From 43747c15c7a33a5fe8e750ba452d5b70c64210dd Mon Sep 17 00:00:00 2001
From: Quentin Gliech <quenting@element.io>
Date: Tue, 2 Nov 2021 09:57:04 +0100
Subject: [PATCH] Setup cross-compilation when building Docker image

---
 .github/workflows/check.yaml       |  3 --
 Dockerfile                         | 69 ++++++++++++++++++++++++++----
 misc/docker-arch-to-rust-target.sh | 15 +++++++
 3 files changed, 75 insertions(+), 12 deletions(-)
 create mode 100755 misc/docker-arch-to-rust-target.sh

diff --git a/.github/workflows/check.yaml b/.github/workflows/check.yaml
index ee8cc0de1..41ab8116b 100644
--- a/.github/workflows/check.yaml
+++ b/.github/workflows/check.yaml
@@ -332,9 +332,6 @@ jobs:
             type=semver,pattern={{major}}
             type=sha
 
-      - name: Set up QEMU
-        uses: docker/setup-qemu-action@v1
-
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1
 
diff --git a/Dockerfile b/Dockerfile
index a6fa255fa..539baa524 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,22 +1,73 @@
-ARG RUSTC_VERSION=1.56.0
+# Builds a minimal image with the binary only. It is multi-arch capable,
+# cross-building to aarch64 and x86_64. When cross-compiling, Docker sets two
+# implicit BUILDARG: BUILDPLATFORM being the host platform and TARGETPLATFORM
+# being the platform being built.
+#
+# Docker platform definitions look like this: linux/arm64 and linux/amd64, so
+# there is a small script that translates those platforms to LLVM triples,
+# respectively x86-64-unknown-linux-gnu and aarch64-unknown-linux-gnu
 
+ARG RUSTC_VERSION=1.56.1
+
+## Base image with cargo-chef and the right cross-compilation toolchain ##
 # cargo-chef helps with caching dependencies between builds
-FROM docker.io/library/rust:${RUSTC_VERSION}-slim AS chef
+FROM --platform=${BUILDPLATFORM} docker.io/library/rust:${RUSTC_VERSION}-slim AS chef
+
+# Install x86_64 and aarch64 cross-compiling stack
+RUN apt update && apt install -y --no-install-recommends \
+  g++-x86-64-linux-gnu \
+  g++-aarch64-linux-gnu \
+  libc6-dev-arm64-cross \
+  libc6-dev-amd64-cross \
+  && rm -rf /var/lib/apt/lists/*
+
 WORKDIR /app
 RUN cargo install --locked cargo-chef
 
-FROM chef AS planner
+ENV \
+  CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER=aarch64-linux-gnu-gcc \
+  CC_aarch64_unknown_linux_gnu=aarch64-linux-gnu-gcc \
+  CXX_aarch64_unknown_linux_gnu=aarch64-linux-gnu-g++ \
+  CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_LINKER=x86_64-linux-gnu-gcc \
+  CC_x86_64_unknown_linux_gnu=x86_64-linux-gnu-gcc \
+  CXX_x86_64_unknown_linux_gnu=x86_64-linux-gnu-g++
+
+ARG RUSTC_VERSION
+ARG TARGETPLATFORM
+
+# Helper script that transforms docker platforms to LLVM triples
+COPY ./misc/docker-arch-to-rust-target.sh /
+# Install the right toolchain for cross-compilation
+RUN rustup target add `/docker-arch-to-rust-target.sh "${TARGETPLATFORM}"` --toolchain "${RUSTC_VERSION}"
+
+## Run the planner from cargo-chef ##
+FROM --platform=${BUILDPLATFORM} chef AS planner
 COPY . .
 RUN cargo chef prepare --recipe-path recipe.json
 
-FROM chef AS builder 
-COPY --from=planner /app/recipe.json recipe.json
+## Actual build stage ##
+FROM --platform=${BUILDPLATFORM} chef AS builder 
+
+ARG TARGETPLATFORM
+
 # Build dependencies
-RUN cargo chef cook --release --recipe-path recipe.json
+COPY --from=planner /app/recipe.json recipe.json
+RUN cargo chef cook \
+  --release \
+  --recipe-path recipe.json \
+  --target $(/docker-arch-to-rust-target.sh "${TARGETPLATFORM}")
+
 # Build the rest
 COPY . .
-RUN cargo build --release --bin mas-cli
+RUN cargo build \
+  --release \
+  --bin mas-cli \
+  --target $(/docker-arch-to-rust-target.sh "${TARGETPLATFORM}")
+# Move the binary to avoid having to guess its name in the next stage
+#
+RUN mv target/$(/docker-arch-to-rust-target.sh "${TARGETPLATFORM}")/release/mas-cli /mas-cli
 
-FROM gcr.io/distroless/cc
-COPY --from=builder /app/target/release/mas-cli /mas-cli
+## Runtime stage ##
+FROM --platform=${TARGETPLATFORM} gcr.io/distroless/cc
+COPY --from=builder /mas-cli /mas-cli
 ENTRYPOINT ["/mas-cli"]
diff --git a/misc/docker-arch-to-rust-target.sh b/misc/docker-arch-to-rust-target.sh
new file mode 100755
index 000000000..76bafaa11
--- /dev/null
+++ b/misc/docker-arch-to-rust-target.sh
@@ -0,0 +1,15 @@
+#!/bin/sh
+
+if [ "$#" -ne 1 ]; then
+  echo "usage: $0 [platform]" >&2
+  exit 1
+fi
+
+if [ "$1" = "linux/arm64" ]; then
+  echo "aarch64-unknown-linux-gnu"
+elif [ "$1" = "linux/amd64" ]; then
+  echo "x86_64-unknown-linux-gnu"
+else
+  echo "unsupported platform $1" >&2
+  exit 2
+fi