From louis.krupp at gmail.com  Mon Feb 19 04:51:40 2018
From: louis.krupp at gmail.com (Louis Krupp)
Date: Mon, 19 Feb 2018 04:51:40 -0700
Subject: [lug] Fwd: Save 55% During Our Presidents' Day Sale
In-Reply-To: <0.1.A2.2BE.1D3A96CA95E896A.0@om-fen.rsys2.com>
References: <0.1.A2.2BE.1D3A96CA95E896A.0@om-fen.rsys2.com>
Message-ID: <CAGRzHDjY-Nuao2np21wiRngWYkr9i+GEf4zLzF66OAwm1iwSvg@mail.gmail.com>

FYI...

---------- Forwarded message ----------
From: InformIT <imprints at e.informit.net>
Date: Mon, Feb 19, 2018 at 3:30 AM
Subject: Save 55% During Our Presidents' Day Sale
To: louis.krupp at gmail.com


Save 55% when you buy 2 or more books, eBooks, video training, or software
through February 23, 2018. Offer includes free U.S. shipping.
[image: InformIT]
<http://e.pearson.info/pub/cc?_ri_=X0Gzc2X%3DYQpglLjHJlTQGpuBtjhkLTpCzdjqc1Xqr4dGsfbirczdizbYvkYyMkyXzggmhPGhmACVXtpKX%3DSSRUBRUD&_ei_=EolaGGF4SNMvxFF7KucKuWNLtK1vtHkmR4aVFNFEo33dr2lUzL9kTf7eoZ5pCuP9OZ32Nwylmn3xCh0ulcGLqclmUphn2IWofoEFJCABWExRuBzvUJG9Hg.>
[image: Buy 2, Save 55% in the Presidents' Day Sale from InformIT]
<http://e.pearson.info/pub/cc?_ri_=X0Gzc2X%3DYQpglLjHJlTQGpuBtjhkLTpCzdjqc1Xqr4dGsfbirczdizbYvkYyMkyXzggmhPGhmACVXtpKX%3DSSRUBRWR&_ei_=EolaGGF4SNMvxFF7KucKuWNLtK1vtHkmR4aVFNFEo33dr2lUzL9kTf7eoZ5pCuP9OZ32Nwylmn3xCh0ulcGLqclmUphn2IWofoEFJCABWExRuBzvUJG9Hg.>
For a limited time, *buy 2, save 55%*
<http://e.pearson.info/pub/cc?_ri_=X0Gzc2X%3DYQpglLjHJlTQGpuBtjhkLTpCzdjqc1Xqr4dGsfbirczdizbYvkYyMkyXzggmhPGhmACVXtpKX%3DSSRUBRWS&_ei_=EolaGGF4SNMvxFF7KucKuWNLtK1vtHkmR4aVFNFEo33dr2lUzL9kTf7eoZ5pCuP9OZ32Nwylmn3xCh0ulcGLqclmUphn2IWofoEFJCABWExRuBzvUJG9Hg.>*
off the list price of books, eBooks, and video training during our
Presidents' Day Sale. Learn a new skill or get even better at what you
already know with the highest-quality learning resources from the best
trainers and instructors in the community.

Use discount code *PD2018* during checkout, to secure your savings—and get
free shipping within the U.S. Sale ends February 23.
SHOP THE STORE
<http://e.pearson.info/pub/cc?_ri_=X0Gzc2X%3DYQpglLjHJlTQGpuBtjhkLTpCzdjqc1Xqr4dGsfbirczdizbYvkYyMkyXzggmhPGhmACVXtpKX%3DSSRUBRWS&_ei_=EolaGGF4SNMvxFF7KucKuWNLtK1vtHkmR4aVFNFEo33dr2lUzL9kTf7eoZ5pCuP9OZ32Nwylmn3xCh0ulcGLqclmUphn2IWofoEFJCABWExRuBzvUJG9Hg.>
*FREE SHIPPING* WITHIN THE U.S.
* Discount code PD2018 confers a 55% discount off the list price of two or
more eligible titles purchased on informit.com in a single transaction.
Eligible titles include most print books, eBooks, Web Editions, and
full-course video titles. Book + eBook bundles, book/eBook + video bundles,
individual video lessons, Rough Cuts, Safari Books Online, non-discountable
titles, titles on promotion with our retail partners, and any title
featured as eBook or Video Deal of the Week are not eligible for discount.
If any items are canceled or returned, and the order is no longer eligible
for a volume-based discount code that was applied to the purchase,
remaining items will be invoiced at the standard discounted price shown on
the website. Offer ends 11:59 p.m. EST February 23, 2018. Discount may not
be combined with any other offer and is not redeemable for cash. Offer
subject to change.
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From jeffrey.haemer at gmail.com  Mon Feb 19 06:37:01 2018
From: jeffrey.haemer at gmail.com (Jeffrey S. Haemer)
Date: Mon, 19 Feb 2018 06:37:01 -0700
Subject: [lug] Fwd: Save 55% During Our Presidents' Day Sale
In-Reply-To: <CAGRzHDjY-Nuao2np21wiRngWYkr9i+GEf4zLzF66OAwm1iwSvg@mail.gmail.com>
References: <0.1.A2.2BE.1D3A96CA95E896A.0@om-fen.rsys2.com>
 <CAGRzHDjY-Nuao2np21wiRngWYkr9i+GEf4zLzF66OAwm1iwSvg@mail.gmail.com>
Message-ID: <CAABvdFy7sAW=q4KMOoTY9AkDUw0GvMjOOurqfR-D6ToNtfMEyg@mail.gmail.com>

