by Malcolm Rowe

pip install --isolated fails on Python 3.0–3.3

(This is a quick post for search-engine fodder, since I didn’t manage to find anything relevant myself.)

If you’re using pip install --isolated to install Python packages and find that it fails with an error like the following:

Complete output from command python setup.py egg_info:
usage: -c [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
   or: -c --help [cmd1 cmd2 ...]
   or: -c --help-commands
   or: -c cmd --help

error: option --no-user-cfg not recognized

… then you might have run into an incompatibility between pip and Python versions 3.0–3.3.

pip version 6.0 added an isolated mode (activated by the --isolated flag) that avoids looking at per-user configuration (pip.conf and environment variables).

Running pip in isolated mode also passes the --no-user-cfg flag to Python’s distutils to disable reading the per-user ~/.pydistutils.cfg. But that flag isn’t available in Python versions 3.0–3.3, causing the error above.

I ran into this because I recently migrated the Python code that generates this site to run under Python 3.x. I’m using a virtualenv setup, so once I had everything working under both Python versions, I was reasonably confident that I could switch ‘production’ (i.e. the Compute Engine instance that serves this site) to Python 3 and discard the 2.x-compatibility code.

Good thing I tested it out first, since it didn’t even install.

It turns out that:

  1. virtualenv ships an embedded copy of pip and setuptools, but setuptools will use the system version of distutils1, and
  2. --no-user-cfg was added in Python 2.7, but wasn’t ported to 3.x until 3.42, and
  3. the distribution I’m using on my real server (Debian 7) ships with Python 3.2.3, rather than the 3.4.x I’m using elsewhere.

I worked around this by just omitting the --isolated flag for Python verions [3.0, 3.4) — though since I don’t actually have any system config files in practice, I probably could have set PIP_CONFIG_FILE=/dev/null instead (which has the effect of ignoring all config files).

I’m not the first person to have noticed that virtualenv isn’t actually hermetic. Though some of that rant is out of date now (Python wheel files provide prebuilt binaries), and some isn’t relevant to the way I’m using virtualenv/pip, it’s definitely true that the dependency on the system Python libraries is the main reasons I’d look to something more like Docker or Vagrant for deployment were I doing this professionally.

So did I finally manage to switch to Python 3.x after that? Not even close: Python 3.x didn’t gain the ability to (redundantly) use the u'foo' syntax for Unicode strings until 3.3, and some of my dependencies use that syntax. So I’m waiting until I can switch to Debian 8 on Compute Engine3, at which point I can cleanly assume Python 3.4 or later.

  1. This is a rant for another day, but it looks like virtualenv monkeypatches pip, which monkeypatches setuptools, which either monkeypatches or builds upon distutils. Debugging through this edifice of patched abstractions is… not easy. 

  2. It’s a bit more complex than that: Python 3.0 and 3.1 were released first, then the feature was implemented in both 2.7 and 3.2, but then distutils as a whole was rolled back to its 3.1 state before 3.2 was released. That rollback was reverted for Python 3.4. 

  3. I can apt-get dist-upgrade from the Debian 7 image just fine, but it’s a bit slow and hacky, so I’d rather wait for official images. (I also need to fix some custom mail-related configuration that appears to have broken under Debian 8.) 

Noda Time 1.3.1

It’s been a while since the last Noda Time release, and while we’re still working towards 2.0, we’ve been collecting a few bug fixes that can’t really wait. So last Friday1, we released Noda Time 1.3.1.

Noda Time 1.3.1 updates the built-in version of TZDB from 2014e to 2015a, and fixes a few minor bugs, two of which were triggered by recent data changes.

Since it’s been a while since the previous release, it may be worth pointing out that new Noda Time releases are not the only way to get new time zone data: applications can choose to load an external version of the time zone database rather than use the embedded version, and so use up-to-date time zone data with any version of the Noda Time assemblies.

If you’re in a hurry, you can get Noda Time 1.3.1 from the NuGet repository (core, testing, JSON support packages), or from the links on the Noda Time home page. The rest of this post talks about the changes in 1.3.1 in a bit more detail.

