[Bug 1110331] Re: Enhance glibc to monitor mtime of resolv.conf

[Thomas Hood]

Oops, I was confused when I wrote the initial Bug Description. The
stat() function *does* follow symbolic links; it's lstat() that doesn't.
So eglibc should, as it stands, also notice changes to
/run/resolvconf/resolv.conf. I will revise the Description now.

** Description changed:

  Browsing the eglibc mailing list archives I notice[0] that the eglibc
  resolver has been enhanced[1] such that if /etc/resolv.conf's mtime
  changes then the client is asked to re-initialize the resolver state. I
  haven't tested the code, but if I understand the feature correctly it
  gives us an opportunity to eliminate from /etc/resolvconf/update.d/libc
  the code that restarts nscd. That would be a desirable simplification.
  Ditto for some other resolvconf update scripts.
  
- We can't go ahead and eliminate the nscd-restart code yet for a simple
- reason. The eglibc resolver monitors /etc/resolv.conf and thus watches
- the mtime of the symlink, not the target, whereas of course resolvconf
- modifies the target.[2]
- 
-                 ret = stat (_PATH_RESCONF, &statbuf);
-                 [...]
-                 if ((ret == 0) && (last_mtime != statbuf.st_mtime)) {
-                         last_mtime = statbuf.st_mtime;
-                         atomicinc (__res_initstamp);
-                 }
- 
- The first question is, can we take advantage of this eglibc enhancement
- in order to simplify resolvconf update scripts?  Second question: do we
- want to?
- 
- If so, then the third question is, what's the best way to enhance the
- new eglibc feature so that we can take advantage of it? Should the
- eglibc code be changed so that, in addition to noticing a change in the
- mtime of /etc/resolv.conf, it also notices a change in the mtime of the
- target of /etc/resolv.conf if the latter is a symbolic link?
+ Can we take advantage of this eglibc enhancement in order to simplify
+ resolvconf update scripts?  Do we want to?
  
  [0]http://www.eglibc.org/archives/patches/msg00977.html
  [1]http://patch-tracker.debian.org/patch/series/view/eglibc/2.11.3-4/any/submitted-resolv.conf-thread.diff
- [2]Quoted code is at line 102 of resolv/res_libc.c

** Summary changed:

- Enhance glibc to monitor mtime of resolv.conf
+ Eliminate nscd restart from update script?

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https://bugs.launchpad.net/bugs/1110331

Title:
  Eliminate nscd restart from update script?

Status in “resolvconf” package in Ubuntu:
  New

Bug description:
  Browsing the eglibc mailing list archives I notice[0] that the eglibc
  resolver has been enhanced[1] such that if /etc/resolv.conf's mtime
  changes then the client is asked to re-initialize the resolver state.
  I haven't tested the code, but if I understand the feature correctly
  it gives us an opportunity to eliminate from
  /etc/resolvconf/update.d/libc the code that restarts nscd. That would
  be a desirable simplification. Ditto for some other resolvconf update
  scripts.

  Can we take advantage of this eglibc enhancement in order to simplify
  resolvconf update scripts?  Do we want to?

  [0]http://www.eglibc.org/archives/patches/msg00977.html
  [1]http://patch-tracker.debian.org/patch/series/view/eglibc/2.11.3-4/any/submitted-resolv.conf-thread.diff

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