settings: fix key-value duplication check

[nvlsianpu]

Whenever a new key-value is about to be stored, the settings
perform check whether the value really changes. This check
after #9521 patch should work differently as \0 is not
the value terminator anymore. Because of above any value which
starts from \0 will be treated mistakenly as a NULL.

This patch uses check-length callback instead read-callback which
fix the issue and simplify the code a little.

Signed-off-by: Andrzej Puzdrowski [email protected]

[carlescufi]

LGTM

[jhedberg]

Gives me these build warnings:

/Users/jhedberg/src/zephyr/subsys/settings/src/settings_store.c: In function 'settings_dup_check_cb':
/Users/jhedberg/src/zephyr/subsys/settings/src/settings_store.c:114:6: warning: unused variable 'rc' [-Wunused-variable]
  int rc;
      ^~
/Users/jhedberg/src/zephyr/subsys/settings/src/settings_store.c:113:7: warning: unused variable 'temp' [-Wunused-variable]
  char temp;
       ^~~~
[217/228] Linking C executable zephyr/zephyr_prebuilt.elf

[jhedberg]

@nvlsianpu this solves the issue I had with mesh_shell, thanks for fixing it! I'll wait for the compilation warnings to be fixed before approving, though.

[jhedberg]

Giving my approval so that this can be merged as soon as @nvlsianpu fixes the build warnings (which are now also causing CI to fail).

[jhedberg]

@nvlsianpu any update on fixing the compilation warnings? It'd good to get this merged ASAP.

[codecov-io]

Codecov Report

Merging #12026 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #12026     +/-   ##
=========================================
- Coverage   48.05%   47.96%   -0.1%     
=========================================
  Files         281      281             
  Lines       43412    43399     -13     
  Branches    10404    10403      -1     
=========================================
- Hits        20863    20815     -48     
- Misses      18400    18432     +32     
- Partials     4149     4152      +3

Impacted Files	Coverage Δ
subsys/logging/log_output.c	59.81% <0%> (-9.59%)	⬇️
subsys/logging/log_core.c	69.86% <0%> (-4.81%)	⬇️
misc/printk.c	81.81% <0%> (-3.13%)	⬇️
include/logging/log_msg.h	89.09% <0%> (-1.82%)	⬇️
include/logging/log_core.h	100% <0%> (ø)	⬆️

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[nvlsianpu]

@jhe green now