{sentinel}", hex_arr(SENTINEL))
+impl = impl.replace("{fuse_version}", c_hex(fuse_version))
+impl = impl.replace("{fuse_wire_length}", c_hex(len(fuses)))
+impl = impl.replace("{initial_config}", hex_arr(initial_config))
+impl = impl.replace("{getters}", getters_cc.strip())
+
+with open(sys.argv[1], 'w') as f:
+  f.write(header)
+
+with open(sys.argv[2], 'w') as f:
+  f.write(impl)