[CentOS-devel] Fwd: Using probe dependencies for ordering

Tue Mar 31 10:35:13 UTC 2009
Niels de Vos <niels.devos at wincor-nixdorf.com>

Jeff Johnson wrote:
> Niels: I've been assembling the necessary implementations
> to __REALLY__ solve the useradd ordering issue for the last few years.
> Thanks for the impetus to finally attempt to deploy a solution.

Thanks for sending the details :)

> Flaps down, headed on final approach pattern to the release runway ...
> Gonna take a few months still ...
> 73 de Jeff
> Begin forwarded message:
>> From: Jeff Johnson <n3npq at mac.com>
>> Date: March 30, 2009 12:47:57 PM EDT
>> To: rpm-devel at rpm5.org
>> Subject: Using probe dependencies for ordering
>> This thread (I chimed in late solely because using PreReq:
>> never solves any problem in reliable packaging imho)
>>    http://lists.centos.org/pipermail/centos-devel/2009-March/004250.html
>> leads me to attempting to ensure that RPM orders packages
>> so that dynamic events (in this case adding a user) occur
>> before they are needed.
>> There's a whole class of these run-time ordering issues, from
>> daemon restart to cache rebuilds, that are often not ordered
>> correctly, leading to endless discussions of the best way
>> to solve rather mundane issues.
>> The problem is intrinsically run-time because an event
>> (like a cache rebuild or adding a user or ...) needs
>> to precede the installation/erasure of some other package.
>> There's all sorts of solutions that have been proposed,
>> some solutions work better than others ;-) The typical
>> approach is to overload dependencies like
>> 	Provides: user(foo)
>> and then attach some semantic to the existence of a
>> value in rpm dependencies. That approach can take you
>> only so far because the string represents an
>> intrinsically run-time condition without actually
>> testing that the run-time condition is satisfied.
>> E.g. adding a user through a %pre script represented by
>> 	Provides: user(add)
>> is only as good (or bad) as the execution of the
>> script itself. One never knows whether the user
>> was actually added, only that the user was *supposed*
>> to have been added, tying the success of an end-luser
>> upgrade to the quality of the packagers scripting
>> abilities.
>> However, the fundamental problem of representing a dependency
>> on a dynamic event (think: adding a user) that usually occurs in the  
>> middle of
>> rather large upgrade transactions using static dependency
>> assertions is rather confusing.
>> I'm gonna call the (solution to the) problem a "Dynamically Ordered  
>> Event"
>> (or DOE for short) because I gotta call it something.
>> There are already several DOE relations that are used for
>> (partially) ordering packages installs and erases already.
>> The most important DOE event (now handled in @rpm5.org code) is
>> 	Install before erasing a package on upgrade.
>> RPM does not compute file dispositions correctly
>> unless install occurs before erase. The ordering
>> relation is a DOE because (obviously) the installation
>> of the new instance before the old instance is removed
>> is dynamic and solely within the scope of running a transaction.
>> Other DOE events (that were added to RPM5 in February) include
>> ensuring that a package that contains a directory is installed
>> before any file is installed in that directory.
>> Similarly, the end-point of a symlink is used as a DOE relation
>> when ordering packages to ensure that the end-point of a symlink is
>> installed before the symlink itself.
>> (aside)
>> There's another _HUGELY_ important DOE condition in RPM that
>> should be instantly obvious:
>> 	Never erase a package instance if the new instance
>> 	was not installed successfully.
>> But that DOE condition is handled through other means (i.e.
>> doubly linked package upgrade chains) than by permuting the
>> order of package installation and erasure.
>> (another aside)
>> Whether DOE relations (like parentdir/linkto dependencies)
>> are _NECESSARY_ or _USEFUL_ for installing *.rpm packages is quite
>> controversial years after implementation. Adding DOE relations is
>> demonstrably not _NECESSARY_, RPM has survived (and @rpm.org ;-)
>> without using that information for more than a decade, there
>> are other means to constrain ordering with existing dependency  
>> relations.
>> I do claim that the the added DOE relations are _USEFUL_ because
>> of the increased determinism in transactions, with less "partial" in  
>> the
>> package ordering. Additionally, the data in the added DOE
>> relations is *guaranteed* to be accurate because hierarchical
>> file paths and symlink end-points always exist, no fuss, no muss,
>> no packaging policies or reviews are ever needed.
>> So I believe its time to start using DOE events (like adding
>> users, restarting daemons, rebuilding caches, etc) to
>> package ordering much like parentdir/linkto and install-before-erase
>> are currently being used.
>> The major problem with treating useradd like parentdir dependencies
>> is how to detect the condition while installing. Unlike parentdir
>> dependencies, useradd is run by a script that can/will fail.
>> Which brings me to run-time probe dependencies. As implemented,
>> run-time probe dependencies are strings that map to a dynamic run-time
>> test. E.g. (as implemented @rpm5.org, ymmv) this dependency
>> 	Requires: user(foo)
>> tests that getpwnam("foo") has a return code of 0 rather than whether
>> some package some where contains a matching string
>> 	Provides: user(foo)
>> (Note: I'm deliberately ignoring details that both the strcmp(3) and
>> the getpwnam(3) probe will be done if necessary, and also that  
>> probes are not currently
>> run against added packages but only against the "system" whatever  
>> that means).
>> The important point is that (as implemented @rpm5.org) the user(foo)
>> probe tests the condition that indeed, "foo" can be looked up rather
>> than that some package happens to contain a matching Provides: of a  
>> string.
>> The remaining pieces of the puzzle have to do with establishing
>> the points in time where DOE relations are evaluated. At the
>> point where packages are ordered, the script containing the
>> useradd has not yet been run (and may fail when it actually is
>> run). So there's a need to attach a post-condition to the
>> running of %pre (where useradd is typically done) to verify
>> that indeed a user was added. There's also a need to test
>> the precondition when installing a package that needs to do
>> chown(2) on some file path with the uid returned by getpwnam(3)
>> (i.e. exactly what the Requires: user(foo) dependency tests).
>> The above pins down the exact points in time, using pre-/post- 
>> conditions
>> where the probe is meaningful.
>> What remains is to attempt to use the DOE relation while ordering
>> packages. As currently implemented, that basically means that
>> %pre scripts need to be parsed to detect that useradd is being
>> attempted, with whatever user, and a DOE relation needs to be
>> added before the user is actually added so that the package
>> that is attempting to add the user is installed before other
>> packages that need to do getpwnam(3) in order to do chown(2).
>> I think I've covered all the necessary details to attempt a
>> DOE relation for adding users. Adding groups is not significantly
>> different.
>> I also claim that other probes, like /sbin/ldconfig cache rebuilds,
>> can be mapped into DOE relations. Note that the code base @rpm5.org
>> already has the run-time probe
>> 	Requires: soname(libfoo.so)
>> and there are further run-time probes that can be evaluated as
>> pre-/post-conditions during package installation/erasure as well
>> as being used as DOE relations for ordering.
>> Opinions?
>> 73 de Jeff
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