maintainers/scripts: add get-maintainer-pings-between.sh

Sometimes it is useful to see which maintainers your change will ping,
to know who you are going to spam in advance.

maintainers/scripts/README.md

@@ -57,6 +57,17 @@ The maintainer is designated by a `selector` which must be one of:
 
 [`maintainer-list.nix`]: ../maintainer-list.nix
 
+### `get-maintainer-pings-between.sh`
+
+Gets which maintainers would be pinged between two Nixpkgs revisions.
+Outputs a JSON object on stdout mapping GitHub usernames to the attributes
+that they would be getting pinged for.
+
+Example:
+
+```sh
+maintainers/scripts/get-maintainer-pings-between.sh HEAD^ HEAD
+```
 
 ## Conventions
 

maintainers/scripts/get-maintainer-pings-between.sh

@@ -0,0 +1,78 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash -p git jq
+
+# Outputs a list of maintainers that would be pinged across two nixpkgs revisions.
+# Authors:
+#  Morgan Jones (@numinit)
+#  Tristan Ross (@RossComputerGuy)
+
+set -euo pipefail
+
+if [ $# -lt 2 ]; then
+    echo "Usage: $0 <rev-from> <rev-to>" >&2
+    exit 1
+fi
+
+repo="$(git rev-parse --show-toplevel)"
+system="$(nix-instantiate --eval --expr builtins.currentSystem)"
+rev1="$(git -C "$repo" rev-parse "$1")"
+rev2="$(git -C "$repo" rev-parse "$2")"
+
+echo "Touched files:" >&2
+git -C "$repo" diff --name-only "$rev1" "$rev2" \
+    | jq --raw-input --slurp 'split("\n")[:-1]' | tee "$TMPDIR/touched-files.json" >&2
+
+# Runs an eval in the given worktree, outputting the path to $TMPDIR/$1.path.
+# $1: The revision SHA.
+eval_in_worktree() (
+    mkdir -p .worktree
+    local rev="$1"
+    local tree=".worktree/$rev"
+    if [ ! -d "$tree" ]; then
+        git -C "$repo" worktree add -f -d "$tree" "$rev" >&2
+    fi
+    cd "$tree"
+
+    local workdir="$TMPDIR/$rev"
+    rm -rf "$workdir"
+    mkdir -p "$workdir"
+
+    nix-build ci -A eval.attrpathsSuperset -o "$workdir/paths" >&2
+    mkdir -p "$workdir/intermediates"
+    nix-build ci -A eval.singleSystem \
+        --arg evalSystem "$system" \
+        --arg attrpathFile "$workdir/paths/paths.json" \
+        --arg chunkSize ${CHUNK_SIZE:-10000} \
+        -o "$workdir/intermediates/.intermediate-1" >&2
+
+    # eval.combine nix-build needs a directory, not a symlink
+    cp -RL "$workdir/intermediates/.intermediate-1" "$workdir/intermediates/intermediate-1"
+    chmod -R +w "$workdir/intermediates/intermediate-1"
+    rm -rf "$workdir/intermediates/.intermediate-1"
+
+    nix-build ci -A eval.combine \
+        --arg resultsDir "$workdir/intermediates" \
+        -o "$workdir/result" >&2
+)
+
+eval_in_worktree "$rev1" &
+pid1=$!
+eval_in_worktree "$rev2" &
+pid2=$!
+
+wait $pid1
+wait $pid2
+
+path1="$TMPDIR/$rev1"
+path2="$TMPDIR/$rev2"
+
+# Use the repo this script was executed in to get accurate maintainer info
+nix-build "$repo/ci" -A eval.compare \
+    --arg beforeResultDir "$path1/result" \
+    --arg afterResultDir "$path2/result" \
+    --arg touchedFilesJson "$TMPDIR/touched-files.json" \
+    --arg byName true \
+    -o comparison
+
+echo "Pinged maintainers (check $repo/comparison for more details)" >&2
+jq < comparison/maintainers.json