tools: ynl-gen: don't touch the output file if content is the same

I often regenerate all YNL files in the tree to make sure they
are in sync with the codegen and specs. Generator rewrites
the files unconditionally, so since make looks at file modification
time to decide what to rebuild - my next build takes longer.

We already generate the code to a tempfile most of the time,
only overwrite the target when we have to.

Before:

  $ stat include/uapi/linux/netdev.h
    File: include/uapi/linux/netdev.h
    Size: 2307      	Blocks: 8          IO Block: 4096   regular file
  Access: 2023-10-27 15:19:56.347071940 -0700
  Modify: 2023-10-27 15:19:45.089000900 -0700
  Change: 2023-10-27 15:19:45.089000900 -0700
   Birth: 2023-10-27 15:19:45.088000894 -0700

  $ ./tools/net/ynl/ynl-regen.sh -f
  [...]

  $ stat include/uapi/linux/netdev.h
    File: include/uapi/linux/netdev.h
    Size: 2307      	Blocks: 8          IO Block: 4096   regular file
  Access: 2023-10-27 15:19:56.347071940 -0700
  Modify: 2023-10-27 15:22:18.417968446 -0700
  Change: 2023-10-27 15:22:18.417968446 -0700
   Birth: 2023-10-27 15:19:45.088000894 -0700

After:

  $ stat include/uapi/linux/netdev.h
    File: include/uapi/linux/netdev.h
    Size: 2307      	Blocks: 8          IO Block: 4096   regular file
  Access: 2023-10-27 15:22:41.520114221 -0700
  Modify: 2023-10-27 15:22:18.417968446 -0700
  Change: 2023-10-27 15:22:18.417968446 -0700
   Birth: 2023-10-27 15:19:45.088000894 -0700

  $ ./tools/net/ynl/ynl-regen.sh -f
  [...]

  $ stat include/uapi/linux/netdev.h
    File: include/uapi/linux/netdev.h
    Size: 2307      	Blocks: 8          IO Block: 4096   regular file
  Access: 2023-10-27 15:22:41.520114221 -0700
  Modify: 2023-10-27 15:22:18.417968446 -0700
  Change: 2023-10-27 15:22:18.417968446 -0700
   Birth: 2023-10-27 15:19:45.088000894 -0700
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20231027223408.1865704-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/ynl-gen-c.py

@@ -3,6 +3,7 @@
 
 import argparse
 import collections
+import filecmp
 import os
 import re
 import shutil
@@ -1168,7 +1169,7 @@ class CodeWriter:
         if out_file is None:
             self._out = os.sys.stdout
         else:
-            self._out = tempfile.TemporaryFile('w+')
+            self._out = tempfile.NamedTemporaryFile('w+')
             self._out_file = out_file
 
     def __del__(self):
@@ -1177,6 +1178,10 @@ class CodeWriter:
     def close_out_file(self):
         if self._out == os.sys.stdout:
             return
+        # Avoid modifying the file if contents didn't change
+        self._out.flush()
+        if os.path.isfile(self._out_file) and filecmp.cmp(self._out.name, self._out_file, shallow=False):
+            return
         with open(self._out_file, 'w+') as out_file:
             self._out.seek(0)
             shutil.copyfileobj(self._out, out_file)