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Subject: Re: Testers needed:  New passwd/group handling in Cygwin
From: Warren Young <warren AT etr-usa DOT com>
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Date: Sun, 16 Feb 2014 03:30:56 -0700
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On Feb 15, 2014, at 5:50 AM, Corinna Vinschen wrote:

> I grabbed all of this including your followup
> change shamelessly and added it to the text.

Happy to provide it.

>> It seems to me that we really only need a single Boolean setting:
>> 
>>   ignore_db=true
> 
> No, that's not right.  We have two mechanisms implemented you can
> choose three out of four possible combinations:
> 
> files only
> db only
> files, then db

Yes, I realize that.

While composing the previous email, I considered a 2-Boolean design:

   ignore_db=false
   ignore_files=false

I rejected that design when I realized that ignore_files is redundant with respect to /etc/foo file existence.  If the files are present, use them.  If not, you have a DB-only configuration.

I consider it an advantage of this design that it is not possible for the files to be present but ignored.  It avoids confusion, both human and software.

A person modifying /etc/passwd expects the change to take immediate effect.  (Obviously SAM or AD could mask it, but…)

Software that scans /etc/passwd expects that the data found within have relevance to system operation.

The nonsense configuration — ignore files *and* ignore DB — is still possible to achieve with the 1-Boolean design.  Set ignore_db=true and remove the /etc files.  Cygwin should simply treat this as a “DB-only” configuration, since that’s the only place it *can* get answers.

> It drops any check for existence, too, which
> is one code point less which has to run for each getpwXXX/getgrXXX
> invocation.

Cygwin should check for file existence at the process tree start, where it reads /etc/nsswitch.conf.  If the files are missing at that time, set the “ignore_files” flag.

Cygwin should use that flag instead of explicit file existence tests for the same reason it doesn’t keep re-reading nsswitch.conf.  If the files are missing at process tree start, then later created, you have to create a new process tree for the new files to be used.

This is no sacrifice.  If you use a 2-Boolean design, you only end up in the code fork where file checking is done if ignore_files == false.  That means there is no point to checking for file existence explicitly. Just blindly try to read the files; if the file is missing, the open will fail, so your check is implicit in the attempt.
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