This improves our Bundler integration (i.e. `bundlerEnv`).
Before describing the implementation differences, I'd like to point a
breaking change: buildRubyGem now expects `gemName` and `version` as
arguments, rather than a `name` attribute in the form of
"<gem-name>-<version>".
Now for the differences in implementation.
The previous implementation installed all gems at once in a single
derivation. This was made possible by using a set of monkey-patches to
prevent Bundler from downloading gems impurely, and to help Bundler
find and activate all required gems prior to installation. This had
several downsides:
* The patches were really hard to understand, and required subtle
interaction with the rest of the build environment.
* A single install failure would cause the entire derivation to fail.
The new implementation takes a different approach: we install gems into
separate derivations, and then present Bundler with a symlink forest
thereof. This has a couple benefits over the existing approach:
* Fewer patches are required, with less interplay with the rest of the
build environment.
* Changes to one gem no longer cause a rebuild of the entire dependency
graph.
* Builds take 20% less time (using gitlab as a reference).
It's unfortunate that we still have to muck with Bundler's internals,
though it's unavoidable with the way that Bundler is currently designed.
There are a number improvements that could be made in Bundler that would
simplify our packaging story:
* Bundler requires all installed gems reside within the same prefix
(GEM_HOME), unlike RubyGems which allows for multiple prefixes to
be specified through GEM_PATH. It would be ideal if Bundler allowed
for packages to be installed and sourced from multiple prefixes.
* Bundler installs git sources very differently from how RubyGems
installs gem packages, and, unlike RubyGems, it doesn't provide a
public interface (CLI or programmatic) to guide the installation of a
single gem. We are presented with the options of either
reimplementing a considerable portion Bundler, or patch and use parts
of its internals; I choose the latter. Ideally, there would be a way
to install gems from git sources in a manner similar to how we drive
`gem` to install gem packages.
* When a bundled program is executed (via `bundle exec` or a
binstub that does `require 'bundler/setup'`), the setup process reads
the Gemfile.lock, activates the dependencies, re-serializes the lock
file it read earlier, and then attempts to overwrite the Gemfile.lock
if the contents aren't bit-identical. I think the reasoning is that
by merely running an application with a newer version of Bundler, you'll
automatically keep the Gemfile.lock up-to-date with any changes in the
format. Unfortunately, that doesn't play well with any form of
packaging, because bundler will immediately cause the application to
abort when it attempts to write to the read-only Gemfile.lock in the
store. We work around this by normalizing the Gemfile.lock with the
version of Bundler that we'll use at runtime before we copy it into
the store. This feels fragile, but it's the best we can do without
changes upstream, or resorting to more delicate hacks.
With all of the challenges in using Bundler, one might wonder why we
can't just cut Bundler out of the picture and use RubyGems. After all,
Nix provides most of the isolation that Bundler is used for anyway.
The problem, however, is that almost every Rails application calls
`Bundler::require` at startup (by way of the default project templates).
Because bundler will then, by default, `require` each gem listed in the
Gemfile, Rails applications are almost always written such that none of
the source files explicitly require their dependencies. That leaves us
with two options: support and use Bundler, or maintain massive patches
for every Rails application that we package.
Closes#8612
vcunat did some cosmetic changes, such as joining lines
because we seem to rarely use one-identifier-per-line style,
or fixing hyena description to conform to our rules.
From the changelog:
```
Version 0.7.80, 2015-11-30
+ Matroska: support of MKVMerge statistics tags (duration frame count,
stream size, bit rate) per track, thanks to ndjamena
+ FLAC: Channel positions, thanks to ndjamena
+ FLAC: difference between detected bit depth and stored bit depth
+ MPEG-TS: if DTVCC transport stream is present and no DTVCC service
descriptor, scan also in the middle of the file in order to detect
more caption services
+ Subtitle frame rate computing if frame count and duration are
available (hidden by default)
+ Subtitles in Matroska: count of elements
+ Matroska, MXF and MP4/MOV: detection of truncated files
+ DTS: difference between ES Matrix and ES Discrete
+ DTS: display ES Matrix or ES Discrete even if HRA or MA is present
+ DTS: difference between DTS-HRA with 96k option and pure DTS-96/24
+ DTS: detection of DTS:X
+ Samples per frame info
+ AC-3: detection of Atmos inside TrueHD
+ Video frame rate: showing precision of 1/1.001 frame rates (e.g.
"23.976 (24000/1001) fps" and "23.976 (23976/1000) fps")
+ MPEG-4/MOV: showing the complete list of compatible brands in the
CodecID field
+ MPEG-4/MOV: Alternate groups
+ MPEG-4/MOV: "Disabled" tag
+ MPEG-4/MOV: "Forced" tag
+ MPEG-4/MOV: showing links between tracks (chapters for, subtitles for,
fallback for)
+ MXF: handling of more acquisition metadata items
+ MXF: Package name
+ AVC: Store method of interlaced content (Interleaved Fields or
Separated Fields)
+ EBUCore: acquisition metadata (Proof of concept, for feedback only)
x Matroska: frame rate detection algorithm revisited, less wrong numbers
are expected
x SDP/Teletext: some pages were sometimes (when present in 2 different
SDP lines) displayed several times
x MPEG-4/MOV: some hint tracks were not displayed
+ Hongkongese language added
+ Option "Full parsing"
```
