Substitute downstream placeholders when realising a "string with context"

[amarshall]

To account for float content-addressed derivation outputs, we use "downstream placeholder" to refer to output deriving paths in a abstract way (by hash) and then substitute those hashes for the store paths once those are known.

We do this when building derivations, but do not yet do when realising strings with context as part of IFD.

We should rework the below test as part of the tests/functional/ca/import-from-derivation.sh / tests/functional/import-from-derivation.sh test, probably.

---

original issue

Describe the bug

Cannot use content-addressed derivation as path for builtins.path

Steps To Reproduce

1. Enable ca-derivations feature
2. Create file (e.g. test.nix) with

   let
     pkgs = (import <nixpkgs> {});
     drv = pkgs.runCommand "source" { __contentAddressed = true; } "mkdir $out; touch $out/{foo,bar}";
     filtered = builtins.path {
       path = drv;
       name = "source";
       filter = path: type: path == "${drv}/foo";
     };
   in pkgs.runCommand "source" { } "ln -s ${filtered} $out"

3. Run nix-build test.nix

Results in error:

error (ignored): error: end of string reached
error:
       … while calling the 'derivationStrict' builtin
         at <nix/derivation-internal.nix>:34:12:
           33|
           34|   strict = derivationStrict drvAttrs;
             |            ^
           35|

       … while evaluating derivation 'source'
         whose name attribute is located at /nix/store/920a6ivyd50598z8djw9x3mr33gys0j5-source/pkgs/stdenv/generic/make-derivation.nix:334:7

       … while evaluating attribute 'buildCommand' of derivation 'source'
         at /nix/store/920a6ivyd50598z8djw9x3mr33gys0j5-source/pkgs/build-support/trivial-builders/default.nix:68:17:
           67|         enableParallelBuilding = true;
           68|         inherit buildCommand name;
             |                 ^
           69|         passAsFile = [ "buildCommand" ]

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: path '/0y6ypwm99vdjmfincll2xx8y1d145ml711vryyvmandlxrg98kca' does not exist

Expected behavior

Succeeds.

nix-env --version output

nix-env (Nix) 2.24.1

Additional context

Works in Nix 2.18.5 & 2.19.6, fails in current patch releases of versions 2.20 through 2.24.

Priorities

Add 👍 to issues you find important.

[Ericson2314]

Hmm interesting.

1. We don't know what the output path of a CA derivation is until its built, so we use a placeholder. /0y6ypwm99vdjmfincll2xx8y1d145ml711vryyvmandlxrg98kca
2. We cannot run the builtins.path until that output is built.

But if the builtins.path triggers IFD, then this output will be built. We should substitute placeholders when realizing a string with context, so the placeholders will go away and the real path remains.

Looks like regression has something to do with that last part.