1 files changed, 55 insertions, 0 deletions
diff --git a/nix/buildkite/fetch-parent-targets.sh b/nix/buildkite/fetch-parent-targets.sh
new file mode 100755
index 000000000000..08c2d1f3ab65
--- /dev/null
+++ b/nix/buildkite/fetch-parent-targets.sh
@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+set -ueo pipefail
+
+# Each Buildkite build stores the derivation target map as a pipeline
+# artifact. To reduce the amount of work done by CI, each CI build is
+# diffed against the latest such derivation map found for the
+# repository.
+#
+# Note that this does not take into account when the currently
+# processing CL was forked off from the canonical branch, meaning that
+# things like nixpkgs updates in between will cause mass rebuilds in
+# any case.
+#
+# If no map is found, the failure mode is not critical: We simply
+# build all targets.
+
+readonly REPO_ROOT=$(git rev-parse --show-toplevel)
+
+: ${DRVMAP_PATH:=pipeline/drvmap.json}
+: ${BUILDKITE_TOKEN_PATH:=~/buildkite-token}
+
+# Runs a fairly complex Buildkite GraphQL query that attempts to fetch all
+# pipeline-gen steps from the default branch, as long as one appears within the
+# last 50 builds or so. The query restricts build states to running or passed
+# builds, which means that it *should* be unlikely that nothing is found.
+#
+# There is no way to filter this more loosely (e.g. by saying "any recent build
+# matching these conditions").
+#
+# The returned data structure is complex, and disassembled by a JQ script that
+# first filters out all builds with no matching jobs (e.g. builds that are still
+# in progress), and then filters those down to builds with artifacts, and then
+# to drvmap artifacts specifically.
+#
+# If a recent drvmap was found, this returns its download URL. Otherwise, it
+# returns the string "null".
+function latest_drvmap_url {
+    set -u
+    curl 'https://graphql.buildkite.com/v1' \
+         --silent \
+         -H "Authorization: Bearer $(cat ${BUILDKITE_TOKEN_PATH})" \
+         -H "Content-Type: application/json" \
+         -d "{\"query\": \"{ pipeline(slug: \\\"$BUILDKITE_ORGANIZATION_SLUG/$BUILDKITE_PIPELINE_SLUG\\\") { builds(first: 50, branch: [\\\"%default\\\"], state: [RUNNING, PASSED]) { edges { node { jobs(passed: true, first: 1, type: [COMMAND], step: {key: [\\\"pipeline-gen\\\"]}) { edges { node { ... on JobTypeCommand { url artifacts { edges { node { downloadURL path }}}}}}}}}}}}\"}" | tee out.json | \
+        jq -r '[.data.pipeline.builds.edges[] | select((.node.jobs.edges | length) > 0) | .node.jobs.edges[] | .node.artifacts[][] | select(.node.path == "pipeline/drvmap.json")][0].node.downloadURL'
+}
+
+readonly DOWNLOAD_URL=$(latest_drvmap_url)
+
+if [[ ${DOWNLOAD_URL} != "null" ]]; then
+    mkdir -p tmp
+    curl -o tmp/parent-target-map.json ${DOWNLOAD_URL} && echo "downloaded parent derivation map" \
+            || echo "failed to download derivation map!"
+else
+    echo "no derivation map found!"
+fi