I spend some time in adding support for Bazel to OpenEXR. OpenEXR v3.0.1 comes now with support for Bazel. OpenEXR has a dependencies to zlib and to Imath. If you want to build OpenEXR you also need to build some state of zlib and Imath.

If you do not have a monorepo you have always the dilemma of how to combining different repositories. A focus of Bazel is to have reproducible builds. Therefore, Bazel is not happy about using master it rather expects a commit hash.

# Option 1

In the case we want always to use the newest and greatest master of Imath, we could simply do it this way:

maybe(
new_git_repository,
name = "Imath",
build_file = "@openexr//:bazel/third_party/Imath.BUILD",
branch = "master",
)


Nevertheless, this would trigger a Bazel warning:

DEBUG: Rule 'Imath' indicated that a canonical reproducible form can be obtained by modifying arguments commit = "b1b5c7cb5f104bcf904d0925cca3abf3cc0f403f", shallow_since = "1614791708 -0800" and dropping ["branch"]


This is only a warning to the user to pin the Imath version. It does not block building or anything else. The user can pin in the WORKSPACE file a specific Imath version. Somewhere in the users WORKSPACE file, before including OpenEXR:

# Pin Imath - to get red of the warning of using always newest master
http_archive(
name = "Imath",
build_file = "@openexr//:bazel/third_party/Imath.BUILD",
sha256 = "6943dce2b2e8737d7cf1ae58103195b64c51c6330237ec07c12d829745b4c9c0",
strip_prefix = "Imath-f21e31a85a4a0b4ddcd980a19e448fd7eca72cce",
)


BTW: This works because we use the keyword maybe which means fetch Imath only when it was not fetched before in a different way.

• No need to bump Imath version
• User gets a Bazel warning to pin it (can be seen as an advantage or disadvantage)

• master of Imath could be broken -> OpenEXR also broken (maybe you see this as an advantage)
• In a few years it will not be clear which Imath version has been used exactly since we have only the information “master” (think about checking out a one year old OpenEXR for some reason - who knows what Imath version should be used with this old OpenEXR state - we do not have this knowledge in the repo -> maybe this is not important for you or users of OpenEXR)
• A Bazel warning is generated. User has to specify a Imath version to get rid of it
• If the CMakeList.txt file of Imath changes this is maybe not reflected in the Imath.BUILD file and this would break the Bazel build (I could also add a WORKSPACE and BUILD file to the Imath repo and add a Bazel build test there to make sure that this does not happen. Bumping Imath manually gives me here some control to check for such issues)

# Option 2

The second option is bump Imath manually.

• Manual bumping needed (-> I will care about it, even if I do not care about it I would expect that an older version of Imath works some time together with a newer OpenEXR version - only if you do incompatible changes this leads to a problem)

• User has not to care which OpenEXR master can be combined with which Imath master
• Checking out an old state of OpenEXR will use than an old version of Imath that was created around the same time
• No Bazel warning - no need for users to pin version to get rid of the warning
• Overriding Imath by a different version is still possible if a user wishes this
• Change of master CMakeLists.txt in Imath repo does not have an effect -> using a controlled way of bumping the version enables me to check if some change in Imath (especially CMakeLists.txt) affects the build

# Option 3

Extension of Option 1: I could think about introducing some variables:

IMATH_REP = "https://github.com/AcademySoftwareFoundation/Imath.git"
IMATH_TAG = "master"

maybe(
new_git_repository,
name = "Imath",
build_file = "@openexr//:bazel/third_party/Imath.BUILD",
tag = IMATH_TAG,
remote = IMATH_REP,
)


Similar to option 1.

# Conclusion

When going for Option 1 it would make sense to add in the long run a WORKSPACE and BUILD file to the Imath repo and a corresponding Bazel build test in the Imath repo. If this is not wished I would stay with the current setting.

Once Imath is officially released and has release tags, it makes sense to pin the version to the Imath release.

Nevertheless, in the case that master of OpenEXR is not anymore compatible with the latest Imath release a fallback to Option 1 will happen.