Upgrading to Ruby 2.1 (and why complex regexes will inevitably hurt you)
engineering.freeagent.com
engineering.freeagent.com
> This tree of strings is then converted to YAML tokens using the class Psych::ScalarScanner that internally uses regular expressions to identify YAML types. The regular expression used to identity time objects changed in the new version of Psych, which meant our nanosecond precision time was now correctly converted to a YAML Time type rather than being left as a string.
Apparently, the "hurt" mentionned in the title refers to a bug being fixed.
In ruby 1.9, a String containing a timestamp gets serialized as a String -- and parsed as a String in 1.9 or 2.1.
In ruby 2.1, a String containing a timestamp gets serialized as a Time (I'm not sure I'd consider this correct behavior), -- and parsed in again as a String anyway in ruby 2.1 (due to new YAML-safety behavior), but as a Time in ruby 1.9.
The ruby 2.1 behavior of serializing out as a Time but then parsing back in to a String anyway -- seems particularly sketchy, and ripe for roundtrip bugs where the string it gets turned into on parse isn't _quite_ the same one that was original before serialization. (Say, winds up with a different timezone or something).
I'm not sure what the lesson here is, but I don't think it's about regexen. It may be about how too much cleverness/magic will get you -- just keep it simple. Shared gem/library code should be predictable and understandable with simple mental models, and gems used as core dependencies should be very stable from version to version (yaml and yaml serialization rules are neither).
It may be, yet again, about how using YAML the way we use YAML is a big mistake -- either use ruby marshal if you actually want exact roundtrip ruby objects, or serialize only to JSON-compatible datatypes (whether using JSON or maybe that's what YAML should have done all along). This middle ground of YAML where it's hard to predict what is round trippable and what isn't, what is safe and what isn't, and it can change from version-to-version -- is just asking for trouble. But it's hard to decide to stop doing it, as so many of your gem dependencies might be.
It's not serializing it out as a time, but serializing it as an "implicit string" (a string with no quotes). When Psych goes to dump the string out, it checks to see if the string could be interpreted as something else. Since the string being dumped doesn't match a YAML timestamp, it doesn't add explicit quoting on the string.
Here's an example. From the blog post, they are doing this:
Psych.dump Time.now.utc.strftime("%Y-%m-%d %H:%M:%S.%6N %Z")
"--- 2014-05-23 15:42:29.882127 UTC\n...\n"
If we change the `strftime` to produce a string that does look like a YAML timestamp like this:
Psych.dump Time.now.utc.strftime("%Y-%m-%d %H:%M:%S.%6N %z")
"--- '2014-05-23 15:46:31.197338 +0000'\n"
irb(main):012:0> Psych.load "--- '2014-05-23 15:46:31.197338 +0000'\n"
=> "2014-05-23 15:46:31.197338 +0000"
irb(main):013:0> Psych.load "--- 2014-05-23 15:46:31.197338 +0000\n"
=> 2014-05-23 08:46:31 -0700
>The same would happen in Psych 2.0.5 if it were not for a new feature called Safe Load which was introduced after the recent Rails YAML deserialisation security vulnerabilities.
I have no idea where they got this. You have to explicitly call `safe_load`, and it doesn't sound like they're doing that. If your format the timestamp correctly in your YAML, it will happily deserialize as a Time object. :-/
...is leading me back to suggesting YAMLs data model is overly complex, and not understandable with a simple developer mental model, and that's the root of a lot of problems.
I previously thought leaving quotes off string literals in YAML was simply a convenience for hand-writing YAML and producing YAML with less noise for human readability; I didn't realize it resulted in an 'implicit string' which would be de-serialized by YAML parsers differently than quoted strings.
(Are these patterns by which 'implicit strings' are recognized to be coerceable to certain types part of the YAML standard, or up to the particular YAML parsing implementation as to how/whether to do it? If the latter... woah, just asking for trouble, no wonder we got it.)
update Oh wait, I just realized I totally misunderstood what you were saying. Strings are Strings, but on serializing, the serializer is supposed to ensure quotes are used if the string, were it unquoted would be misinterpreted as something other than a string. Okay, seems reasonable -- but OP is reporting this is failing somehow, right? And in some cases something meant to be a string is winding up unquoted, and being interpreted as something else on load?
Or I'm probably still not quite getting it. Okay, yeah, tldr, we can stick with: YAML's data/processing model ends up being way too complex making it's behavior hard to predict and hard to maintain consistent between parsers/versions.
In this particular case, a timestamp with "UTC" in it should not be interpreted as a timestamp, and older versions of psych would interpret it as a timestamp even though the spec says it's not valid.
