bug#42053: hidden derivation build with inferiors

[Julien Lepiller]

Hi,

someone (kernel-help) on IRC today was having issues using an inferior
kernel with their os configuration and reported that guix was
"hanging". It turns out that guix was actually building the kernel in
the background, but did not report anything to the user. kernel-help
was kind enough to share their config, and I can confirm that guix is
building the kernel, but doesn't tell anything.

I was able to track this done to a call to operating-system-derivation,
which is supposed to return the path to a .drv file. I did not expect
it to have to build anything, since it's only computing a derivation
name.

Using the attached operating system declaration, defining it as "os",
you can see that calling ((operating-system-derivation os)
(open-connection)) "hangs" with no message, and a build user is
building the kernel (unless you already have the kernel built).

Note that the build process continues even if you interrupt that
procedure, and only stops after you exit guix repl entirely.

Going one step further, operating-system-derivation calls fold-services
(which is fine), then service-value (which is also fine) and calls that
value with the store, which builds the kernel without saying anything.
That value comes from the root service type, system-service-type and its
system-derivation procedure.

With some pk, I was able to find that mapm/accumulate-builds was the
culprit, but I don't know how to investigate further.

test-inferior.scm
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