This Week I Learned - 2016 Week 38

Last week post or the whole series. Interesting stuff learned this week.

Encountered this error message when checking a USB thumbdrive with `fdisk` command. The particular thumb drive was burned with an ISO file through the `dd` command.
$ sudo fdisk -l
......
GPT PMBR size mismatch (1432123 != 15765503) will be corrected by w(rite).
Disk /dev/sdc: 7.5 GiB, 8071938048 bytes, 15765504 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 8C18967D-CB41-4EF1-8958-4E495054958D

Device     Start     End Sectors   Size Type
/dev/sdc1     64   17611   17548   8.6M Microsoft basic data
/dev/sdc2  17612   23371    5760   2.8M EFI System
/dev/sdc3  23372 1432075 1408704 687.9M Microsoft basic data

Follow the instructions given, running the device through `gparted` seems to resolve the issue.



Perl's hash initialization, referencing, and de-referencing. Seriously, I need to get this correctly and read more Perl's FAQs.
# Normal way, without referencing.
%foobar = (a => '1', b => '2');
say $foobar{a};

# Using referencing. More readable.
$foobar = {a => '1', b => '2'};
say $foobar->{a};

# Alternatively.
$foobar_ref = \%foobar;
say $foobar_ref->{a};

Finding properties of the event target in Javascript.
$('foo').bind('click', function () {
    // inside here, `this` will refer to the foo that was clicked
});

How do you add a trailing slash if none found? Regex, regex.
$string =~ s!/*$!/!; # Add a trailing slash

Protocol-relative URL. While we're on HTTP protocol, it was made aware to me that the anchor tag should be the last item on the URL.

CSS image sprite technique using HTML unordered list. One of the issue encountered is if you have single line text link, how do you align the text link vertically in the middle? Make sure the `line-height` is equal to `height` for the `li`` element.

Git merge conflict? Just abort the whole process.

Similarly discard all changes on a diverged local branch, two ways. First method is to my liking.
# Method 1
$ git branch -D phobos
$ git checkout --track -b phobos origin/phobos

# Method 2
$ git checkout phobos
$ git reset --hard origin/phobos

Debugging Dockerfile. Something I learned this week but in a separate and longer post.

Starting a new software project but not sure about which technology stack to use? Read this slide as a guide.






Debugging Dockerfile

While building Docker image through Dockerfile, I've encountered an error in one of the build step, shown below. It seemed that that one of the Perl's module failed to install due to some unknown reason.
$ docker build -t ang:dist-zilla .

Sending build context to Docker daemon 3.072 kB
Step 1 : FROM perl:latest
 ---> a9d757d1a33b
Step 2 : RUN cpanm install Term::ReadKey
 ---> Running in 78760f841b26
 ---> Working on install
......
Building and testing TermReadKey-2.33 ... ! Installing Term::ReadKey failed. 
See /root/.cpanm/work/1473691870.7/build.log for details. Retry with --force to force install it.
......

Since there is an error, the particular layer of image was not created. Hence, there is no way for me to debug and trace the error. Fortunately, you can force it through the build by marking it to be a success build. The changes is minor, you just need to append an OR condition (also known as Short-circuit evaluation) as shown.
RUN cpanm install Term::ReadKey || true

Rebuild the image. Regardless the error, we have successfully create a layer.
$ docker build -t ang:dist-zilla .
......
Step 2 : RUN cpanm install Term::ReadKey || true
 ---> Running in f3fa83db0ae9
......
Building and testing TermReadKey-2.33 ... ! 
Installing Term::ReadKey failed. See /root/.cpanm/work/1473691397.7/build.log for details. 
Retry with --force to force install it.
FAIL
1 distribution installed
 ---> 62e2b1bd2c61
Removing intermediate container f3fa83db0ae9
Successfully built 62e2b1bd2c61

Log in to the particular snapshot or layer before we troubleshoot the issue.
$ docker run --rm -it 62e2b1bd2c61 bash -il
root@3e47eeff4328:~#

Checking the build log.
root@3e47eeff4328:~# tail .cpanm/work/1473691397.7/build.log -n 25
  
chmod 755 blib/arch/auto/Term/ReadKey/ReadKey.so
"/usr/local/bin/perl" -MExtUtils::Command::MM -e 'cp_nonempty' -- 
ReadKey.bs blib/arch/auto/Term/ReadKey/ReadKey.bs 644
Running Mkbootstrap for Term::ReadKey ()
chmod 644 "ReadKey.bs"
PERL_DL_NONLAZY=1 "/usr/local/bin/perl" 
"-MExtUtils::Command::MM" "-MTest::Harness" "-e" 
"undef *Test::Harness::Switches; test_harness(0, 'blib/lib', 'blib/arch')" t/*.t
t/01_basic.t ............... ok
# Looks like you planned 7 tests but ran 1.
t/02_terminal_functions.t .. 
Dubious, test returned 255 (wstat 65280, 0xff00)
Failed 6/7 subtests 
        (less 1 skipped subtest: 0 okay)

Test Summary Report
-------------------
t/02_terminal_functions.t (Wstat: 65280 Tests: 1 Failed: 0)
  Non-zero exit status: 255
  Parse errors: Bad plan.  You planned 7 tests but ran 1.
Files=2, Tests=2,  0 wallclock secs ( 0.02 usr  0.00 sys +  0.04 cusr  0.00 csys =  0.06 CPU)
Result: FAIL
Failed 1/2 test programs. 0/2 subtests failed.
Makefile:1029: recipe for target 'test_dynamic' failed
make: *** [test_dynamic] Error 255
-> FAIL Installing Term::ReadKey failed. 
See /root/.cpanm/work/1473691397.7/build.log for details. Retry with --force to force install it.
1 distribution installed

Googling result indicates that Term::ReadKey module have issue with one of the unit test where interactive shell is not found. Building Docker image does not needs interactive shell available.

The workaround is to downgrade and install the previous working version.
RUN cpanm install Term::ReadKey@2.32