> Okay, yeah, tldr, we can stick with: YAML's data/processing model ends up being way too complex making it's behavior hard to predict and hard to maintain consistent between parsers/versions.
Absolutely. If we eliminated implicit strings, it would make life lots easier. OTOH, you could just use JSON. :-D
Of course, I realize the devil is in the details, and that may be what those developing YAML thought they were doing, but always adding just one more tiny thing that would be useful too.
Complexity vs flexibility, one of the fundamental tensions of software design. I see how we got here through good intentions, but in retrospect YAML made some decisions on this balance that most of us would do differently. Or, did those designing YAML realize what a fundamental part of the ruby ecosystem it was to become, and what the implications of that were? Maybe part of the lesson is that some things really do need to be carefully designed, not just iteratively designed "you can always fix it later" by an ever-changing community of volunteers; is that part of what happened with YAML spec and implementations and uses?
Also, thanks for explaining the bug-fix as part of the story, that makes a lot more sense. I understand that bug fixes will inevitably sometimes be backward-incompat. I think it has maybe been consistently under-estimated how important/useful it would to allow upgrades of ruby itself while keeping the exact same YAML processor and version. The psych/syck stuff ended up being one of the biggest pain points for many people way back in 1.8.7->1.9.3 too. Most gems ended up compatible with multiple ruby versions, so you could upgrade ruby versions while keeping your gem dependencies identical. YAML serializing/parsing, so fundamental, where minor differences in implementation can have disastrous consequences, was a painful exception.
It's there, but I agree it's bad editorializing. A better takeaway is that if you use YAML you will be burned, but even that is a reach since YAML for all its horrors is the best serialization format for Ruby, something for which you'd probably be equally ill-advised to roll your own.
https://github.com/tenderlove/psych/blob/b6e8b10c83720fe73ab...
And if that serialization format were ruby marshal... there's actually a 'marshal version' embedded with marshaled serializations, which changes independently of ruby version, can change to indicate backwards incompat, but I don't believe has changed in quite some time, not between 1.9.3 and 2.1.x. So again, if the marshal version is the same, we'd be shocked (and it would unambiguously be a bug) if there were anything handled differently between versions. (Okay, that's not entirely true, I can think of some counter-examples too).
I think it really is about YAML, and the mistaken design decisions in the way much of rubydom has historically used it, which continue to pile up.
Note that I think ActiveRecord `serialized` uses YAML in the db, so that's another place you might find yourself "sharing state" via YAML between diff ruby versions -- if you upgrade to ruby 2.1 and have it reading a database created with an earlier ruby version. Oops.
Maybe the lesson is really that serialization is way harder to get right than you think, in terms of both security and forwards compatibility (future-proofness).
Are you sure Activerecord uses YAML in the DB? Everything I've used it for is stored as the native database types. What is the need to serialize the object into the DB rather than create a new one based on the record in the DB?
Nowhere did I say serialisation was easy. My comment was that running multiple versions of platform and libraries at once sharing state was brave. To do that I would want to be completely explicit about the state format and have tests to ensure it is correct even though it is an internal API.
Better not to share anything between the servers and just have them all accessing the same service that owns the state format (probably the DB but possibly also caching layers[risky needs managing] and other services).
ActiveRecord use YAML (by default) to serialize non scalar values like hashes and arrays.
http://apidock.com/rails/ActiveRecord/AttributeMethods/Seria...
In ruby 2.1:
irb(main):018:0> "2014-99-99 99:99:99.999999 UTC".to_yaml
=> "--- 2014-99-99 99:99:99.999999 UTC\n...\n"
irb(main):006:0> YAML.load "--- 2014-99-99 99:99:99.999999 UTC\n...\n"
ArgumentError: argument out of range
from /usr/lib/ruby/1.9.1/psych/scalar_scanner.rb:111:in `utc'
PS: If you think at this point "who would send an incorrect timestamp?", think about some definition of a field validator for example. "format: 9999-99-99 99:99:99.999999 UTC"
Because that's what the YAML spec says to do. I will try to write better code. :-(
This is a needless bash on Regex (which is not even mentioned in the article at all, except as a YAML footnote!). I have a tip, though- If you DO need to write a complex regex (hint: it will run much faster than doing the same thing in code), document it using things like the /x switch! Here is an example I wrote of a complex password validator- I don't think most developers would have a problem understanding what's going on:
I love them for some reason because 1) they're like puzzles, 2) they're extremely fast if you know what you're doing, vs. doing the same things in interpreted code.