Design Document: Distribution Artifact for Bazel
Design documents are not descriptions of the current functionality of Bazel. Always go to the documentation for current information.
Status: Implemented
Author: Klaus Aehlig
Design document published: 11 October 2016
Current State and Shortcomings
Dependency on protoc
Bazel depends on a protobuffer compiler to generate code, especially
java code, from an abstract description of the protocol buffer;
in particular, files generated by protoc
are machine-independent.
In fact, Bazel most of the time uses the latest version of protoc
.
New versions of protoc
that contain incompatible changes to the
programming interface are released frequently.
Current approach to this dependency
The current approach to the protoc
dependency is to have checked-in
statically-linked executables for all the supported platforms (where
some platforms, like FreeBSD, have to use Linux-compatibility features).
The full source tree of the protobuf compiler is also part of the repository.
However, for generating files, the committed binaries are always used.
Shortcomings
The current approach as certain shortcomings.
-
Having up-to-date binaries for all the supported platforms does not scale well as the number of platforms Bazel should run on is increasing.
-
The requirement of having a suitable executable in the code base adds additional complexity to the process of bootstrapping a new architecture.
-
Binaries in the code base do not follow standard open-source principles; in fact, meaningful reviews for changes updating them are hard and in practise often boil down to a question of trust in the person making the change.
-
Committed binaries make the “source” repository unnecessary big. Currently, a checkout at head contains over 250MB in committed
.exe
and.dll
files.
Proposed solution
Change BUILD
to compile protoc
from source
This BUILD
file for the third_party/protobuf
is changed in such a
way, that the protoc
is compiled from source instead of selecting from
the committed pre-built binaries; the pre-built binaries are removed from
the source tree. As the protoc
sources are already part of the repository,
this is not a huge change; also, as protoc
is written in C++
, no additional
dependencies are introduced that way.
Note that then, every user who already has a working (bootstrap) bazel
, can
build bazel from source, without depending on committed binaries or having
a protoc
already on the machine. The problem of building your first bazel
will be addressed in the next sections.
This change also removes an internal consistency requirement from the code base. It was always assumed that the binaries actually match the accompanying sources.
Distribution artifact
A new target //:bazel-distfile
will be added. This will be an archive
containing
-
all source files in their respective places, including the files under
third_party
,site
,scripts
, etc, as well as -
under a subdirectory
derived
all the files generated byprotoc
that are needed to compile a bootstrap version ofbazel
.
For convenience, the derived
subdirectory may also contain other
generated architecture-independent files, like an HTML-version of the
documentation for local browsing. A corrollary of the archive layout is that
by removing the derived
directory a checkout of the upstream sources is
obtained.
This new artifact will be built for every release and made available together along with the other release artifacts (like packages, installers, executables). The same means of certifying integrity (like hashes, SSL-certificates) will be used.
Bootstrapping Bazel
The compile.sh
will be modified to first check if a derived
directory exists
and if this is the case assume that all the files generated by protoc
are
already present there; only if not present, it will try to generate the needed
output of protoc
for bootstrapping, assuming that the PROTOC
environment
variable points to a good protoc
binary.
So, there will be three ways to build bazel
.
-
If one has an old
bazel
binary already, a new one can be built from a checkout of the source repository. This approach is useful for developers. It might also be used by users who want to upgrade their oldbazel
binary to the next release. -
By downloading the distribution artifact, the
compile.sh
script can be used to build bazel. Again, noprotoc
has to be installed ahead of time. This approach is useful for source distributions, as well as for bringing Bazel to a new platform. -
If one already has the correct version of
protoc
on the machine, thecompile.sh
script can be used by setting thePROTOC
environment variable. This approach is useful for distributions that want to provide snapshots ofbazel
between official releases and maintain aprotoc
package anyway.
Other approaches considered
Requiring users to have the correct version of the protoc
binary installed
This would be the standard open-source approach of requiring the user to have
the required dependencies installed ahead of time. Unfortunately, protoc
contains incompatible changes too frequently, so that this would be an
unreasonable
burden. Note that the bootstrapping from your own protoc
and a repository
checkout is still possible with the suggested approach.
Committing the protoc
output
Another approach would be to make the output of protoc
part of the versioned
sources instead of generating them for the distribution file. As with all
approaches based on committing generated files, this would
introduce another consistency requirement to the repository. In this case, the
requirement would be that the generated files be up-to-date with respect to the
respective .proto
files. Of course, such a consistency could be verified by
an appropriate test. Nevertheless, it seems the cleaner and probably more
manageable to only version true source files and generate derived files from
the respective sources.