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I have been trying to build an OpenGL-based image processing library with GLFW, and need it to be testable. Unfortunately, I ran into this bug - GLFW needs its initialization functions to be called from the main thread, but Cargo tests are run on a background thread.

Lukas Kalbertodt
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jasongrlicky
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  • Just ran into this trying to use egui, guess it's still not Just Works in 2022? – Max Jun 17 '22 at 02:16

1 Answers1

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No, to the best of my knowledge, it's not easily possible right now. There is a --test-threads argument you can pass to the test harness, but it sets the number of threads in addition to the main thread. So --test-threads=1 results in two threads still.

So you can't really use the default test-harness. Luckily, you can disable it in Cargo.toml. Possibly the best solution is to create a new folder (e.g. gltests) and place all tests that require being run in main thread in there. Then we just have to declare those tests in Cargo.toml:

[[test]]
name = "gltests"
path = "gltests/main.rs"
harness = false

This means that cargo will try to compile gltests/main.rs as an executable (expecting a main() function) and execute this executable, whenever you say cargo test. This way you won't get any of the fancy output you usually get from cargo tests. You just have to do everything yourself in main.rs, but at least you can start tests in the main thread.

Lukas Kalbertodt
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  • Great answer, this also helps anyone writing an automated tests of a desktop GUI application. Just like GLFW in the question, many GUI toolkits also require being initialized or used from the main thread. – user4815162342 Apr 17 '17 at 22:05