enso/lib/rust/launcher-shims

Launcher Shims

This small project is used to aid with testing the Enso Launcher, primarily its self-upgrade mechanism.

See UpgradeSpec.scala to see the relevant tests.

Our self-upgrade tests need a way to work with multiple launcher binaries which report different versions. That is needed to be able to test scenarios where an older version of the launcher downloads a newer one and replaces itself with it. The launcher version is a part of the launcher binary. We cannot use the same binary as then it would be impossible to test if the upgrade has actually happened. We cannot rely on passing any parameters as the upgrade process does not do that - the inherent property of launcher update is switching the binary executable that is being used.

One way to achieve different versions is to instrument launcher builds and create multiple builds with changed versions. However, with how our build infrastructure works, it is not easily possible to trigger multiple builds with changed parameters from within a test suite and would require significant changes that do not seem justified by a single test. Other than that, the launcher is built using Native Image which is not incremental and takes a significant amount of time. Building the big binary multiple times would increase the test time very significantly.

The chosen solution is to build small Rust binaries which act as a proxy - all that they do is run the original launcher binary, they forward all passed parameters but modify one special (hidden in normal operations) parameter that overrides the reported version.

We could not use Bash scripts to achieve the same purpose as it is necessary for the launcher executables to be really executables - for example on Windows there are complexities of moving the currently running executable that we want to test and this would not be properly achieved with a Bash script (which is just a script opened by a shell/interpreter). So we need actual binary executables. And given that the language of choice for such usecases in our project is Rust, this subproject has been created. One more advantage is that the shims compile extremely fast.

So the purpose of this project is to build small Rust executables whose purpose is just to run another executable at some fixed path and pass the parameters. The only difference between each binary is the version that it will use to override the original launcher version. To see how the emulation of launcher version works, see InternalOpts.scala.

The launcher shim runs the actual launcher executable as its subprocess, ensuring that the shim is running the whole time the original executable is running, thus emulating well the conditions of moving a running executable. The upgrade mechanism also relies on the launcher detecting the location of the binary being run. Since the subprocess will still find the location of the original binary which would be wrong, we use a similar mechanism as for overriding the version, to override the perceived executable location to point at the location of the shim - thus any operations that need to localize the launcher binary or move it will work with the shim - as intended for the tests. This mechanism is also implemented within InternalOpts.scala.