From 11a986ae11d25614819adbf9aa64c6b7839efbbf Mon Sep 17 00:00:00 2001
From: Fabrizio Ferrai <fabrizio.ferrai@gmail.com>
Date: Tue, 4 Sep 2018 11:11:40 +0300
Subject: [PATCH] Fix #32: include the git revision in the Hydra build (#33)

---
 README.md            | 11 +++++++++--
 docs/README.md.dhall | 11 +++++++++--
 release.nix          | 10 ++++++++++
 3 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/README.md b/README.md
index df3052f0..b059648c 100644
--- a/README.md
+++ b/README.md
@@ -274,8 +274,15 @@ metadata:
 ### Tests
 
 All tests are defined in `release.nix`. We run these tests in CI in a [Hydra
-project][hydra-project]. You can run the tests locally with `nix build --no-link
-release.nix`.
+project][hydra-project].
+
+You can run the tests locally with the following somewhat magic command:
+```bash
+nix-build --no-link -E 'with import <nixpkgs> {}; callPackage ./release.nix { src.rev = ""; }'
+```
+
+It is so because `release.nix` expects Hydra to provide the git revision in `src.rev`,
+but locally we don't have/want that, so we just call the function with an empty string.
 
 ### Changing the README
 
diff --git a/docs/README.md.dhall b/docs/README.md.dhall
index fa1fa1c7..fece2c7e 100644
--- a/docs/README.md.dhall
+++ b/docs/README.md.dhall
@@ -90,8 +90,15 @@ ${../examples/out/ingress.yaml as Text}
 ### Tests
 
 All tests are defined in `release.nix`. We run these tests in CI in a [Hydra
-project][hydra-project]. You can run the tests locally with `nix build --no-link
-release.nix`.
+project][hydra-project].
+
+You can run the tests locally with the following somewhat magic command:
+```bash
+nix-build --no-link -E 'with import <nixpkgs> {}; callPackage ./release.nix { src.rev = ""; }'
+```
+
+It is so because `release.nix` expects Hydra to provide the git revision in `src.rev`,
+but locally we don't have/want that, so we just call the function with an empty string.
 
 ### Changing the README
 
diff --git a/release.nix b/release.nix
index c34c9fe2..9b9308f1 100644
--- a/release.nix
+++ b/release.nix
@@ -1,3 +1,4 @@
+{ src, ... }:
 let
   pkgs = import ./nixpkgs.nix;
 
@@ -43,6 +44,14 @@ let
         LC_ALL=en_US.UTF-8 ./scripts/build-examples.py $out
       '';
 
+  # Derivation that trivially depends on the input source code revision.
+  # As this is included in the "dhall-lang" aggregate, it forces every
+  # commit to have a corresponding GitHub status check, even if the
+  # commit doesn't make any changes (which can happen when merging
+  # master in).
+  rev = pkgs.runCommand "rev" {} ''echo "${src.rev}" > $out'';
+
+
 in {
   dhall-kubernetes = pkgs.releaseTools.aggregate {
     name = "dhall-kubernetes";
@@ -50,6 +59,7 @@ in {
       generatedFilesCompile
       validateExamples
       buildReadme
+      rev
     ];
   };
 }