improve docker cross-compilation

* support cross-compiling an x86-64 target on an arm64 host. This
  it turns out is a matter of *removing* an unnecessary dependency.
  (aarch64-linux-gnu-pkg-config exists but x86_64-linux-gnu-pkg-config
  doesn't. Turns out neither is necessary.) Added a comment explaining
  where ${gcc_target}-pkg-config comes from now.
* documentation tweaks
* improve debug output a bit

docker/Dockerfile

@@ -4,7 +4,7 @@
 # SPDX-License-Identifier: GPL-v3.0-or-later WITH GPL-3.0-linking-exception.
 
 # See documentation here:
-# https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/syntax.md
+# https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/reference.md
 
 # "dev-common" is the portion of "dev" (see below) which isn't specific to the
 # target arch. It's sufficient for building the non-arch-specific webpack.

docker/build-server.bash

@@ -13,9 +13,8 @@ mkdir /docker-build-debug/build-server
 exec > >(tee -i /docker-build-debug/build-server/output) 2>&1
 date
 uname -a
-ls -laFR /cargo-cache > /docker-build-debug/build-server/cargo-cache-before
-ls -laFR /var/lib/moonfire-nvr/target \
-    > /docker-build-debug/build-server/target-before
+find /cargo-cache -ls > /docker-build-debug/build-server/cargo-cache-before
+find ~/target -ls > /docker-build-debug/build-server/target-before
 
 source ~/.buildrc
 
@@ -26,12 +25,13 @@ sudo chmod 777 /cargo-cache /var/lib/moonfire-nvr/target
 mkdir -p /cargo-cache/{git,registry}
 ln -s /cargo-cache/{git,registry} ~/.cargo
 
+build_profile=release-lto
 cd src/server
 time cargo test
-time cargo build --profile=release-lto
-sudo install -m 755 ~/moonfire-nvr /usr/local/bin/moonfire-nvr
-
-ls -laFR /cargo-cache > /docker-build-debug/build-server/cargo-cache-after
-ls -laFR /var/lib/moonfire-nvr/target \
-    > /docker-build-debug/build-server/target-after
+time cargo build --profile=$build_profile
+find /cargo-cache -ls > /docker-build-debug/build-server/cargo-cache-after
+find ~/target -ls > /docker-build-debug/build-server/target-after
+sudo install -m 755 \
+    ~/platform-target/$build_profile/moonfire-nvr \
+    /usr/local/bin/moonfire-nvr
 date

docker/build-ui.bash

@@ -19,6 +19,6 @@ npm --version
 time npm ci
 time npm run build
 
-ls -laFR /var/lib/moonfire-nvr/src/ui/node_modules \
+find /var/lib/moonfire-nvr/src/ui/node_modules -ls \
     > /docker-build-debug/build-ui/node_modules-after
 date

docker/deploy.bash

@@ -11,8 +11,7 @@ set -o xtrace
 
 mkdir -p /docker-build-debug/deploy
 exec > >(tee -i /docker-build-debug/deploy/output) 2>&1
-ls -laFR /var/cache/apt \
-    > /docker-build-debug/deploy/var-cache-apt-before
+find /var/cache/apt -ls > /docker-build-debug/deploy/var-cache-apt-before
 
 date
 uname -a
@@ -29,6 +28,5 @@ time apt-get install --assume-yes --no-install-recommends \
 rm -rf /var/lib/apt/lists/*
 ln -s moonfire-nvr /usr/local/bin/nvr
 
-ls -laFR /var/cache/apt \
-    > /docker-build-debug/deploy/var-cache-apt-after
+find /var/cache/apt -ls > /docker-build-debug/deploy/var-cache-apt-after
 date

docker/dev-common.bash

@@ -15,7 +15,7 @@ exec > >(tee -i /docker-build-debug/dev-common/output) 2>&1
 
 date
 uname -a
-ls -laFR /var/cache/apt > /docker-build-debug/dev-common/var-cache-apt-before
+find /var/cache/apt -ls > /docker-build-debug/dev-common/var-cache-apt-before
 
 export DEBIAN_FRONTEND=noninteractive
 
@@ -78,5 +78,5 @@ export CARGO_BUILD_TARGET_DIR=/var/lib/moonfire-nvr/target
 EOF
 chown moonfire-nvr:moonfire-nvr /var/lib/moonfire-nvr/.buildrc
 
