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Incremental improvements to source-build in official builds

We can add official source-build to repos one at a time. This incrementally creates a tree of cached builds.

Dependency leaves

The following diagram is the source-build intermediate output flow in 3.1, with leaf nodes colored gold and the Core-SDK (dotnet/installer) product at the top:

source (img/official-leaves.dot)

To build up the tree of cached builds, source-build must be distributed amongst the repos. We should do this by repeatedly adding source-build functionality to leaf repos (removing completed repos from the graph) until every repo runs source-build.

Note: the .NET Core build does have circular dependencies, so technically no repos are pure leaves. For example, compiling C# in the leaves above requires a .NET Core SDK toolset. Source-build cuts the dependencies as shown above and uses a bootstrap flow to avoid prebuilts. The bootstrap flow is out of the scope of this incremental official build doc. See "The speculative SDK" in README.md.

Leaf first

Reasons to implement ArPow in leaves first:

  • This most closely resembles an actual source-build. Either the repo has no dependencies, or it gets all its dependencies from upstream ArPow builds.
  • If there are differences in the source-built artifacts from the upstreams, they are exercised.
  • Fewer or no prebuilts are used.

Exceptions to leaf-first

Some repos have very high impact to source-build and may introduce unexpected problems, such as dotnet/runtime. It is useful to implement ArPow in these repos even if upstreams aren't ready yet. For example, working on dotnet/runtime before it was a leaf exposed issues with artifact size.

This makes sense for dotnet/runtime, but not for dotnet/aspnetcore. The behavior of aspnetcore depends a lot on the non-portable source-build RID, and needs artifacts to be available specifically for that RID. Building aspnetcore for centos.7-x64 requires runtime artifacts with that non-portable RID, which are only built by source-build.