refactor(nix/buildkite): don't calculate deps for skipped targets

We don't need to calculate dependencies between and on targets that are
part of the parent target map since they will be skipped by buildkite
anyways. This speeds up 🦙 considerably for pipeline runs that have a
limited number of changed targets and a parent target map passed
in (i.e. pipeline runs of most CLs, but not canon runs). In my testing
it was about a minute faster (1/6 of the time 🦙 takes currently) for a
pipeline where under five drv targets changed. For the full
pipeline (i.e. no parentTargetMap) 🦙 takes about the same time as
before (it's a few seconds slower as is to be expected, but nothing
significant).

Change-Id: Ia5a80e142da8f40bc591e2c6cfaf48c325b2f577
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12818
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

nix/buildkite/default.nix

@@ -99,11 +99,7 @@ rec {
   #
   # See //nix/dependency-analyzer for documentation on the structure of `targetDepMap`.
   getTargetPipelineDeps = targetDepMap: drvPath:
-    # Sanity check: We should only call this function on targets explicitly
-    # passed to mkPipeline. Thus it should have been passed as a “known” drv to
-    # dependency-analyzer.
-    assert targetDepMap.${drvPath}.known;
-    builtins.map keyForDrv targetDepMap.${drvPath}.knownDeps;
+    builtins.map keyForDrv (targetDepMap.${drvPath}.knownDeps or [ ]);
 
   # Create a pipeline step from a single target.
   mkStep = { headBranch, parentTargetMap, targetDepMap, target, cancelOnBuildFailing }:
@@ -231,7 +227,19 @@ rec {
       buildEnabled = elem "build" enabledPhases;
 
       # Dependency relations between the `drvTargets`. See also //nix/dependency-analyzer.
-      targetDepMap = dependency-analyzer (dependency-analyzer.drvsToPaths drvTargets);
+      targetDepMap =
+        let
+          # Only calculate dependencies between drvTargets that were not part of
+          # the previous pipeline (per parentTargetMap). Unchanged targets will
+          # be skipped (assumed already built), so it's useless to emit deps
+          # on their steps.
+          changedDrvTargets = builtins.filter
+            (target:
+              parentTargetMap.${mkLabel target}.drvPath or null != target.drvPath
+            )
+            drvTargets;
+        in
+        dependency-analyzer (dependency-analyzer.drvsToPaths changedDrvTargets);
 
       # Convert a target into all of its steps, separated by build
       # phase (as phases end up in different chunks).