End of year transitions (Bangladesh)

In the middle of 2009, Bangladesh started observing permanent daylight saving time, as an energy-saving measure. This was abandoned at the end of that year, and the country went back to permanent standard time.

Until recently, that transition back to standard time was actually recorded as happening a minute too early, at 23:59 on December 31st. TZDB 2014g fixed this by changing the transition time to “24:00” — that is, midnight at the end of the last day of the year.

Noda Time could already handle transitions at the end of the day, but would incorrectly ignore this particular transition because it occurred ‘after’ 2009. That’s now fixed, and Noda Time 1.3.1 returns the correct offset for Asia/Dhaka when using data from TZDB 2014g or later.

BCL provider: historical changes to the base offset (Russia)

In October 2014, most of Russia switched from permanent daylight saving time to permanent standard time, effectively moving local time back one hour. These changes were included in TZDB 2014f.

For people using the BCL provider instead of the TZDB provider (and using Windows), Microsoft delivered a hotfix in September 2014. However, our BCL provider depends upon the .NET framework’s TimeZoneInfo class, and the .NET framework — unlike TZDB — is unable to represent historical changes to the ‘base’ offset of a time zone (as happened here).

The result is that Noda Time (and other applications using TimeZoneInfo in .NET 4.5.3 and earlier) incorrectly compute the offset for dates before October 26th, 2014.

A future update of the .NET framework should correct this limitation, but without a corresponding change in Noda Time, the extra information wouldn’t be used; Noda Time 1.3.1 prepares for this change, and will use the correct offset for historical dates when TimeZoneInfo does.

BCL provider: time zone equality

The time zones returned by the BCL provider have long had a limitation in the way time zone equality was implemented: a BCL time zone was considered equal to itself, and unequal to a time zone returned by a different provider, but attempting to compare two different BCL time zone instances for equality always threw a NotImplementedException. This was particularly annoying for ZonedDateTime, as its equality is defined in terms of the contained DateTimeZone.

This was documented, but we always considered it a bug, as it wasn’t possible to predict whether testing for equality would throw an exception. Noda Time 1.3.1 fixes this by implementing equality in terms of the underlying TimeZoneInfo: BCL time zones are considered equal if they wrap the same underlying TimeZoneInfo instance.

Note that innate time zone equality is not really well defined in general, and is something we’re planning to reconsider for Noda Time 2.0. Rather than rely on DateTimeZone.Equals(), we’d recommend that applications that want to compare time zones for equality use ZoneEqualityComparer to specify how two time zones should be compared.

And finally…

There are a handful of other smaller fixes in 1.3.1: the NodaTime assembly correctly declares a dependency on System.Xml, so you won’t have to; the NuGet packages now work with ASP.NET’s kpm tool, and declare support for Xamarin’s Xamarin.iOS (for building iOS applications using C#) in addition to Xamarin.Android, which was already listed; and we’ve fixed a few reported documentation issues along the way.

As usual, see the User Guide and 1.3.1 release notes for more information about all of the above.

Work is still continuing on 2.0 along the lines described in our 1.3.0 release post, and we’re also planning a 1.4 release to act as a bridge between 1.x and 2.0. This will deprecate members that we plan to remove in 2.0 and introduce the replacements where feasible.

  1. Release late on Friday afternoon? What could go wrong? Apart from running out of time to write a blog post, I mean. 

Just how big is Noda Time anyway?

Some years back, I posted a graph showing the growth of Subversion’s codebase over time, and I thought it might be fun to do the same with Noda Time. The Subversion graph shows the typical pattern of linear growth over time, so I was expecting to see the same thing with Noda Time. I didn’t1.

Noda Time’s repository is a lot simpler than Subversion’s (it’s also at least an order-of-magnitude smaller), so it wasn’t that difficult to come up with a measure of code size: I just counted the lines in the .cs files under src/NodaTime/ (for the production code) and src/NodaTime.Test/ (for the test code).