Thank you, Louis.

On Mon, Feb 19, 2018 at 4:51 AM, Louis Krupp <louis.krupp at gmail.com> wrote:

> FYI...
>
> ---------- Forwarded message ----------
> From: InformIT <imprints at e.informit.net>
> Date: Mon, Feb 19, 2018 at 3:30 AM
> Subject: Save 55% During Our Presidents' Day Sale
> To: louis.krupp at gmail.com
>
>
> Save 55% when you buy 2 or more books, eBooks, video training, or software
> through February 23, 2018. Offer includes free U.S. shipping.
> [image: InformIT]
> <http://e.pearson.info/pub/cc?_ri_=X0Gzc2X%3DYQpglLjHJlTQGpuBtjhkLTpCzdjqc1Xqr4dGsfbirczdizbYvkYyMkyXzggmhPGhmACVXtpKX%3DSSRUBRUD&_ei_=EolaGGF4SNMvxFF7KucKuWNLtK1vtHkmR4aVFNFEo33dr2lUzL9kTf7eoZ5pCuP9OZ32Nwylmn3xCh0ulcGLqclmUphn2IWofoEFJCABWExRuBzvUJG9Hg.>
> [image: Buy 2, Save 55% in the Presidents' Day Sale from InformIT]
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> For a limited time, *buy 2, save 55%*
> <http://e.pearson.info/pub/cc?_ri_=X0Gzc2X%3DYQpglLjHJlTQGpuBtjhkLTpCzdjqc1Xqr4dGsfbirczdizbYvkYyMkyXzggmhPGhmACVXtpKX%3DSSRUBRWS&_ei_=EolaGGF4SNMvxFF7KucKuWNLtK1vtHkmR4aVFNFEo33dr2lUzL9kTf7eoZ5pCuP9OZ32Nwylmn3xCh0ulcGLqclmUphn2IWofoEFJCABWExRuBzvUJG9Hg.>*
> off the list price of books, eBooks, and video training during our
> Presidents' Day Sale. Learn a new skill or get even better at what you
> already know with the highest-quality learning resources from the best
> trainers and instructors in the community.
>
> Use discount code *PD2018* during checkout, to secure your savings—and
> get free shipping within the U.S. Sale ends February 23.
> SHOP THE STORE
> <http://e.pearson.info/pub/cc?_ri_=X0Gzc2X%3DYQpglLjHJlTQGpuBtjhkLTpCzdjqc1Xqr4dGsfbirczdizbYvkYyMkyXzggmhPGhmACVXtpKX%3DSSRUBRWS&_ei_=EolaGGF4SNMvxFF7KucKuWNLtK1vtHkmR4aVFNFEo33dr2lUzL9kTf7eoZ5pCuP9OZ32Nwylmn3xCh0ulcGLqclmUphn2IWofoEFJCABWExRuBzvUJG9Hg.>
> *FREE SHIPPING* WITHIN THE U.S.
> * Discount code PD2018 confers a 55% discount off the list price of two or
> more eligible titles purchased on informit.com in a single transaction.
> Eligible titles include most print books, eBooks, Web Editions, and
> full-course video titles. Book + eBook bundles, book/eBook + video bundles,
> individual video lessons, Rough Cuts, Safari Books Online, non-discountable
> titles, titles on promotion with our retail partners, and any title
> featured as eBook or Video Deal of the Week are not eligible for discount.
> If any items are canceled or returned, and the order is no longer eligible
> for a volume-based discount code that was applied to the purchase,
> remaining items will be invoiced at the standard discounted price shown on
> the website. Offer ends 11:59 p.m. EST February 23, 2018. Discount may not
> be combined with any other offer and is not redeemable for cash. Offer
> subject to change.
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-- 
Jeffrey Haemer <jeffrey.haemer at gmail.com>
720-837-8908 [cell], @goyishekop [twitter]
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From bcarr at purgatoire.org  Wed Feb 21 17:38:14 2018
From: bcarr at purgatoire.org (BC)
Date: Wed, 21 Feb 2018 17:38:14 -0700
Subject: [lug] selinux
In-Reply-To: <1517182067.3001918.1251124376.179A68B0@webmail.messagingengine.com>
References: <L3tg_bg--3-0@tutanota.com>
 <1517100008.1647334.1250439216.732250C0@webmail.messagingengine.com>
 <CAJB=V00Q=jyzFHYq7bRNsAN303x66Ff+_JbfasOyDUb1uPDmLw@mail.gmail.com>
 <1517182067.3001918.1251124376.179A68B0@webmail.messagingengine.com>
Message-ID: <31f545da-7b9f-e4c2-f878-69bf8bf77b30@purgatoire.org>


