Some other repos apply the condition where they set the flag:
https://github.com/dotnet/machinelearning/blob/adf0cecfa8adecfbc299b9eada5e0db27bb27fe8/build/vsts-ci.yml#L34
https://github.com/dotnet/aspnetcore/blob/7f1e7a582243aba0ec9a94cafde7f645f15dda77/.azure/pipelines/ci.yml#L52-L53
https://github.com/dotnet/winforms/blob/fd670ec808ed3b20c69f923a1fe0bf34fe1cae23/azure-pipelines.yml#L14-L15

But I am OK with us doing this. I wonder if there is anyone, like NuGet for instance, that might have a default branch name that's not main. Can we make the condition be building in default branch?

there's no good way to check what the default branch is. I don't think anyone currently enabling source-indexing is using a different name, my thinking was that if repo is using a different name they can just override the condition.

Here's the list of repos indexed today -- https://github.com/dotnet/source-indexer/blob/3733d33b8050743a7034c3e2c10deb06c3a2b78d/src/index/repositories.props

I think you're right, but I'd like to add more (like NuGet). I guess we just need to make sure they update the condition. I think we hadn't added NuGet in the past due to it building outside dnceng. I think we might need to do some work in azdo around permissions to let them publish to the source index's internal blob storage.

/backport to release/10.0

Started backporting to release/10.0 (link to workflow run)

/backport to release/9.0

Started backporting to release/9.0 (link to workflow run)

@akoeplinger backporting to release/9.0 failed, the patch most likely resulted in conflicts. Please backport manually!

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: Only run source index for main builds
Using index info to reconstruct a base tree...
M	eng/common/core-templates/job/source-index-stage1.yml
Falling back to patching base and 3-way merge...
Auto-merging eng/common/core-templates/job/source-index-stage1.yml
CONFLICT (content): Merge conflict in eng/common/core-templates/job/source-index-stage1.yml
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0001 Only run source index for main builds
Error: The process '/usr/bin/git' failed with exit code 128