trivial-builders: refactor writeTextFile to be overridable

This fixes #126344, specifically with the goal of enabling overriding the
checkPhase argument. See `design notes` at the end for details.

This allows among other things, enabling bash extension for the `checkPhase`.
Previously using such bash extensions was prohibited by the `writeShellScript`
code because there was no way to enable the extension in the checker.

As an example:

```nix
(writeShellScript "foo" ''
  shopt -s extglob
  echo @(foo|bar)
'').overrideAttrs (old: {
  checkPhase = ''
    # use subshell to preserve outer environment
    (
      export BASHOPTS
      shopt -s extglob
      ${old.checkPhase}
    )
  '';
})
```

This commit also adds tests for this feature to `pkgs/tests/default.nix`,
under `trivial-overriding`. The test code is located at
`pkgs/build-support/trivial-builders/test-overriding.nix`.

Design notes:
-------------

Per discussion with @sternenseemann, the original approach of just wrapping
`writeTextFile` in `makeOverridable` had the issue that combined with `callPackage`
in the following form, would shadow the `.override` attribute of the `writeTextFile`:

```nix
with import <nixpkgs>;
callPackage ({writeShellScript}: writeShellScript "foo" "echo foo")
```

A better approach can be seen in this commit, where `checkPhase` is moved
from an argument of `writeTextFile`, which is substituted into `buildCommand`,
into an `mkDerivation` argument, which is substituted from the environment
and `eval`-ed. (see the source)

This way we can simple use `.overideAttrs` as usual, and this also makes
`checkPhase` a bit more conformant to `mkDerivation` naming, with respect to
phases generally being overridable attrs.

Co-authored-by: sterni <sternenseemann@systemli.org>
Co-authored-by: Naïm Favier <n@monade.li>

1 files changed, 1 insertions, 0 deletions

diff --git a/pkgs/test/default.nix b/pkgs/test/default.nix
index 8c9e7b6f281..ebf732839ce 100644
--- a/pkgs/test/default.nix
+++ b/pkgs/test/default.nix
@@ -52,6 +52,7 @@ with pkgs;
   cuda = callPackage ./cuda { };
 
   trivial = callPackage ../build-support/trivial-builders/test.nix {};
+  trivial-overriding = callPackage ../build-support/trivial-builders/test-overriding.nix {};
 
   writers = callPackage ../build-support/writers/test.nix {};
 }