I'm converting a [bash] shell script to golang in an effort to learn 
to program in Go.  I'm having lots of success but this script 
statement has me confused:


     _value="$(__get_jail_prop $_property default)"


The puzzling word is 'default'.  __get_jail_prop is a function to 
which is passed the parameter $_property (defined elsewhere).  The 
function returns _value.

But what does the word 'default' do?  I tried passing the word 
"default" to the Go-equivalent function and the program bombs.  So I 
don't think it is a parameter to be passed.

Any thoughts on what that word does?

TIA,

Bucky

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From blug-mail at duboulder.com  Wed Feb 21 17:47:45 2018
From: blug-mail at duboulder.com (Lee Woodworth)
Date: Wed, 21 Feb 2018 17:47:45 -0700
Subject: [lug] selinux
In-Reply-To: <31f545da-7b9f-e4c2-f878-69bf8bf77b30@purgatoire.org>
References: <L3tg_bg--3-0@tutanota.com>
 <1517100008.1647334.1250439216.732250C0@webmail.messagingengine.com>
 <CAJB=V00Q=jyzFHYq7bRNsAN303x66Ff+_JbfasOyDUb1uPDmLw@mail.gmail.com>
 <1517182067.3001918.1251124376.179A68B0@webmail.messagingengine.com>
 <31f545da-7b9f-e4c2-f878-69bf8bf77b30@purgatoire.org>
Message-ID: <2851074d-7731-14dc-79b5-cefacd019a2c@duboulder.com>

On 02/21/2018 05:38 PM, BC wrote:
> 
> I'm converting a [bash] shell script to golang in an effort to learn to program in Go.  
> I'm having lots of success but this script statement has me confused:
> 
> 
>      _value="$(__get_jail_prop $_property default)"
> 
> 
> The puzzling word is 'default'.  __get_jail_prop is a function to which is passed the 
> parameter $_property (defined elsewhere).  The function returns _value.
> 
> But what does the word 'default' do?  I tried passing the word "default" to the 
> Go-equivalent function and the program bombs.  So I don't think it is a parameter to be 
> passed.

I don't recall the string 'default' being special to the shell in an
argument list. It looks to me like a second argument to the command/function.

info bash usually has all the gory details

What could happen is that since _property is not quoted, when it is empty then
the command expansion is equivalent to:
   "$(__get_jail_prop default)"

From bcarr at purgatoire.org  Wed Feb 21 17:56:09 2018
From: bcarr at purgatoire.org (BC)
Date: Wed, 21 Feb 2018 17:56:09 -0700
Subject: [lug] Bash shell script question
In-Reply-To: <2851074d-7731-14dc-79b5-cefacd019a2c@duboulder.com>
References: <L3tg_bg--3-0@tutanota.com>
 <1517100008.1647334.1250439216.732250C0@webmail.messagingengine.com>
 <CAJB=V00Q=jyzFHYq7bRNsAN303x66Ff+_JbfasOyDUb1uPDmLw@mail.gmail.com>
 <1517182067.3001918.1251124376.179A68B0@webmail.messagingengine.com>
 <31f545da-7b9f-e4c2-f878-69bf8bf77b30@purgatoire.org>
 <2851074d-7731-14dc-79b5-cefacd019a2c@duboulder.com>
Message-ID: <9e662b14-b3a0-95fb-09fd-0693dbf18a47@purgatoire.org>



On 2/21/2018 5:47 PM, Lee Woodworth wrote:
> On 02/21/2018 05:38 PM, BC wrote:
>>
>>
>>      _value="$(__get_jail_prop $_property default)"
>>
>>
>
> I don't recall the string 'default' being special to the shell in an
> argument list. It looks to me like a second argument to the 
> command/function.
>
> info bash usually has all the gory details

Yeah, yeah - I've studied the bash man page (and what Google offers 
me) for months now.  I'm having to learn sh scripting alongside 
golang.  Bletch.

