Introduction

The standard Go build process

Schematically, the standard toolchain’s process for creating an executable (e.g, to fulfill a go build command) is:

flowchart LR
  start([Start])

  start --> plan[Plan]
  plan --> cache[Check for cached artifacts]
  cache --> compile[Compile code]
  compile --> link[Link final binary]
  link --> finish

  finish([End])

Specifically:

• During the Plan phase, the toolchain lists all Go packages that need to be compiled in order to build the final executable. It constructs a dependency tree from all necessary compilation units;
• It computes a build ID for each package, based on the package’s dependency tree as well as version information for all Go toolchain program used; and checks the $GOCACHE for already available objects matching these build IDs;
• It then proceeds to Compile anything that was not found in the GOCACHE, and then Links the final binary.

When computing build IDs, the Go toolchain invokes all required programs with the -V=full argument, and factors the output into the build ID, so that any change in a toolchain program results in invalidation of all cached outputs from the previous version(s). The regular output of this looks something like the following:

$ go tool compile -V=full
compile version go1.22.5
$ go tool asm -V=full
asm version go1.22.5

Among other information, the build IDs also factor in information about the dependencies of the object, so that changing a package invalidates all its dependents when relevant.

Enter orchestrion toolexec

At the core, Orchestrion is interfacing with the standard Go toolchain using the -toolexec mechanism:

-toolexec 'cmd args'
	a program to use to invoke toolchain programs like vet and asm.
	For example, instead of running asm, the go command will run
	'cmd args /path/to/asm <arguments for asm>'.
	The TOOLEXEC_IMPORTPATH environment variable will be set,
	matching 'go list -f {{.ImportPath}}' for the package being built.

This mechanism allows orchestrion to integrate into the Go build process to modify the source code about to be compiled. In particular:

• The compile command is provided all .go files that are compiled into the final executable; which orchestrion will modify to insert instrumentation code at all relevant places;
• The link command builds the ifnal executable by linking together all the Go packages that contribute to the main entry point; to which orchestrion adds any library required by injected code that was not already present in the dependency tree.

Integrating with GOCACHE

The attentive reader will have noticed that this means orchestrion changes the dependency tree of packages being compiled by possibly adding new branches to it; but the build ID has already been calculated before compile and link are involved… To properly integrate with the Go build artifact cache, orchestrion intercepts the -V=full invocations of toolchain commands, and appends versioning information including:

• its own version (a development build of v0.7.2 in the example below)
• the transitive closure of packages it may inject (resulting in the hash listed after injectables= below)
• the checksum of the built-in injection rules (listef after aspects= below)

The Go toolchain expects a resulting string composed of three fields, so Orchestrion composes into a rather long output:

$ orchestrion toolexec $(go env GOTOOLDIR)/compile -V=full
compile version go1.22.5:orchestrion@v0.7.2+MqXURZSvaKZl7setr4REn5Jn6AlQBABEe3QuUlyYTzW4yJ2XhUTMdsUnd1xjjnvTSxcV76mP7mquaAQCo7nwow==;injectables=lGUc8QV91HuOK1yWcSxkfmUFLQbKekTyy0eANpJE0rmeGmHR5D61VXn04/XX2kjuPbo8Nrdo+dFBmKPgpKV9jQ==;aspects=sha512:M1yO7gdlnh5Uy2ySDJZp1/QbFL97hY5HGKHYpIq2r561weEn4pAbseW7yBGNuQAP8lTpY4Id8M5jC1ItvVcj2w==

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