Nixpkgs: services: Switch to Type=exec wherever possible with systemd 240

[nh2]

systemd 240 introduces a new Type=exec, which like Type=simple, but better in the sense that it communicates failures in exec() back to systemd, as opposed failures only in fork(). See

https://github.com/systemd/systemd/blob/09e5c07fbd23d098edf2e0b51fbf402f07474dae/NEWS#L5

for details.

Using Type=exec everywhere should mean that we can detect many issues earlier, increasing the reliability of systems built with NixOS, and in particular NixOps deployments.

This change might fix many problems I've encountered, where a new configuration first apparently activates successfully, only to crash shortly after.

[orivej]

Type=exec doesn't seem useful for nix-generated services. What kind of issues have you encountered that would benefit from it?

[arianvp]

Why doesn't it seem useful? In this talk Lennart explains well what kind of edge cases are handled well in exec but not in simple https://media.ccc.de/v/ASG2018-230-systemd_in_2018#t=491

Bugs like: "the binary is absent, but the service is still reported successfully started" are caught with exec but not with simple. And those kind of bugs are bugs we ran in before. (For example, someone makes a typo in a binary name, but all the NixOS tests pass anyway because the service successfully started)

A service being generated from nix and the service's type seem like two totally orthogonal things to me. We have services of many types in Nix. From simple, oneshot, and forking.

Simple has buggy semantics but because people now might depend on the buggy semantics, systemd will introduce a new type exec which fixes these issues while keeping the old simple type as well for backwards compatibility. Exec will become the default in some later release. Switching now already seems like a good investment.

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