>
> What could happen is that since _property is not quoted, when it is 
> empty then
> the command expansion is equivalent to:
>   "$(__get_jail_prop default)"

This I will have to chew on and study with the resources until I 
understand what you are saying...  From what I can tell of the script, 
$_property is never empty at this point.  I've been stepping through 
my Go version of the program and it this call that causes failure.

Thanks.  Oh, in case you couldn't tell, I'm a hobbyist.

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From blug-mail at duboulder.com  Wed Feb 21 18:17:32 2018
From: blug-mail at duboulder.com (Lee Woodworth)
Date: Wed, 21 Feb 2018 18:17:32 -0700
Subject: [lug] Bash shell script question
In-Reply-To: <9e662b14-b3a0-95fb-09fd-0693dbf18a47@purgatoire.org>
References: <L3tg_bg--3-0@tutanota.com>
 <1517100008.1647334.1250439216.732250C0@webmail.messagingengine.com>
 <CAJB=V00Q=jyzFHYq7bRNsAN303x66Ff+_JbfasOyDUb1uPDmLw@mail.gmail.com>
 <1517182067.3001918.1251124376.179A68B0@webmail.messagingengine.com>
 <31f545da-7b9f-e4c2-f878-69bf8bf77b30@purgatoire.org>
 <2851074d-7731-14dc-79b5-cefacd019a2c@duboulder.com>
 <9e662b14-b3a0-95fb-09fd-0693dbf18a47@purgatoire.org>
Message-ID: <0dab9af6-a83a-fb56-5740-a8d64b3eeb09@duboulder.com>

On 02/21/2018 05:56 PM, BC wrote:
> 
> 
> On 2/21/2018 5:47 PM, Lee Woodworth wrote:
>> On 02/21/2018 05:38 PM, BC wrote:
>>>
>>>
>>>      _value="$(__get_jail_prop $_property default)"
>>>
>>>
>>
>> I don't recall the string 'default' being special to the shell in an
>> argument list. It looks to me like a second argument to the command/function.
>>
>> info bash usually has all the gory details
> 
> Yeah, yeah - I've studied the bash man page (and what Google offers me) for months now.  
> I'm having to learn sh scripting alongside golang.  Bletch.
> 
>>
>> What could happen is that since _property is not quoted, when it is empty then
>> the command expansion is equivalent to:
>>   "$(__get_jail_prop default)"
> 
> This I will have to chew on and study with the resources until I understand what you are 
> saying...  From what I can tell of the script, $_property is never empty at this point.  
> I've been stepping through my Go version of the program and it this call that causes failure.
> 
> Thanks.  Oh, in case you couldn't tell, I'm a hobbyist.

If you change your bash from
     _value="$(__get_jail_prop $_property default)"
to
     _value="$(__get_jail_prop '$_property' default)"
and the behavior changes it would suggest _property is an
empty or blank string. at some point.

Oh. And when building command lines for os.Cmd make sure you have separate
slice elements for each arg so they will be passed as separate args to the
command:

     property := "the prop string"
     tmpdir := "value for TMPDIR in the passed environment"
     cmd := &exec.Cmd{
         Path:       "/path/to/_get_jail_prop",
         Dir:        "/cd/to/this/dir/,
         Args:       []string{
                        "/path/to/_get_jail_prop",
                        property,
                        "default",
                     },
         Env:        []string{
                        "PATH=/usr/local/bin:/usr/bin",
                        "TMPDIR=" + tmpdir,
                     },
     }

From jeffrey.haemer at gmail.com  Wed Feb 21 18:20:29 2018
From: jeffrey.haemer at gmail.com (Jeffrey S. Haemer)
Date: Wed, 21 Feb 2018 18:20:29 -0700
Subject: [lug] selinux
In-Reply-To: <31f545da-7b9f-e4c2-f878-69bf8bf77b30@purgatoire.org>
References: <L3tg_bg--3-0@tutanota.com>
 <1517100008.1647334.1250439216.732250C0@webmail.messagingengine.com>
 <CAJB=V00Q=jyzFHYq7bRNsAN303x66Ff+_JbfasOyDUb1uPDmLw@mail.gmail.com>
 <1517182067.3001918.1251124376.179A68B0@webmail.messagingengine.com>
 <31f545da-7b9f-e4c2-f878-69bf8bf77b30@purgatoire.org>
Message-ID: <CAABvdFwiFiE1V7LtdkqSJUHtdCYBX8BYJe5R9tnSyG6wJMpT2w@mail.gmail.com>

Bucky,

Unless the shell script is being far too clever, the string "default" is
being passed to __get_jail_prop() as a parameter.  I might be able to help
more if you included the function definition.

