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59 lines
3.3 KiB
Markdown
59 lines
3.3 KiB
Markdown
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# Launcher Shims
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This small project is used to aid with testing the Enso Launcher, primarily its
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self-upgrade mechanism.
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See
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[UpgradeSpec.scala](../../../engine/launcher/src/test/scala/org/enso/launcher/upgrade/UpgradeSpec.scala)
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to see the relevant tests.
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Our self-upgrade tests need a way to work with multiple launcher binaries which
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report different versions. That is needed to be able to test scenarios where an
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older version of the launcher downloads a newer one and replaces itself with it.
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The launcher version is a part of the launcher binary. We cannot use the same
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binary as then it would be impossible to test if the upgrade has actually
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happened. We cannot rely on passing any parameters as the upgrade process does
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not do that - the inherent property of launcher update is switching the _binary
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executable_ that is being used.
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One way to achieve different versions is to instrument launcher builds and
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create multiple builds with changed versions. However, with how our build
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infrastructure works, it is not easily possible to trigger multiple builds with
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changed parameters from within a test suite and would require significant
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changes that do not seem justified by a single test. Other than that, the
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launcher is built using Native Image which is not incremental and takes a
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significant amount of time. Building the big binary multiple times would
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increase the test time very significantly.
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The chosen solution is to build small Rust binaries which act as a proxy - all
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that they do is run the original launcher binary, they forward all passed
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parameters but modify one special (hidden in normal operations) parameter that
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overrides the reported version.
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We could not use Bash scripts to achieve the same purpose as it is necessary for
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the launcher executables to be really executables - for example on Windows there
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are complexities of moving the currently running executable that we want to test
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and this would not be properly achieved with a Bash script (which is just a
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script opened by a shell/interpreter). So we need actual binary executables. And
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given that the language of choice for such usecases in our project is Rust, this
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subproject has been created. One more advantage is that the shims compile
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extremely fast.
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So the purpose of this project is to build small Rust executables whose purpose
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is just to run another executable at some fixed path and pass the parameters.
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The only difference between each binary is the version that it will use to
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override the original launcher version. To see how the emulation of launcher
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version works, see
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[InternalOpts.scala](engine/launcher/src/main/scala/org/enso/launcher/cli/InternalOpts.scala).
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The launcher shim runs the actual launcher executable as its subprocess,
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ensuring that the shim is running the whole time the original executable is
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running, thus emulating well the conditions of moving a running executable. The
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upgrade mechanism also relies on the launcher detecting the location of the
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binary being run. Since the subprocess will still find the location of the
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original binary which would be wrong, we use a similar mechanism as for
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overriding the version, to override the perceived executable location to point
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at the location of the shim - thus any operations that need to localize the
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launcher binary or move it will work with the shim - as intended for the tests.
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This mechanism is also implemented within `InternalOpts.scala`.
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