Build logs are missing when using nixUnstable

[roberth]

Description

No build logs appear in the dashboard, except perhaps some lines about substitution from Nix itself.

To Reproduce

Deploy an agent system with

nix.package = pkgs.nixUnstable;
services.hercules-ci-agent.enable = true;
# etcetera

Solution

Soon: update Nixpkgs. For now:

Apply the following configuration to the system:

NixOS configuration.nix or nix-darwin configuration:

  nixpkgs.overlays = [
    (final: prev: {
      nixUnstable = prev.nixUnstable.overrideAttrs (o: {
        patches = (o.patches or []) ++ [
          # Fix #4234, send build logs to Nix 2.3 / Hercules CI 0.8
          # https://github.com/NixOS/nix/pull/4418
          ./nix-issue-4234-build-output-for-nix-2.3.diff
        ];
      });

      # Pick up our overrides (Nixpkgs uses the same definition
      # instead of a reference to the final nixUnstable attribute)
      nixFlakes = final.nixUnstable;
    })
  ];

Add to the same directory the file nix-issue-4234-build-output-for-nix-2.3.diff:

commit aae705f9fc6f118315bf6d2bb49c98e2558fa190
Author: Robert Hensing <[email protected]>
Date:   Sat Jan 2 19:27:34 2021 +0100

    daemon.cc: Duplicate build output as general logs for Nix 2.3
    
    The conditional on clientVersion isn't accurate for a certain
    range of master commits, but those are unsupported anyway.
    
    It's not accurate for repeated builds either, because that
    would complicate the code for too little gain.

diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index ba5788b64..14e5b1b9b 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -142,6 +142,14 @@ struct TunnelLogger : public Logger
 
     void result(ActivityId act, ResultType type, const Fields & fields) override
     {
+        if (GET_PROTOCOL_MINOR(clientVersion) < 22 && type == resBuildLogLine && settings.verboseBuild) {
+            // Nix < 2.4 expects build logs to be logged as general logs at lvlError.
+            // We probably shouldn't do this for repeated builds, but that's not
+            // something we can know here without complicating the implementation
+            // unnecessarily.
+            auto lastLine = fields[0].s;
+            log(lvlError, lastLine);
+        }
         if (GET_PROTOCOL_MINOR(clientVersion) < 20) return;
         StringSink buf;
         buf << STDERR_RESULT << act << type << fields;

Platform / Version

Hercules CI Agent: all versions
Nix: nixUnstable / master between NixOS/nix#3073 and NixOS/nix#4418

[roberth]

Versions >= 0.8.4 build against Nix 2.4 by default, with the possible exception of some Nixpkgs versions. This means that the Nix client version has caught up with the NIx daemon version and it won't exhibit this problem anymore.