feat(nix/buildkite): Check target map of parent to determine skips r/3602

This changes the logic for build pipeline generation to inspect
an (optional) parentTargetMap attribute which contains the derivation
map of a target commit.

Targets that existed in a parent commit with the same drv hash will be
skipped, as they are not considered to have changed.

This does not yet wire up any logic for retrieving the target map from
storage, meaning that at this commit all targets are always built.

The intention is that we will have logic to fetch the target
map (initially from Buildkite artefact storage), which we then pass to
the depot via externalArgs when actually generating the pipeline.

Change-Id: I3373c60aaf4b56b94c6ab64e2e5eef68dea9287c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4946
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

1 files changed, 6 insertions, 2 deletions

diff --git a/ops/pipelines/depot.nix b/ops/pipelines/depot.nix
index 8cc4b5691f59..232d229b90d8 100644
--- a/ops/pipelines/depot.nix
+++ b/ops/pipelines/depot.nix
@@ -1,6 +1,6 @@
 # This file configures the primary build pipeline used for the
 # top-level list of depot targets.
-{ depot, pkgs, ... }:
+{ depot, pkgs, externalArgs, ... }:
 
 let
   # Protobuf check step which validates that changes to .proto files
@@ -17,11 +17,15 @@ let
     command = "${depot.tools.depotfmt.check}";
     label = ":evergreen_tree: (tools/depotfmt)";
   };
+
   pipeline = depot.nix.buildkite.mkPipeline {
     headBranch = "refs/heads/canon";
     drvTargets = depot.ci.targets;
-    skipIfBuilt = true;
     additionalSteps = [ depotfmtCheck protoCheck ];
+
+    parentTargetMap = if (externalArgs ? parentTargetMap)
+      then builtins.fromJSON (builtins.readFile externalArgs.parentTargetMap)
+      else {};
   };
 
   drvmap = depot.nix.buildkite.mkDrvmap depot.ci.targets;