Add a script that builds a static Linux executable

scripts/build-static.sh builds the daemon for x86_64 Linux against
musl, as one executable without dependencies to copy to a router. It
finds a musl C compiler for jemalloc (musl-cross on macOS, musl-gcc on
Linux), and uses its ar and ranlib too: with the host's, a build from
macOS gets an empty libjemalloc.a and fails to link. It prints the
revision and the SHA-256 of the result.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/content/docs/getting-started/installation.md

@@ -57,6 +57,35 @@ To build and install the daemon:
 
    The command displays the installed version, for example `bgp 0.1.0`.
 
+## Building a Static Executable
+
+The script `scripts/build-static.sh` builds the daemon as a single static
+executable for x86_64 Linux, linked against musl. The executable runs on any
+x86_64 Linux distribution without other dependencies.
+
+The script requires the Rust target and a musl C compiler for jemalloc:
+
+```bash
+rustup target add x86_64-unknown-linux-musl
+```
+
+On macOS, install the cross compiler with
+`brew install FiloSottile/musl-cross/musl-cross`. On x86_64 Linux, install
+`musl-tools` (Debian, Ubuntu) or `musl` (Arch Linux).
+
+```bash
+scripts/build-static.sh
+```
+
+The script displays the path of the executable, the source revision, and
+the SHA-256 checksum of the executable:
+
+```
+target/x86_64-unknown-linux-musl/release/bgp: ELF 64-bit LSB pie executable, x86-64, static-pie linked
+revision 9ce81a8
+2faa32d3...  target/x86_64-unknown-linux-musl/release/bgp
+```
+
 ## Building a Container Image
 
 The `Dockerfile` in the repository builds the daemon in a Rust build stage

scripts/build-static.sh

@@ -0,0 +1,49 @@
+#!/bin/sh
+# Builds the daemon as one static executable for x86_64 Linux, which runs
+# on any x86_64 Linux distribution with nothing else installed.
+#
+#   scripts/build-static.sh
+#
+# The executable is linked against musl, with jemalloc as the memory
+# allocator (see bin/bgp/src/main.rs). jemalloc is C, so a C compiler for
+# the target is needed as well as the Rust target:
+#
+#   rustup target add x86_64-unknown-linux-musl
+#   macOS:  brew install FiloSottile/musl-cross/musl-cross
+#   Debian, Ubuntu (on x86_64):  apt-get install musl-tools
+#   Arch (on x86_64):  pacman -S musl
+set -eu
+target=x86_64-unknown-linux-musl
+repository="$(cd "$(dirname "$0")/.." && pwd)"
+
+# The cross compiler from musl-cross, or musl's own wrapper on Linux.
+if command -v x86_64-linux-musl-gcc >/dev/null; then
+    cc=x86_64-linux-musl-gcc
+    ar=x86_64-linux-musl-ar
+    ranlib=x86_64-linux-musl-ranlib
+elif command -v musl-gcc >/dev/null; then
+    cc=musl-gcc
+    ar=ar
+    ranlib=ranlib
+else
+    echo "No C compiler for $target: see the comment at the top of $0" >&2
+    exit 1
+fi
+
+cd "$repository"
+# jemalloc is built by its own configure script, which looks for an
+# archiver named after the target (x86_64-unknown-linux-musl-ar) and, not
+# finding one, would take the host's: on macOS that one cannot index Linux
+# objects, and the library comes out empty. So AR and RANLIB are named.
+CC_x86_64_unknown_linux_musl=$cc \
+AR_x86_64_unknown_linux_musl=$ar \
+AR=$ar \
+RANLIB=$ranlib \
+CARGO_TARGET_X86_64_UNKNOWN_LINUX_MUSL_LINKER=$cc \
+    cargo build --release --locked -p bgp --target "$target"
+
+binary="target/$target/release/bgp"
+echo
+file "$binary"
+echo "revision $(git rev-parse --short HEAD)$(git diff --quiet HEAD || echo ', with uncommitted changes')"
+if command -v sha256sum >/dev/null; then sha256sum "$binary"; else shasum -a 256 "$binary"; fi