I decided to exclude comments and blank lines this time round, because I wanted to know about the functional code, not whether we’d expanded our documentation. As it turns out, the proportion of comments has stayed about the same over time, but that ratio is very different for the production code and test code: comments and blank lines make up approximately 50% of the production code, but only about 20–25% of the test code.

Here’s the graph. It’s not exactly up-and-to-the-right, more… wibbly-wobbly-timey-wimey.

Code and test size are both approximately
flat over time, though with dips before some of the major releases.
Noda Time’s codesize in KLOC over time

There are some thing that aren’t surprising: during the pre-1.0 betas (the first two unlabelled points) we actively pruned code that we didn’t want to commit to for 1.x2, so the codebase shrinks until we release 1.0. After that, we added a bunch of functionality that we’d been deferring, along with a new compiled TZDB file format for the PCL implementation. So the codebase grows again for 1.1.

But then with 1.2, it shrinks. From what I can see, this is mostly due to an internal rewrite that removed the concept of calendar ‘fields’ (which had come along with the original mechanical port from Joda Time). This seems to counterbalance the fact that at the same time we added support for serialization3 and did a bunch of work on parsing and formatting.

1.3 sees an increase brought on by more features (new calendars and APIs), but then 2.0 (at least so far) sees an initial drop, a steady increase due to new features, and (just last month) another significant drop.

The first decrease for 2.0 came about immediately, as we removed code that was deprecated in 1.x (particularly, the handling for 1.0’s non-PCL-compatible compiled TZDB format). Somewhat surprisingly, this doesn’t come with a corresponding decrease in our test code size, which has otherwise been (roughly speaking) proportional in size to the production code (itself no real surprise, as most of our tests are unit tests). It turns out that the majority of this code was only covered by an integration test, so there wasn’t much test code to remove.

The second drop is more interesting: it’s all down to new features in C# 6.

For example, in Noda Time 1.3, Instant has Equals() and GetHashCode() methods that are written as follows:

public override bool Equals(object obj)
    if (obj is Instant)
        return Equals((Instant)obj);
    return false;
public override int GetHashCode()
    return Ticks.GetHashCode();

In Noda Time 2.0, the same methods are written using expression-bodied members, in two lines (I’ve wrapped the first line here):

public override bool Equals(object obj) =>
    obj is Instant && Equals((Instant)obj);
public override int GetHashCode() => duration.GetHashCode();

That’s the same functionality, just written in a terser syntax. I think it’s also clearer: the former reads more like a procedural recipe to me; the latter, a definition.

Likewise, ZoneRecurrence.ToString() uses expression-bodied members and string interpolation to turn this:

public override string ToString()
    var builder = new StringBuilder();
    builder.Append(" ").Append(Savings);
    builder.Append(" ").Append(YearOffset);
    builder.Append(" [").Append(fromYear).Append("-").Append(toYear).Append("]");
    return builder.ToString();

into this:

public override string ToString() =>
    $"{Name} {Savings} {YearOffset} [{FromYear}-{ToYear}]";

There’s no real decrease in test code size though: most of the C# 6 features are really only useful for production code.

All in all, Noda Time’s current production code is within 200 lines of where it was back in 1.0.0-beta1, which isn’t something I would have been able to predict. Also, while we don’t quite have more test code than production code yet, it’s interesting to note that we’re only about a hundred lines short.

Does any of this actually matter? Well, no, not really. Mostly, it was a fun little exercise in plotting some graphs.

It did remind me that we have certainly simplified the codebase along the way — removing undesirable APIs before 1.0 and removing concepts (like fields) that were an unnecessary abstraction — and those are definitely good things for the codebase.

And it’s also interesting to see how effective the syntactic sugar in C# 6 is in reducing line counts, but the removal of unnecessary text also improves readability, and it’s that that’s the key part here rather than the number of lines of code that results.

But mostly I just like the graphs.

  1. Or, if you prefer BuzzFeed-style headlines, “You won’t believe what happened to this codebase!”. 

  2. To get to 1.0, we removed at least: a verbose parsing API that tried to squish the Noda Time and BCL parsing models together, an in-code type-dependency graph checker, and a very confusingly-broken CultureInfo replacement. 

