Add a post-build-hook

Passing `--post-build-hook /foo/bar` to a nix-* command will cause
`/foo/bar` to be executed after each build with the following
environment variables set:

    DRV_PATH=/nix/store/drv-that-has-been-built.drv
    OUT_PATHS=/nix/store/...build /nix/store/...build-bin /nix/store/...build-dev

This can be useful in particular to upload all the builded artifacts to
the cache (including the ones that don't appear in the runtime closure
of the final derivation or are built because of IFD).

This new feature prints the stderr/stdout output to the `nix-build`
and `nix build` client, and the output is printed in a Nix 2
compatible format:

    [nix]$ ./inst/bin/nix-build ./test.nix
    these derivations will be built:
      /nix/store/ishzj9ni17xq4hgrjvlyjkfvm00b0ch9-my-example-derivation.drv
    building '/nix/store/ishzj9ni17xq4hgrjvlyjkfvm00b0ch9-my-example-derivation.drv'...
    hello!
    bye!
    running post-build-hook '/home/grahamc/projects/github.com/NixOS/nix/post-hook.sh'...
    post-build-hook: + sleep 1
    post-build-hook: + echo 'Signing paths' /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
    post-build-hook: Signing paths /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
    post-build-hook: + sleep 1
    post-build-hook: + echo 'Uploading paths' /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
    post-build-hook: Uploading paths /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation
    post-build-hook: + sleep 1
    post-build-hook: + printf 'very important stuff'
    /nix/store/qr213vjmibrqwnyp5fw678y7whbkqyny-my-example-derivation

    [nix-shell:~/projects/github.com/NixOS/nix]$ ./inst/bin/nix build -L -f ./test.nix
    my-example-derivation> hello!
    my-example-derivation> bye!
    my-example-derivation (post)> + sleep 1
    my-example-derivation (post)> + echo 'Signing paths' /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
    my-example-derivation (post)> Signing paths /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
    my-example-derivation (post)> + sleep 1
    my-example-derivation (post)> + echo 'Uploading paths' /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
    my-example-derivation (post)> Uploading paths /nix/store/c263gzj2kb2609mz8wrbmh53l14wzmfs-my-example-derivation
    my-example-derivation (post)> + sleep 1
    my-example-derivation (post)> + printf 'very important stuff'
    [1 built, 0.0 MiB DL]

Co-authored-by: Graham Christensen <graham@grahamc.com>
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index 0af8215d1f..7dea689214 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -315,6 +315,9 @@ public:
         "pre-build-hook",
         "A program to run just before a build to set derivation-specific build settings."};
 
+    Setting<std::string> postBuildHook{this, "", "post-build-hook",
+        "A program to run just after each succesful build."};
+
     Setting<std::string> netrcFile{this, fmt("%s/%s", nixConfDir, "netrc"), "netrc-file",
         "Path to the netrc file used to obtain usernames/passwords for downloads."};