Add example with extra target dependencies (libpcap)

- ARMv7 target running Debian Stretch
- Depends on libpcap0.8-dev:armhf
- Adds dummy crate to force linking as an example

docker/armhf-stretch-crate

@@ -0,0 +1,19 @@
+# syntax=docker/dockerfile:1
+FROM debian:stretch
+
+ARG DEBIAN_FRONTEND=noninteractive
+ARG TZ=UTC
+
+RUN dpkg --add-architecture armhf && \
+    apt-get update && \
+    apt-get --yes install \
+    crossbuild-essential-armhf \
+    curl \
+    libpcap0.8-dev:armhf && \
+    apt clean all && \
+    curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y && \
+    /root/.cargo/bin/rustup target add armv7-unknown-linux-gnueabihf
+
+WORKDIR /opt/build
+
+CMD ["/bin/bash"]

examples/armhf-stretch-crate.md

@@ -0,0 +1,41 @@
+# ARMv7 Dynamic Rust Project from Cargo Package
+
+Using this [Dockerfile](../docker/armhf-stretch-crate) one can build a dynamically
+linked Rust project from a Cargo package (`.crate` file, a gzipped tarball) that
+depends on `libpcap0.8` in a Debian Stretch container targetting ARMv7.
+
+Note that the Dockerfile does a few special things needed to account for the
+*target* architecture's development headers:
+
+1. The `armhf` architecture is added before `apt` is updated using `dpkg --add-architecture armhf`
+2. Since `libpcap-dev` itself has no `armhf` installation candidate, the specific package `libpcap0.8-dev:armhf` is specified instead
+
+Without these two steps, cross compliation dependent on libraries that differ across architectures is hopeless.
+
+### Container Build
+
+`$ podman build -t crossbuild-armhf -f armhf-stretch-crate`
+
+### Build Example
+
+This assumes `PROJECT_DIR` is set to the path to which this repo was cloned, and that you have
+the Rust toolchain installed with `cargo` on your `PATH`.
+
+The project built in this example is [here](./pcap-example/).
+
+```bash
+$ mkdir ~/build_dir
+$ cd ${PROJECT_DIR}/examples/pcap-example/
+$ cargo package --no-verify --allow-dirty
+$ cp target/package/pcap-example-0.1.0.crate ~/build_dir/
+$ podman run --rm -it e BIN_NAME="pcap-example" \
+	-e BIN_VERSION="0.1.0" \
+	-v "${HOME}/build_dir:/opt/build:z \
+	crossbuild-armhf:latest \
+	/opt/build/armhf-stretch-crate.sh
+
+# Build output redacted
+
+$ file build_dir/*_pcap-example
+build_dir/armv7-unknown-linux-gnueabihf_pcap-example: ELF 32-bit LSB pie executable, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-armhf.so.3, for GNU/Linux 3.2.0, BuildID[sha1]=40ed6d3beeed98797558d4024fbdb468f58abd49, stripped
+```

examples/armhf-stretch-crate.sh

@@ -0,0 +1,47 @@
+#!/bin/bash
+
+set -e
+
+ARMV7="armv7-unknown-linux-gnueabihf"
+
+if [ -z "${BIN_NAME}" ]; then
+    echo "Missing BIN_NAME, exiting" >&2
+    exit 1
+fi
+
+if [ -z "${BIN_VERSION}" ]; then
+    echo "Missing BIN_VERSION, exiting" >&2
+    exit 1
+fi
+
+
+source ${HOME}/.cargo/env
+
+cat >>"${HOME}/.cargo/config" <<EOF
+[build]
+rustflags = ["-C", "strip=symbols"]
+
+[target."${ARMV7}"]
+linker = "arm-linux-gnueabihf-gcc"
+
+EOF
+
+PROJ_DIR="${BIN_NAME}-${BIN_VERSION}"
+CRATE="${PROJ_DIR}.crate"
+
+if [ ! -f "${CRATE}" ]; then
+    echo "Expected crate not found" >&2
+    exit 1
+fi
+
+tar xf "${CRATE}"
+
+cd "${PROJ_DIR}"
+
+cargo build --release --target "${ARMV7}"
+BIN_PATH="target/${ARMV7}/release/${BIN_NAME}"
+mv "${BIN_PATH}" "/opt/build/${ARMV7}_${BIN_NAME}"
+
+cd /opt/build
+
+rm -rf "${PROJ_DIR}"

examples/pcap-example/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "pcap-example"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+pcap = "0.10"

examples/pcap-example/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    let _cap = pcap::Capture::dead(pcap::Linktype::ETHERNET).unwrap();
+}