[PATCH] docs: license-rules.txt: cover SPDX headers on Python scripts

[Mauro Carvalho Chehab <mchehab+samsung@kernel.org>]

The author of the license-rules.rst file wanted to be very restrict
with regards to the location of the SPDX header. It says that
the SPDX header "shall be added at the first  possible line in
a file which can contain a comment". Not happy with this already
restrictive requiement, it goes further:

"For the majority  of files this is the first line, except for
scripts", opening an exception to have the SPDX header at the
second line, if the first line starts with "#!".

Well, it turns that this is too restrictive for Python scripts,
and may cause regressions if this would be enforced.

As mentioned on:
	https://stackoverflow.com/questions/728891/correct-way-to-define-python-source-code-encoding

Python's PEP-263 [1] dictates that an script that needs to default to
UTF-8 encoding has to follow this rule:

	'Python will default to ASCII as standard encoding if no other
	 encoding hints are given.

	 To define a source code encoding, a magic comment must be placed
	 into the source files either as first or second line in the file'

And:
	'More precisely, the first or second line must match the following
	 regular expression:

	 ^[ \t\f]*#.*?coding[:=][ \t]*([-_.a-zA-Z0-9]+)'

[1] https://www.python.org/dev/peps/pep-0263/

If a script has both "#!" and the charset encoding line, we can't place
a SPDX tag without either violating license-rules.rst or breaking the
script by making it crash with non-ASCII characters.

So, add a sort notice saying that, for Python scripts, the SPDX
header may be up to the third line, in order to cover the case
where both "#!" and "# .*coding.*UTF-8" lines are found.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
---
 Documentation/process/license-rules.rst | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/Documentation/process/license-rules.rst b/Documentation/process/license-rules.rst
index 2ef44ada3f11..5d23e3498b1c 100644
--- a/Documentation/process/license-rules.rst
+++ b/Documentation/process/license-rules.rst
@@ -64,9 +64,12 @@ License identifier syntax
    possible line in a file which can contain a comment.  For the majority
    of files this is the first line, except for scripts which require the
    '#!PATH_TO_INTERPRETER' in the first line.  For those scripts the SPDX
-   identifier goes into the second line.
+   identifier goes into the second line\ [1]_.
 
-|
+.. [1] Please notice that Python scripts may also need an encoding rule
+   as defined on PEP-263, which should be defined either at the first
+   or the second line. So, for such scripts, the SPDX identifier may
+   go up to the third line.
 
 2. Style:
 
-- 
2.21.0