This adds a new configuration option to Nix, `always-allow-substitutes`,
whose effect is simple: it causes the `allowSubstitutes` attribute in
derivations to be ignored, and for substituters to always be used.
This is extremely valuable for users of Nix in CI, where usually
`nix-build-uncached` is used. There, derivations which disallow
substitutes cause headaches as the inputs for building already-cached
derivations need to be fetched to spuriously rebuild some simple text
file.
This option should be a good middle-ground, since it doesn't imply
rebuilding the world, such as the approach I took in
https://github.com/NixOS/nixpkgs/pull/221048
Using abstract types like can help cut down on compilation time, both
from scratch, and especially incremental builds during development. The
idea is that `worker-protocol.hh` can declare all the (de)serializers, but
only again abstract types; when code needs to use some (de)serializers, it can
include headers just for the data types it needs to (de)serialize.
`store-api.hh` in particular is a bit of a sledgehammer, and the data
types we want to serialize have their own headers.
This is the more typically way to do [Argument-dependent
lookup](https://en.cppreference.com/w/cpp/language/adl)-leveraging
generic serializers in C++. It makes the relationship between the `read`
and `write` methods more clear and rigorous, and also looks more
familiar to users coming from other languages that do not have C++'s
libertine ad-hoc overloading.
I am returning to this because during the review in
https://github.com/NixOS/nix/pull/6223, it came up as something that
would make the code easier to read --- easier today hopefully already,
but definitely easier if we were have multiple codified protocols with
code sharing between them as that PR seeks to accomplish.
If I recall correctly, the main criticism of this the first time around
(in 2020) was that having to specify the type when writing, e.g.
`WorkerProto<MyType>::write`, was too verbose and cumbersome. This is
now addressed with the `workerProtoWrite` wrapper function.
This method is also the way `nlohmann::json`, which we have used for a
number of years now, does its serializers, for what its worth.
This reverts commit 45a0ed82f0. That
commit in turn reverted 9ab07e99f5.
This is good in general, but in particular ensures when we heavily
refactor it in the next commit there is less likelihood for an
unintentional change in behavior to sneak in.
These items are not templates, and they declared in
`worker-protocol.hh`; therefore they should live in a
`worker-protocol.cc`.
Anything else needlessly diverges from convention. After all, it is not
like this code is only used in `remote-store.cc`; it is also used in
`daemon.cc`. There is no good reason to place it with the client
implementation or the server implementation when it used equally by
both.
it's more likely for readers to find it right there.
this also slightly rewords examples to make them stand out better.
in the long run there probably needs to be a dedicated section on formal syntax, and better highlighting of examples.