-ls -laFR /var/cache/apt > /docker-build-debug/dev-common/var-cache-apt-after
+find /var/cache/apt -ls > /docker-build-debug/dev-common/var-cache-apt-after
 date

docker/dev.bash

@@ -14,7 +14,7 @@ exec > >(tee -i /docker-build-debug/dev/output) 2>&1
 
 date
 uname -a
-ls -laFR /var/cache/apt > /docker-build-debug/dev/var-cache-apt-before
+find /var/cache/apt -ls > /docker-build-debug/dev/var-cache-apt-before
 
 export DEBIAN_FRONTEND=noninteractive
 
@@ -69,7 +69,6 @@ if [[ "${BUILDARCH}" != "${TARGETARCH}" ]]; then
     packages+=(
         g++-${gcc_target/_/-}
         libc6-dev-${dpkg_arch}-cross
-        pkg-config-${gcc_target}
         qemu-user
     )
 fi
@@ -85,12 +84,12 @@ time apt-get update
 time apt-get install --assume-yes --no-install-recommends "${packages[@]}"
 
 # Set environment variables for cross-compiling.
-# Also set up a symlink that points to the release binary, because the
-# release binary's location varies when cross-compiling, as described here:
+# Also set up a symlink that points to the output platform's target dir, because
+# the target directory layout varies when cross-compiling, as described here:
 # https://doc.rust-lang.org/cargo/guide/build-cache.html
 if [[ -n "${rust_target}" ]]; then
     su moonfire-nvr -lc "rustup target install ${rust_target} &&
-                         ln -s target/${rust_target}/release-lto/moonfire-nvr ."
+                         ln -s target/${rust_target} platform-target"
     underscore_rust_target="${rust_target//-/_}"
     uppercase_underscore_rust_target="${underscore_rust_target^^}"
     cat >> /var/lib/moonfire-nvr/.buildrc <<EOF
@@ -99,6 +98,10 @@ if [[ -n "${rust_target}" ]]; then
 export CARGO_BUILD_TARGET=${rust_target}
 export CARGO_TARGET_${uppercase_underscore_rust_target}_LINKER=${gcc_target}-gcc
 
+# Use a pkg-config wrapper for the target architecture. This wrapper was
+# automatically created on "dpkg --add-architecture"; see
+# /etc/dpkg/dpkg.cfg.d/pkgconf-hook-config.
+#
 # https://github.com/rust-lang/pkg-config-rs uses the "PKG_CONFIG"
 # variable to to select the pkg-config binary to use. As of pkg-config 0.3.19,
 # it unfortunately doesn't understand the <target>_ prefix that the README.md
@@ -112,8 +115,8 @@ export CC_${underscore_rust_target}=${gcc_target}-gcc
 export CXX_${underscore_rust_target}=${gcc_target}-g++
 EOF
 else
-    su moonfire-nvr -lc "ln -s target/release-lto/moonfire-nvr ."
+    su moonfire-nvr -lc "ln -s target platform-target"
 fi
 
-ls -laFR /var/cache/apt > /docker-build-debug/dev/var-cache-apt-after
+find /var/cache/apt -ls > /docker-build-debug/dev/var-cache-apt-after
 date

guide/build.md

@@ -114,8 +114,11 @@ caveats:
 On Linux hosts (as opposed to when using Docker Desktop on macOS/Windows),
 you'll likely see errors like the ones below. The solution is to [install
 emulators](https://github.com/tonistiigi/binfmt#installing-emulators).
+You may need to reinstall emulators on each boot of the host.
 
 ```
+Exec format error
+
 Error while loading /usr/sbin/dpkg-split: No such file or directory
 Error while loading /usr/sbin/dpkg-deb: No such file or directory
 ```
@@ -127,7 +130,7 @@ Moonfire NVR's `Dockerfile` has some built-in debugging tools:
     info includes:
     *    output (stdout + stderr) from the build script, running long operations
          through the `time` command.
-    *    `ls -laFR` of cache mounts before and after.
+    *    `find -ls` output on cache mounts before and after.
 *   Each stage accepts a `INVALIDATE_CACHE_<stage>` argument. You can use eg
     `--build-arg=INVALIDATE_CACHE_BUILD_SERVER=$(date +%s)` to force the
     `build-server` stage to be rebuilt rather than use cached Docker layers.
@@ -191,8 +194,9 @@ $ sudo apt-get install \
                tzdata
 ```
 
-Ubuntu 20.04 (the latest LTS as of this writing) bundles node 10, which has
-reached end-of-life (see [node.js: releases](https://nodejs.org/en/about/releases/)).
+Ubuntu 20.04 LTS (still popular, supported by Ubuntu until April 2025) bundles
+node 10, which has reached end-of-life (see
+[node.js: releases](https://nodejs.org/en/about/releases/)).
 So rather than install the `nodejs` and `npm` packages from the built-in
 repository, see [Installing Node.js via package
 manager](https://nodejs.org/en/download/package-manager/#debian-and-ubuntu-based-linux-distributions).