Task dependencies

An Engage file with a handful of tasks (but only one group) might look like this:

interpreter = ["bash", "-euo", "pipefail", "-c"]

[[task]]
name = "cargo"
group = "versions"
script = "cargo --version"

[[task]]
name = "cargo-fmt"
group = "versions"
script = "cargo fmt --version"

[[task]]
name = "cargo-clippy"
group = "versions"
script = "cargo clippy --version"

[[task]]
name = "rustc"
group = "versions"
script = "rustc --version"

Which results in this graph:

In this configuration, Engage will run all four tasks at the same time. This is actually ideal for this example, since none of these tasks require side effects from any of the other tasks to happen first.

For the sake of demonstrating how Engage works though, pretend that cargo-fmt and cargo-clippy must be run after cargo, and rustc can only run after cargo-fmt and cargo-clippy have finished. This can be accomplished by making the following changes:

interpreter = ["bash", "-euo", "pipefail", "-c"]

[[task]]
name = "cargo"
group = "versions"
script = "cargo --version"

[[task]]
name = "cargo-fmt"
group = "versions"
script = "cargo fmt --version"
# New field!
depends = ["cargo"]

[[task]]
name = "cargo-clippy"
group = "versions"
script = "cargo clippy --version"
# New field!
depends = ["cargo"]

[[task]]
name = "rustc"
group = "versions"
script = "rustc --version"
# New field!
depends = ["cargo-fmt", "cargo-clippy"]

Which results in this graph:

Here, it can be seen that Engage will first run cargo, and when it's done, it will run cargo-fmt and cargo-clippy in parallel, and after those both finish, it will finally run rustc.

If a task with dependents fails, Engage will not start any of those dependent tasks. In the above Engage file, if cargo-fmt or cargo-clippy fail, then rustc is guaranteed to not run; if cargo fails, then all three other tasks are guaranteed to not run.