On Wed, Feb 21, 2018 at 5:38 PM, BC <bcarr at purgatoire.org> wrote:

>
> I'm converting a [bash] shell script to golang in an effort to learn to
> program in Go.  I'm having lots of success but this script statement has me
> confused:
>
>
>     _value="$(__get_jail_prop $_property default)"
>
>
> The puzzling word is 'default'.  __get_jail_prop is a function to which is
> passed the parameter $_property (defined elsewhere).  The function returns
> _value.
>
> But what does the word 'default' do?  I tried passing the word "default"
> to the Go-equivalent function and the program bombs.  So I don't think it
> is a parameter to be passed.
>
> Any thoughts on what that word does?
>
> TIA,
>
> Bucky
>
>
> _______________________________________________
> Web Page:  http://lug.boulder.co.us
> Mailing List: http://lists.lug.boulder.co.us/mailman/listinfo/lug
> Join us on IRC: irc.hackingsociety.org port=6667 channel=#hackingsociety
>



-- 
Jeffrey Haemer <jeffrey.haemer at gmail.com>
720-837-8908 [cell], @goyishekop [twitter]
*פרייהייט? דאס איז יאַנג דינען וואָרט!*
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From bcarr at purgatoire.org  Wed Feb 21 18:25:14 2018
From: bcarr at purgatoire.org (BC)
Date: Wed, 21 Feb 2018 18:25:14 -0700
Subject: [lug] Bash shell script question
In-Reply-To: <0dab9af6-a83a-fb56-5740-a8d64b3eeb09@duboulder.com>
References: <L3tg_bg--3-0@tutanota.com>
 <1517100008.1647334.1250439216.732250C0@webmail.messagingengine.com>
 <CAJB=V00Q=jyzFHYq7bRNsAN303x66Ff+_JbfasOyDUb1uPDmLw@mail.gmail.com>
 <1517182067.3001918.1251124376.179A68B0@webmail.messagingengine.com>
 <31f545da-7b9f-e4c2-f878-69bf8bf77b30@purgatoire.org>
 <2851074d-7731-14dc-79b5-cefacd019a2c@duboulder.com>
 <9e662b14-b3a0-95fb-09fd-0693dbf18a47@purgatoire.org>
 <0dab9af6-a83a-fb56-5740-a8d64b3eeb09@duboulder.com>
Message-ID: <3b8af6f5-64a7-e611-c264-35fc09a3e01d@purgatoire.org>


My Go code doesn't look remotely like yours, but I do remember seeing 
an os.Cmd structure that takes the environment into account. I 
experimented and found I didn't need to use that form.  But I'll go 
back and take another gander at it, just to be sure.  Go's slices are 
wonderful, aren't they?


On 2/21/2018 6:17 PM, Lee Woodworth wrote:
>
> Oh. And when building command lines for os.Cmd make sure you have 
> separate
> slice elements for each arg so they will be passed as separate args 
> to the
> command:
>
>     property := "the prop string"
>     tmpdir := "value for TMPDIR in the passed environment"
>     cmd := &exec.Cmd{
>         Path:       "/path/to/_get_jail_prop",
>         Dir:        "/cd/to/this/dir/,
>         Args:       []string{
>                        "/path/to/_get_jail_prop",
>                        property,
>                        "default",
>                     },
>         Env:        []string{
>                        "PATH=/usr/local/bin:/usr/bin",
>                        "TMPDIR=" + tmpdir,
>                     },
>     }

