From what I understand, most programs calling vpd_get_value don't look at return code. Instead they check the output, because that's the one really matters.

sysfs vpd does know if a key exists or not, so it returns 1 on non-exist.
'vpd -g non_exist' returns 0, so actually we don't really know if the key exists or not. This is probably because 'vpd' command didn't support "deletion" in the very beginning, I'm not sure.

It is possible to find the difference between empty value and absence in the cache version (via "vpd -l"), but the scripting will be more complicated and I'm not sure if it's worthy.

If you do want the 3 different ways (sysfs, vpd-g, cache) to return same value, always returning 0 may be the most easy solution, and more compatible to existing scripts.

If you do want to know if a key is "missing", that is also possible I think - but will need more changes and I'm not sure if there's any real case that people want to know if a value is empty or missing.

OK, I'll take a quick look at existing callers of vpd_get_value then and if I don't find any problems, I'll prepare a change to exit with status 0 on missing keys for the sysfs case.

Here's a CL: https://chromium-review.googlesource.com/c/chromiumos/platform/vpd/+/643188

It works fine in local testing, but let's see what the CQ says about it.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/vpd/+/bc0fd5a3aca8a2619b5f6abfe537a6b33e60450f

commit bc0fd5a3aca8a2619b5f6abfe537a6b33e60450f
Author: Mattias Nissler <mnissler@chromium.org>
Date: Thu Aug 31 11:52:20 2017

Make sure vpd_get_value succeeds for missing keys

Using attempting vpd_get_value to read a VPD key that isn't present,
it was unclear what the exit status should be. The sysfs read path
would exit with status 1 whereas the legacy cache read would not
consider this an error and exit with status 0. Existing callers of
vpd_get_value are expecting failure for an actual error condition, but
not absence of keys, so adjust the sysfs path exit successful on
absent keys as well.

BUG= chromium:760500 
TEST=vpd_get_value no_such_key ; echo $? should print 0 on all devices.
BRANCH=none

Change-Id: I7c3c08f44fa09eaeb0fe2f5b483199b5313cf7de
Reviewed-on: https://chromium-review.googlesource.com/643188
Commit-Ready: Mattias Nissler <mnissler@chromium.org>
Tested-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>

[modify] https://crrev.com/bc0fd5a3aca8a2619b5f6abfe537a6b33e60450f/util/vpd_get_value

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/vpd/+/573e9d1f6c34df84129222ca18c74d6a6886d7c0

commit 573e9d1f6c34df84129222ca18c74d6a6886d7c0
Author: Mattias Nissler <mnissler@chromium.org>
Date: Fri Sep 08 19:43:50 2017

Make sure vpd_get_value succeeds for missing keys

Using attempting vpd_get_value to read a VPD key that isn't present,
it was unclear what the exit status should be. The sysfs read path
would exit with status 1 whereas the legacy cache read would not
consider this an error and exit with status 0. Existing callers of
vpd_get_value are expecting failure for an actual error condition, but
not absence of keys, so adjust the sysfs path exit successful on
absent keys as well.

BUG= chromium:760500 
TEST=vpd_get_value no_such_key ; echo $? should print 0 on all devices.
BRANCH=none

Reviewed-on: https://chromium-review.googlesource.com/643188
Commit-Ready: Mattias Nissler <mnissler@chromium.org>
Tested-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>

BUG=chromium:761803

Change-Id: I69b8f9b079adb93868dee3784788c946c033abdd
Reviewed-on: https://chromium-review.googlesource.com/657411
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Tested-by: Mattias Nissler <mnissler@chromium.org>

[modify] https://crrev.com/573e9d1f6c34df84129222ca18c74d6a6886d7c0/util/vpd_get_value