python_arch: return "unknown" when file does not exist

python_arch: return "unknown" when file does not exist

The `file` program follows POSIX as outlined here:
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/file.html

Specifically, this part causes troubles:
If the file named by the file operand does not exist, cannot be read, or
the type of the file named by the file operand cannot be determined, this
shall not be considered an error that affects the exit status.

With older versions of `file` (that does not conform to POSIX), this
script will exit 0 with "unknown" as its output when "$1" does not exist.
But with recent releases (that conform to POSIX), this script will exit 1
and can break the build.

Have the script explicitly check for the existence of $1 and if it does
not exist, then runt he unknown logic like normal.

BUG=chromium:332547
TEST=ran python_arch.sh on bogus files with new & old `file` programs

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=244778

[Lei Zhang, 2014-01-13 22:19:59]

Is anyone still using python_arch.py besides PyAuto, which is going away
soonish?

[Mark Mentovai, 2014-01-13 22:39:23]

https://codereview.chromium.org/137433002/diff/1/build/linux/python_arch.sh
File build/linux/python_arch.sh (right):

https://codereview.chromium.org/137433002/diff/1/build/linux/python_arch.sh#n...
build/linux/python_arch.sh:14: if [ $? -ne 0 ] || [ ! -e "$1" ] ; then
A short form of your checkin comment would do a whole lot to explain what the -e
is about. Something like “the spec says that file should exit with status 0 when
a file does not exist.”

https://codereview.chromium.org/137433002/diff/1/build/linux/python_arch.sh#n...
build/linux/python_arch.sh:42: 
I’m inclined to do the -e check way down here, as it should be an uncommon case.

[vapier, 2014-01-13 22:45:18]

the code might be punted in the future, but i think we should fix this known bug
now rather than wait

https://codereview.chromium.org/137433002/diff/1/build/linux/python_arch.sh
File build/linux/python_arch.sh (right):

https://codereview.chromium.org/137433002/diff/1/build/linux/python_arch.sh#n...
build/linux/python_arch.sh:14: if [ $? -ne 0 ] || [ ! -e "$1" ] ; then
done

https://codereview.chromium.org/137433002/diff/1/build/linux/python_arch.sh#n...
build/linux/python_arch.sh:42: 
it's unlikely, but if the error message contains the string the rest of the code
is searching for, it could wrongly flag as an arch that it isn't

[Mark Mentovai, 2014-01-13 22:47:56]

LGTM

[commit-bot: I haz the power, 2014-01-14 00:48:34]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/vapier@chromium.org/137433002/70001

[commit-bot: I haz the power, 2014-01-14 18:52:37]

Change committed as 244778