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From zlynx at acm.org  Wed Feb 21 19:20:30 2018
From: zlynx at acm.org (Zan Lynx)
Date: Wed, 21 Feb 2018 19:20:30 -0700
Subject: [lug] Bash shell script question
In-Reply-To: <3b8af6f5-64a7-e611-c264-35fc09a3e01d@purgatoire.org>
References: <L3tg_bg--3-0@tutanota.com>
 <1517100008.1647334.1250439216.732250C0@webmail.messagingengine.com>
 <CAJB=V00Q=jyzFHYq7bRNsAN303x66Ff+_JbfasOyDUb1uPDmLw@mail.gmail.com>
 <1517182067.3001918.1251124376.179A68B0@webmail.messagingengine.com>
 <31f545da-7b9f-e4c2-f878-69bf8bf77b30@purgatoire.org>
 <2851074d-7731-14dc-79b5-cefacd019a2c@duboulder.com>
 <9e662b14-b3a0-95fb-09fd-0693dbf18a47@purgatoire.org>
 <0dab9af6-a83a-fb56-5740-a8d64b3eeb09@duboulder.com>
 <3b8af6f5-64a7-e611-c264-35fc09a3e01d@purgatoire.org>
Message-ID: <20180222022029.GA28004@catbox.zlynx.org>

On Wed, Feb 21, 2018 at 06:25:14PM -0700, BC wrote:
> Go's slices are wonderful, aren't they?

They are pretty great but like a few other Go constructs, they contain
hidden traps.

You have to always remember that the slice is actually a pointer. If you
pass a slice into a goroutine you either have to not use it from
anywhere else, or you have to use a sync.Mutex.

You also have to remember that appending to a slice returns a
potentially, but not always new copy of the backing array. I've seen
problems where some code kept writing to an old copy of the slice, but
that wasn't actually the array that was currently in use.

And there's potential garbage collection issues. It can surprise some
people that holding a byte slice of six characters can pin a 4K page.
This can happen a lot when parsing out of a file stream, then storing
that slice into a data structure without copying.

The same is true of maps. Maps all hold internal pointers and need to be
single-threaded or guarded by Mutex.

Still, Go has way fewer problems than C or C++. I do like it.

From bcarr at purgatoire.org  Wed Feb 21 19:40:46 2018
From: bcarr at purgatoire.org (BC)
Date: Wed, 21 Feb 2018 19:40:46 -0700
Subject: [lug] Bash shell script question
In-Reply-To: <20180222022029.GA28004@catbox.zlynx.org>
References: <L3tg_bg--3-0@tutanota.com>
 <1517100008.1647334.1250439216.732250C0@webmail.messagingengine.com>
 <CAJB=V00Q=jyzFHYq7bRNsAN303x66Ff+_JbfasOyDUb1uPDmLw@mail.gmail.com>
 <1517182067.3001918.1251124376.179A68B0@webmail.messagingengine.com>
 <31f545da-7b9f-e4c2-f878-69bf8bf77b30@purgatoire.org>
 <2851074d-7731-14dc-79b5-cefacd019a2c@duboulder.com>
 <9e662b14-b3a0-95fb-09fd-0693dbf18a47@purgatoire.org>
 <0dab9af6-a83a-fb56-5740-a8d64b3eeb09@duboulder.com>
 <3b8af6f5-64a7-e611-c264-35fc09a3e01d@purgatoire.org>
 <20180222022029.GA28004@catbox.zlynx.org>
Message-ID: <a1f5230a-3c7d-342c-d680-dabf43dd620d@purgatoire.org>



On 2/21/2018 7:20 PM, Zan Lynx wrote:
> On Wed, Feb 21, 2018 at 06:25:14PM -0700, BC wrote:
>> Go's slices are wonderful, aren't they?
> They are pretty great but like a few other Go constructs, they contain
> hidden traps.
>
> You have to always remember that the slice is actually a pointer. If you
> pass a slice into a goroutine you either have to not use it from
> anywhere else, or you have to use a sync.Mutex.

You are waaaaay ahead of me.  I'm nowhere near ready to learn how to 
use go routines and all they entail.  Same with channels.

> You also have to remember that appending to a slice returns a
> potentially, but not always new copy of the backing array. I've seen
> problems where some code kept writing to an old copy of the slice, but
> that wasn't actually the array that was currently in use.

Yes, I'm taking care to not get caught in that trap.  That is why I'm 
starting with an easy project, converting someone else's working 
"program" into Go.

> And there's potential garbage collection issues. It can surprise some
> people that holding a byte slice of six characters can pin a 4K page.
> This can happen a lot when parsing out of a file stream, then storing
> that slice into a data structure without copying.

I grew up in the era when 4k of RAM was a lot.  But now...  A trifling 
in the big scheme of things.

> The same is true of maps. Maps all hold internal pointers and need to be
> single-threaded or guarded by Mutex.

I haven't learned about maps, yet.

> Still, Go has way fewer problems than C or C++. I do like it.

