Add optional public header checking to GN build

tools/gn/variables.cc

Index: tools/gn/variables.cc
diff --git a/tools/gn/variables.cc b/tools/gn/variables.cc
index 40f60aa525f58ef9b279389df5d6f5a1db5e00a4..8e41e62cbe5ff40aae006202f931939ab42ac022 100644
--- a/tools/gn/variables.cc
+++ b/tools/gn/variables.cc
@@ -678,6 +678,45 @@ const char kOutputs_Help[] =
     "  file(s). For actions, the outputs should be the list of files\n"
     "  generated by the script.\n";
 
+const char kPublic[] = "public";
+const char kPublic_HelpShort[] =
+    "public: [file list] Declare public header files for a target.";
+const char kPublic_Help[] =
+    "public: Declare public header files for a target.\n"
+    "\n"
+    "  A list of files and patterns that other targets can include. These\n"
+    "  permissions are checked via the \"check\" command\n"
+    "  (see \"gn help check\").\n"
+    "\n"
+    "  If no public files are declared, other targets (assuming they have\n"
+    "  visibility to depend on this target) can include any file. If this\n"
+    "  variable is defined on a target, dependent targets may only include\n"
+    "  files on this whitelist.\n"
+    "\n"
+    "  The entries in this list are patterns (see \"gn help patterns\") so\n"
+    "  you can use simple wildcard matching if you have a directory of public\n"
+    "  files.\n"
+    "\n"
+    "  Header file permissions are also subject to visibility. A target\n"
+    "  must be visible to another target to include any files from it at all\n"
+    "  and the public headers indicate which subset of those files are\n"
+    "  permitted.\n"
+    "\n"
+    "  Public files are inherited through the dependency tree. So if there is\n"
+    "  a dependency A -> B -> C, then A can include C's public headers.\n"
+    "  However, the same is NOT true of visibility, so unless A is in C's\n"
+    "  visibility list, the include will be rejected.\n"
+    "\n"
+    "Examples:\n"
+    "  These exact files are public:\n"
+    "    public = [ \"foo.h\", \"bar.h\" ]\n"
+    "\n"
+    "  All files in the \"public\" directory are public:\n"
+    "    public = [ \"public/*\" ]\n"
+    "\n"
+    "  No files are public (no targets may include headers from this one):\n"
+    "    public = []\n";
+
 const char kScript[] = "script";
 const char kScript_HelpShort[] =
     "script: [file name] Script file for actions.";
@@ -870,6 +909,7 @@ const VariableInfoMap& GetTargetVariables() {
     INSERT_VARIABLE(OutputExtension)
     INSERT_VARIABLE(OutputName)
     INSERT_VARIABLE(Outputs)
+    INSERT_VARIABLE(Public)
     INSERT_VARIABLE(Script)
     INSERT_VARIABLE(SourcePrereqs)
     INSERT_VARIABLE(Sources)