  3. I’m not counting the size of the NodaTime.Serialization.JsonNet package here at all (nor the NodaTime.Testing support package), so this serialization support just refers to the built-in XML and binary serialization. 

Simple privilege separation using ssh

If you have a privileged process that needs to invoke a less-trusted child process, one easy way to reduce what the child is able to do is to run it under a separate user account and use ssh to handle the delegation.

This is pretty simple stuff, but as I’ve just wasted a day trying to achieve the same thing in a much more complicated way, I’m writing it up now to make sure that I don’t forget about it again.

(Note that this is about implementing privilege separation using ssh, not about how ssh itself implements privilege separation; if you came here for that, see the paper Preventing Privilege Escalation by Niels Provos et al.)

In my case, I’ve been migrating my home server to a new less unhappy machine, and one of the things I thought I’d clean up was how push-to-deploy works for this site, which is stored in a Mercurial repository.

What used to happen was that I’d push from wherever I was editing, over ssh, to a repository in my home directory on my home server, then a changegroup hook would update the working copy (hg up) to include whatever I’d just pushed, and run a script (from the repository) to deploy to my webserver. The hook script that runs sends stdout back to me, so I also get to see what happened.

(This may sound a bit convoluted, but I’m not always able to deploy directly from where I’m editing to the webserver. This also has the nice property that I can’t accidentally push an old version live by running from the wrong place, since history is serialised through a single repository.)

The two main problems here are that pushing to the repository has the surprising side-effect of updating the working copy in my home directory (and so falls apart if I accidentally leave uncommitted changes lying around), and that the hook script runs as the user who owns the repository (i.e. me), which is largely unnecessary.

For entirely separate reasons, I’ve recently needed to set up shared Mercurial hosting (which I found to be fairly simple, using mercurial-server), so I now have various repositories owned by a single hg user.

I don’t want to run the (untrusted) push-to-deploy scripts directly as that shared user, because they’d then have write access to all repositories on the server. (This doesn’t matter so much for my repositories, since only I can write to them, and it’s my machine anyway, but it will for some of the others.)

In other words, I want a way to allow one privileged process (the Mercurial server-side process running as the hg user) to invoke another (a push-to-deploy script) in such a way that the child process doesn’t retain the first process’s privileges.

There are lots of ways to achieve this, but one of the simplest is to run the two processes under different user accounts, then either find a way to communicate between two always-running processes (named pipes or shared memory, for example), or for one to invoke the other directly.

The latter is more appropriate in this case, and while the obvious way for a (non-root) user to run a process as another is via sudo, the policy specification for that (in /etc/sudoers) is… complicated. Happily, there’s a simpler way that only requires editing configuration files owned by the two users in question: ssh.

The setup is fairly easy: I’ve created a separate user that will run the push-to-deploy script (hg-blog), generated a password-less keypair for the calling (hg) user, and added the public key (with from= and command= options) to /home/hg-blog/.ssh/authorized_keys.

Now the Mercurial server-side process can trigger the push script simply by creating a $REPOS/.hg/hgrc containing:

changegroup.autopush = ssh hg-blog@localhost

This automatically runs the command I specified in the target user’s authorized_keys, so I don’t even have to worry about listing it here1.

In conclusion, ssh is pretty good tool for creating a simple privilege separation between two processes. It’s ubiquitous, and doesn’t require root to do anything special, and while the case I’m using it for here involves two processes on the same machine, there’s actually no reason that they couldn’t be on different machines.

The ‘right’ answer may well be to run each of these as Docker containers, completely isolating them from each other. I’m not at that point yet, and in the meantime, hopefully by writing this up I won’t forget about it the next time I need to do something similar!

  1. In this case, adding a command restriction doesn’t protect against a malicious caller, since the command that’s run immediately turns around and fetches the next script to run from that same caller. It does protect against someone else obtaining the (password-less by necessity) keypair, I suppose, though the main reason is the one listed above: it means that ‘what to do when something changes’ is specified entirely in one place.