Oh yeah.  I've drunk the Koolaid.  Never could stomach learning C 
(never tried C++), but Go is quite welcoming in the way it works.

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From zlynx at acm.org  Thu Feb 22 00:26:11 2018
From: zlynx at acm.org (Zan Lynx)
Date: Thu, 22 Feb 2018 00:26:11 -0700
Subject: [lug] Bash shell script question
In-Reply-To: <a1f5230a-3c7d-342c-d680-dabf43dd620d@purgatoire.org>
References: <L3tg_bg--3-0@tutanota.com>
 <1517100008.1647334.1250439216.732250C0@webmail.messagingengine.com>
 <CAJB=V00Q=jyzFHYq7bRNsAN303x66Ff+_JbfasOyDUb1uPDmLw@mail.gmail.com>
 <1517182067.3001918.1251124376.179A68B0@webmail.messagingengine.com>
 <31f545da-7b9f-e4c2-f878-69bf8bf77b30@purgatoire.org>
 <2851074d-7731-14dc-79b5-cefacd019a2c@duboulder.com>
 <9e662b14-b3a0-95fb-09fd-0693dbf18a47@purgatoire.org>
 <0dab9af6-a83a-fb56-5740-a8d64b3eeb09@duboulder.com>
 <3b8af6f5-64a7-e611-c264-35fc09a3e01d@purgatoire.org>
 <20180222022029.GA28004@catbox.zlynx.org>
 <a1f5230a-3c7d-342c-d680-dabf43dd620d@purgatoire.org>
Message-ID: <0fbca839-c7eb-0a9d-1903-4b40cbd0efe9@acm.org>

On 2/21/2018 7:40 PM, BC wrote:
> On 2/21/2018 7:20 PM, Zan Lynx wrote:
>> And there's potential garbage collection issues. It can surprise some
>> people that holding a byte slice of six characters can pin a 4K page.
>> This can happen a lot when parsing out of a file stream, then storing
>> that slice into a data structure without copying.
> 
> I grew up in the era when 4k of RAM was a lot.  But now...  A trifling 
> in the big scheme of things.

Well, it starts to add up. The situation I saw was a toy program I wrote 
that did something you can do with Awk, really. Read a big log file, 
maybe a few hundred megabytes? And store counts of how often words in 
field 6 appeared. And at the end output the list of words and counts.

It used a LOT more memory than I expected and when I profiled it I 
realized that when I thought I was being clever by storing byte slices, 
I had actually prevented the garbage collector from collecting any of 
the file buffers. I just changed it to store regular strings instead, 
which Go copies out of byte slices and the problem went away.

From bcarr at purgatoire.org  Thu Feb 22 07:16:09 2018
From: bcarr at purgatoire.org (BC)
Date: Thu, 22 Feb 2018 07:16:09 -0700
Subject: [lug] Go and Garbage Collection
In-Reply-To: <0fbca839-c7eb-0a9d-1903-4b40cbd0efe9@acm.org>
References: <L3tg_bg--3-0@tutanota.com>
 <1517100008.1647334.1250439216.732250C0@webmail.messagingengine.com>
 <CAJB=V00Q=jyzFHYq7bRNsAN303x66Ff+_JbfasOyDUb1uPDmLw@mail.gmail.com>
 <1517182067.3001918.1251124376.179A68B0@webmail.messagingengine.com>
 <31f545da-7b9f-e4c2-f878-69bf8bf77b30@purgatoire.org>
 <2851074d-7731-14dc-79b5-cefacd019a2c@duboulder.com>
 <9e662b14-b3a0-95fb-09fd-0693dbf18a47@purgatoire.org>
 <0dab9af6-a83a-fb56-5740-a8d64b3eeb09@duboulder.com>
 <3b8af6f5-64a7-e611-c264-35fc09a3e01d@purgatoire.org>
 <20180222022029.GA28004@catbox.zlynx.org>
 <a1f5230a-3c7d-342c-d680-dabf43dd620d@purgatoire.org>
 <0fbca839-c7eb-0a9d-1903-4b40cbd0efe9@acm.org>
Message-ID: <a66ed327-b84c-3b91-802e-71a05c8d94f4@purgatoire.org>



On 2/22/2018 12:26 AM, Zan Lynx wrote:
>
>> I grew up in the era when 4k of RAM was a lot.  But now...  A 
>> trifling in the big scheme of things.
>
> Well, it starts to add up. The situation I saw was a toy program I 
> wrote that did something you can do with Awk, really. Read a big log 
> file, maybe a few hundred megabytes? And store counts of how often 
> words in field 6 appeared. And at the end output the list of words 
> and counts.
>
> It used a LOT more memory than I expected and when I profiled it I 
> realized that when I thought I was being clever by storing byte 
> slices, I had actually prevented the garbage collector from 
> collecting any of the file buffers. I just changed it to store 
> regular strings instead, which Go copies out of byte slices and the 
> problem went away.

