Add tooling for josh syncs

Create a justfile that handles pulling from and pushing to
rust-lang/rust. This can be invoked with the following:

    $ just rustc-pull
    $ just rustc-push

Author: tgross35

justfile

@@ -0,0 +1,158 @@
+name := "compiler-builtins"
+# Override these environment variables to test against a fork
+upstream_repo := env("UPSTREAM_ORG", "rust-lang") + "/rust"
+upstream_ref := env("UPSTREAM_REF", "HEAD")
+upstream_url := "https://github.com/" + upstream_repo
+
+josh_port := "42042"
+josh_filter := ":/library/compiler-builtins"
+josh_url_base := "http://localhost:" + josh_port
+josh_cache_dir := cache_directory() / "rust-lang.compiler-builtins-josh"
+
+# Set traps to kill Josh on shell exit or failure, then launch Josh.
+start_josh := \
+	"trap 'exit $EXIT_CODE' INT TERM\n" + \
+	"trap 'EXIT_CODE=$?; kill 0' EXIT\n" +  \
+	just_executable() + " start-josh &\n" + \
+	just_executable() + " _ensure-josh-running\n"
+
+# TODO make these messages better
+update_version_msg := """
+	Preparing for merge from rustc
+"""
+rustc_to_builtins_msg := """
+	Merge from rustc
+"""
+
+default:
+	just --list
+
+# Exit with failure if the working directory has uncommitted files
+_ensure-clean:
+	#!/bin/bash
+	set -eaxo pipefail
+	if [ -n "$(git status --untracked-files=no --porcelain)" ]; then
+		echo "working directory must be clean"
+		exit 1
+	fi
+
+# Make sure Josh is reachable.
+_ensure-josh-running:
+	#!/bin/bash
+	set -eaxo pipefail
+	for _ in $(seq 1 100); do
+		sleep 0.01s
+
+		# Exit with success if we can connect to the port
+		if nc -z 127.0.0.1 "{{ josh_port }}"; then
+			exit
+		fi
+	done
+
+	echo "Even after waiting for 1s, josh-proxy is still not available."
+	exit 1
+
+# Launch Josh, for running commands manually
+start-josh:
+	josh-proxy --local '{{ josh_cache_dir }}' \
+		--remote 'https://github.com' \
+		--port '{{ josh_port }}' \
+		--no-background
+
+# Update this repo with changes from rust-lang/rust
+rustc-pull: _ensure-clean
+	#!/bin/bash
+	set -eaxo pipefail
+
+	commit="$(git ls-remote "{{ upstream_url }}" "{{ upstream_ref }}" | cut -w -f1)"
+	josh_url="{{ josh_url_base }}/{{ upstream_repo }}.git@${commit}{{ josh_filter }}.git"
+
+	{{ start_josh }}
+
+	previous_base_commit="$(cat rust-version)"
+	if [ "$previous_base_commit" = "$commit" ]; then
+		echo "Nothing to pull; commit at $commit"
+		exit 1
+	fi
+
+	orig_head="$(git rev-parse HEAD)"
+	echo "$commit" > "rust-version"
+	git commit rust-version --no-verify -m '{{ update_version_msg }}'
+
+	if ! git fetch "$josh_url"; then
+		echo "FAILED to fetch new commits, something went wrong."
+		git reset --hard "$orig_head"
+		echo "(committing the rust-version file has been undone)"
+		exit 1
+	fi
+
+	num_roots_before="$(git rev-list HEAD --max-parents=0 --count)"
+	sha="$(git rev-parse HEAD)"
+	git merge FETCH_HEAD --no-verify --no-ff -m '{{ rustc_to_builtins_msg }}'
+	new_sha="$(git rev-parse HEAD)"
+
+	if [ "$sha" = "$new_sha" ]; then
+		git reset --hard "$orig_head"
+		echo "No merge was performed, no changes to pull were found. Rolled back the preparation commit."
+		exit 1
+	fi
+
+	num_roots="$(git rev-list HEAD --max-parents=0 --count)"
+
+	if [ "$num_roots" -ne "$num_roots_before" ]; then
+		echo "Josh created a new root commit. This is probably not the history you want."
+		exit 1
+	fi
+
+# Create a pull request to rust-lang/rust with changes from this repo
+rustc-push github_user branch="update-builtins": _ensure-clean
+	#!/bin/bash
+	set -eaxo pipefail
+
+	base="$(cat rust-version)"
+	branch="{{ branch }}"
+	github_user="{{ github_user }}"
+	josh_url="{{ josh_url_base }}/{{ github_user }}/rust.git{{ josh_filter }}.git"
+	user_rust_url="[email protected]:{{ github_user }}/rust.git"
+	
+	if [ -z "$RUSTC_GIT" ]; then
+		echo "The RUSTC_GIT environment variable must be set"
+		exit 1
+	fi
+
+	{{ start_josh }}
+
+	(
+		# Execute in the rustc directory
+		cd "$RUSTC_GIT"
+
+		echo "Preparing $github_user/rust (base: $base)..."
+	
+		if git fetch "$user_rust_url" "$branch" 2>&1 > /dev/null; then
+			echo "The branch '$branch' seems to already exist in '$user_rust_url'. \
+				  Please delete it and try again."
+			exit 1
+		fi
+
+		git fetch "https://github.com/{{ upstream_repo }}" "$base"
+		git push "$user_rust_url" "$base:refs/heads/$branch" --no-verify
+	)
+
+	# Do the actual push.
+	echo "Pushing changes..."
+	git push "$josh_url" "HEAD:$branch"
+
+	# Do a round-trip check to make sure the push worked as expected
+	git fetch "$josh_url" "$branch"
+	head="$(git rev-parse HEAD)"
+	fetch_head="$(git rev-parse FETCH_HEAD)"
+
+	if [ "$head" != "$fetch_head" ]; then
+		echo "Josh created a non-roundtrip push! Do NOT merge this into rustc!"
+		echo "Expected '$head', got '$fetch_head'."
+		exit 1
+	fi
+
+	echo "Confirmed that the push round-trips back to {{ name }} properly. Please create a rustc PR:"
+	# Open PR with `subtree update` title to silence the `no-merges` triagebot check
+	echo "    {{ upstream_url }}/compare/$github_user:$branch?quick_pull=1&title=rustc-dev-guide+subtree+update&body=r?+@ghost"