tools: cpplint: fix up build/include_order rule

Change the build/include_order rule to match our preference:
project headers before system headers.

The rationale is that system headers before project headers makes it
easy to slip in bugs where a project header that requires a definition
from a system header, forgets to include the system header but still
compiles because the source files that include the project header
coincidentally include the system header too.

A good example is the size_t type. A project header file that needs the
definition of size_t should include stddef.h but forgetting to do so
will probably go unnoticed for a long time because almost every other
system header includes stddef.h (either directly or indirectly) and
almost every source file includes one or more system headers.

Ergo, project headers before system headers. It's a good thing.

tools/cpplint.py

@@ -280,11 +280,11 @@ for op, inv_replacement in [('==', 'NE'), ('!=', 'EQ'),
 
 # These constants define types of headers for use with
 # _IncludeState.CheckNextIncludeOrder().
-_C_SYS_HEADER = 1
-_CPP_SYS_HEADER = 2
-_LIKELY_MY_HEADER = 3
-_POSSIBLE_MY_HEADER = 4
-_OTHER_HEADER = 5
+_LIKELY_MY_HEADER = 1
+_POSSIBLE_MY_HEADER = 2
+_OTHER_HEADER = 3
+_C_SYS_HEADER = 4
+_CPP_SYS_HEADER = 5
 
 
 _regexp_compile_cache = {}
@@ -377,9 +377,9 @@ class _IncludeState(dict):
   # needs to move backwards, CheckNextIncludeOrder will raise an error.
   _INITIAL_SECTION = 0
   _MY_H_SECTION = 1
-  _C_SECTION = 2
-  _CPP_SECTION = 3
-  _OTHER_H_SECTION = 4
+  _OTHER_H_SECTION = 2
+  _C_SECTION = 3
+  _CPP_SECTION = 4
 
   _TYPE_NAMES = {
       _C_SYS_HEADER: 'C system header',
@@ -453,33 +453,32 @@ class _IncludeState(dict):
 
     last_section = self._section
 
-    if header_type == _C_SYS_HEADER:
-      if self._section <= self._C_SECTION:
-        self._section = self._C_SECTION
-      else:
-        self._last_header = ''
-        return error_message
-    elif header_type == _CPP_SYS_HEADER:
-      if self._section <= self._CPP_SECTION:
-        self._section = self._CPP_SECTION
-      else:
-        self._last_header = ''
-        return error_message
-    elif header_type == _LIKELY_MY_HEADER:
+    if header_type == _LIKELY_MY_HEADER:
       if self._section <= self._MY_H_SECTION:
         self._section = self._MY_H_SECTION
       else:
-        self._section = self._OTHER_H_SECTION
+        self._last_header = ''
+        return error_message
     elif header_type == _POSSIBLE_MY_HEADER:
       if self._section <= self._MY_H_SECTION:
         self._section = self._MY_H_SECTION
       else:
-        # This will always be the fallback because we're not sure
-        # enough that the header is associated with this file.
+        self._last_header = ''
+        return error_message
+    elif header_type == _OTHER_HEADER:
+      if self._section <= self._OTHER_H_SECTION:
         self._section = self._OTHER_H_SECTION
+      else:
+        self._last_header = ''
+        return error_message
+    elif header_type == _C_SYS_HEADER:
+      if self._section <= self._C_SECTION:
+        self._section = self._C_SECTION
+      else:
+        self._section = self._CPP_SECTION
     else:
-      assert header_type == _OTHER_HEADER
-      self._section = self._OTHER_H_SECTION
+      assert header_type == _CPP_SYS_HEADER
+      self._section = self._CPP_SECTION
 
     if last_section != self._section:
       self._last_header = ''
@@ -2312,10 +2311,10 @@ def CheckIncludeLine(filename, clean_lines, linenum, include_state, error):
       # lower type after that.
       error_message = include_state.CheckNextIncludeOrder(
           _ClassifyInclude(fileinfo, include, is_system))
-      # if error_message:
-      #   error(filename, linenum, 'build/include_order', 4,
-      #         '%s. Should be: %s.h, c system, c++ system, other.' %
-      #         (error_message, fileinfo.BaseName()))
+      if error_message:
+        error(filename, linenum, 'build/include_order', 4,
+              '%s. Should be: %s.h, c system, c++ system, other.' %
+              (error_message, fileinfo.BaseName()))
       if not include_state.IsInAlphabeticalOrder(include):
         error(filename, linenum, 'build/include_alpha', 4,
               'Include "%s" not in alphabetical order' % include)