Oooops, guilty here too.  I will store this tidbit away for future 
reference. Thanks.

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From jeffrey.haemer at gmail.com  Wed Feb 21 13:10:57 2018
From: jeffrey.haemer at gmail.com (Jeffrey S. Haemer)
Date: Wed, 21 Feb 2018 13:10:57 -0700
Subject: [lug] Fwd: Two curiosity questions
In-Reply-To: <15889ed5-148a-134a-419e-17280a477e59@scrye.com>
References: <CAABvdFyGuWAytN+=vVpriYEsdiCRu3pQvhUzCti80p3n92wqVQ@mail.gmail.com>
 <15889ed5-148a-134a-419e-17280a477e59@scrye.com>
Message-ID: <CAABvdFx1PbXO4KhON61gBy1emGgmj77kUqvqRC1nKhUa46TWRQ@mail.gmail.com>

An FYI for folks who know Kevin & Nash. (Kevin is the host of the BLUG
mailing list.)

---------- Forwarded message ----------
From: Kevin Fenzi <kevin at scrye.com>
Date: Wed, Feb 21, 2018 at 10:52 AM
Subject: Re: Two curiosity questions
To: "Jeffrey S. Haemer" <jeffrey.haemer at gmail.com>

[...]

In other news here:

a) Nash has been at the big vet hospital in portland the last few weeks.
He had a mass on his kidney that burst, and they had to remoove the
kidney and it's been a very slow recovery since then (he wont eat after
that). To help defray his massive medical bills I started a gofundme:
https://www.gofundme.com/medical-bills-for-nash Feel free to pass it on,
just look at the cute nash pics or skip it. ;)
Last night they put in a e-tube on him, and hopefully we can bring him
home for real later today.

[...]

kevin




-- 
Jeffrey Haemer <jeffrey.haemer at gmail.com>
720-837-8908 <(720)%20837-8908> [cell], @goyishekop [twitter]
*פרייהייט? דאס איז יאַנג דינען וואָרט!*
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From blug-mail at duboulder.com  Thu Feb 22 17:25:38 2018
From: blug-mail at duboulder.com (Lee Woodworth)
Date: Thu, 22 Feb 2018 17:25:38 -0700
Subject: [lug] golang gotchas
Message-ID: <6b9d3aa3-f457-b6ab-2034-df1dce4ed2b6@duboulder.com>

Speaking of golang gotchas, when I started using go, nil
handling was sometimes puzzling.

Given these declarations:

     type T struct { a, b int }

     var ptr1 *T = nil
     var ptr2 *T /* effectively (*T) (0) */
     var ifval1 interface{} = T{ a: 1, b: 2 }
     var ifval2 interface{} = &T{ a: 3, b: 4 }
     var ifval3 interface{} = nil
     var ifval4 interface{} = ptr1

The output of fmt.Printf using format specs of "%T/%p %v" for the values
and "%t" for the comparisons:

     ptr1=*main.T/0x0 <nil>
     ptr2=*main.T/0x0 <nil>
         ptr1 == ptr2 is true
         ptr1 == nil is true
     ifval1=main.T/%!p(main.T={1 2}) {1 2}
     ifval2=*main.T/0xc420018100 &{3 4}
     ifval3=<nil>/%!p(<nil>) <nil>
     ifval4=*main.T/0x0 <nil>
         ifval3 == nil is true
         ifval4 == nil is false

The quirky thing is
     ptr1 = nil; ifval4 = ptr1
but
     ptr1 == nil and ifval4 != nil

This is due to interfaces holding a type and a value and untyped nil constant conversion.
     ptr1 = nil effectively converts the untyped nil to (*T) (0)
     ifval3 = nil converts the untyped nil to a special value "nil"
         with a special system-defined type
     ifval4 = ptr1 stores the type *T and ptr1's value (0) which
         is not the same as the special "nil" value

You can see that interface{} is storing a value and not a pointer
from ifval1. Its type is T, not *T and the %p format uses an error
display instead of printing an address.

Another quirk is that if type T had the pointer-based method M1 defined on it:
     func (t *T) M1 () { fmt.Println ("ok") }
then
     ptr1.M1 ()
is a perfectly valid method call and won't panic since M1 does not dereference
its receiver argument.