Update Third Party Lib

Updating Third-Party Libraries in imodel-native

This guide covers the process for updating most third-party libraries that you can find in iModelCore/libsrc/. There is other third-party code embedded within our code that does not follow this generic process.

Because of the varied nature of third-party builds, we create Bentley build parts and MKE/MKI files and manually build the libraries our way. This ensures they reuse our build chain setup, and that they build in the same way across all platforms. This also makes setup and maintenance more difficult, as you'll have to react to addition/removal of source files, and new preprocessor macros that may have to get defined. Many libraries are Unix-first, and can only be understood by trying to use their standard ./configure & make process on a Unix system to see what they do so you can replicate it. Once a library has already been ported, updates tend to be easier (look for added/removed files and hope macros didn't change).

Background

There are two primary considerations for this process:

1. Third-party code needs to be committed on a separate branch, with all revisions on the same branch, and then merged in to our branches that have our modifications. This keeps us from losing our Bentley changes.
2. The imodel-native repository requires squash commits, which make git forget that it has merged from a branch. Without these merge revisions, it constantly tries to re-merge everything.

With those considerations, we have a libsrc-Vendor branch to commit the raw drops from the vendor, and libsrc-Main where we merge from main and libsrc-Vendor but can keep the merge revisions because this branch is not directly merged into main. Once libsrc-Main is building, then we create a normal PR branch from main and merge in libsrc-Main which will go through the squash commit. All Bentley changes must be done in libsrc-Main; libsrc-Vendor must be the source as downloaded from the vendor.

Branch roles

Flow diagram

libsrc-Vendor  ──vendor drop──►  libsrc-Main  ──PR branch──►  main
                                      ▲
                                      │
                                 merge main

File conventions

Each library under iModelCore/libsrc/ typically has:

• Vendor source files (.cpp, .h, license/notice) - go on libsrc-Vendor
• Bentley build files (.mke, .PartFile.xml) - exist only on main/libsrc-Main
• Bentley config files (e.g. hand-written config.h replacing CMake output) - exist only on main/libsrc-Main
• Bentley patches (inline modifications to vendor source) - applied on libsrc-Main after vendor merge

When copying vendor source, rename files to match existing conventions (e.g. .cc → .cpp if the repo already uses .cpp).

Steps

1. Prepare: Get the vendor source

Before switching branches, read the library's top-level README.md under iModelCore/libsrc/ if it exists. Its library-specific instructions are mandatory and may modify or supplement this workflow. Read it while still on main, because Bentley-maintained instructions do not belong on libsrc-Vendor.

Download the exact release archive from the upstream project. Note the exact URL - you'll need it for the commit message.

2. Update libsrc-Vendor

git checkout libsrc-Vendor
# For an existing library - delete and replace the directory:
rm -rf iModelCore/libsrc/<category>/<library>/
# Copy only the vendor source files (no build system files, no tests unless needed):
cp <downloaded>/<relevant-files> iModelCore/libsrc/<category>/<library>/
# Check for unwanted new directories - add to .gitignore if needed
git status
git add iModelCore/libsrc/<category>/<library>/
git commit -m "Updating <library> to version X.Y.Z from <exact-download-url>"

Important: The commit message MUST include the exact download URL. Git sometimes produces different diffs for zip vs tar.gz due to line endings, so recording the archive format helps future maintainers.

If the library is new to libsrc-Vendor (never tracked before), this will be an add-only commit. Expect add/add merge conflicts when merging to libsrc-Main later.

3. Merge main into libsrc-Main

git checkout libsrc-Main
git merge main
# Usually clean. Commit if needed.

4. Merge libsrc-Vendor into libsrc-Main

git merge libsrc-Vendor

Conflicts are common here, especially for:

• add/add conflicts (library new to vendor branch): resolve by taking the vendor version (git checkout --theirs <file>)
• Content conflicts (library update changed lines near Bentley patches): manually merge, keeping Bentley modifications where they're still needed

After resolving:

git add .
git commit  # or let the merge auto-commit if clean

4a. Update the version in libsrc/README.md

iModelCore/libsrc/README.md contains a table listing every third-party library and its version. Update the Version cell for the library you're updating to the new version. This is a Bentley file, so like every other Bentley change it is authored on libsrc-Main (not libsrc-Vendor); it will reach main via the PR branch merge in step 5.

 | Directory | Library | Version | vcpkg? |
 |-----------|---------|---------|--------|
 | `compress/snappy` | Google Snappy | 1.2.2 | No |

Match the existing version format for that row (some entries include extra detail like a sub-library version, revision number, or SONAME).

Commit the change on libsrc-Main so it's included in the step 5 merge (a merge only picks up committed content):

git add iModelCore/libsrc/README.md
git commit -m "Update <library> version in libsrc/README.md"

5. Create PR branch and merge libsrc-Main

Before building, revisit the library's top-level README.md and verify that all of its integration and NuGet license metadata instructions have been completed.

git checkout main
git checkout -b user/merge<Library>
git merge libsrc-Main
# Build and verify
git push origin user/merge<Library>

Create a PR. The PR will show many commits from other people due to squash history - this is expected.

When completing the PR: Edit the squash commit message to remove all the excess commit messages GitHub auto-adds. Keep only your own commit descriptions.

6. Push all branches

git push origin libsrc-Vendor
git push origin libsrc-Main
# PR branch is pushed in step 5

Practical tips

• If your changes are already on a PR branch against main (e.g. you did the update directly), you can still retroactively update the vendor branches. Stash your work, do the vendor/main branch dance, then restore.
• When resolving add/add conflicts for vendor files, --theirs means the vendor branch version (since you're on libsrc-Main merging libsrc-Vendor).
• Libraries vary wildly in build setup. The .mke files and PartFile.xml are Bentley's build system replacements for whatever the upstream project uses (CMake, autotools, Bazel, etc.). You may need to replicate ./configure behavior manually.
• You cannot build locally without Bentley's initialized shell environment. CI will validate platform builds you couldn't test locally.

Example: snappy 1.2.2 update (March 2026)

Snappy had never been on libsrc-Vendor before (it was committed directly to main in the initial open-source commit). The update to 1.2.2 involved:

1. Vendor files (on libsrc-Vendor): snappy.cpp, snappy-internal.h, snappy-stubs-internal.h, snappy-sinksource.cpp, snappy-stubs-internal.cpp, snappy-notice.txt - all raw from upstream, .cc renamed to .cpp
2. Bentley files (on PR branch / main only): config.h (hand-written replacement for CMake config), snappy.mke (Bentley build), 3-line #include append at end of snappy.cpp to fold compilation units for bmake
3. Conflicts were add/add (new to vendor branch), resolved by taking --theirs (vendor version)

(Note: iModelCore/libsrc/README.md did not exist at the time of this snappy update, so there was no version-table bump for it. For any update today, remember to update that table per step 4a.)