From 4e2d74e21cf47ce4c0165cd1906b5fbbba5f3db0 Mon Sep 17 00:00:00 2001
From: Graham McIntire <graham@mcintire.me>
Date: Tue, 31 Mar 2026 08:55:09 -0500
Subject: [PATCH] Build wgrib2 in Docker with cached build stage
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Add a separate wgrib2-builder stage that compiles wgrib2 from source.
Docker layer caching means this only builds once — subsequent deploys
reuse the cached layer. The binary is copied into the final runtime
image. Also fixed wgrib2 path lookup to be runtime instead of
compile-time so it works in the Docker build pipeline.
---
 Dockerfile                                | 33 ++++++++++++++++++++---
 lib/microwaveprop/weather/grib2/wgrib2.ex |  9 +++----
 2 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/Dockerfile b/Dockerfile
index 87f9b787..8c627aaa 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -18,6 +18,29 @@ ARG DEBIAN_VERSION=trixie-20260316-slim
 ARG BUILDER_IMAGE="docker.io/hexpm/elixir:${ELIXIR_VERSION}-erlang-${OTP_VERSION}-debian-${DEBIAN_VERSION}"
 ARG RUNNER_IMAGE="docker.io/debian:${DEBIAN_VERSION}"
 
+# ---- wgrib2 build stage (cached independently) ----
+FROM ${RUNNER_IMAGE} AS wgrib2-builder
+
+ARG WGRIB2_VERSION=3.6.1
+
+RUN apt-get update \
+  && apt-get install -y --no-install-recommends \
+     build-essential gfortran wget ca-certificates \
+     zlib1g-dev libaec-dev libpng-dev \
+  && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /tmp/wgrib2
+
+RUN wget -q "https://www.ftp.cpc.ncep.noaa.gov/wd51we/wgrib2/wgrib2.tgz.v${WGRIB2_VERSION}" \
+      -O wgrib2.tgz \
+  && tar xzf wgrib2.tgz \
+  && cd grib2 \
+  && export CC=gcc FC=gfortran \
+  && make \
+  && cp wgrib2/wgrib2 /usr/local/bin/wgrib2 \
+  && strip /usr/local/bin/wgrib2
+
+# ---- Elixir build stage ----
 FROM ${BUILDER_IMAGE} AS builder
 
 # install build dependencies
@@ -66,12 +89,13 @@ COPY config/runtime.exs config/
 COPY rel rel
 RUN mix release
 
-# start a new build stage so that the final image will only contain
-# the compiled release and other runtime necessities
+# ---- Final runtime stage ----
 FROM ${RUNNER_IMAGE} AS final
 
 RUN apt-get update \
-  && apt-get install -y --no-install-recommends libstdc++6 openssl libncurses6 locales ca-certificates snmp \
+  && apt-get install -y --no-install-recommends \
+     libstdc++6 openssl libncurses6 locales ca-certificates snmp \
+     libgfortran5 libaec0 zlib1g libpng16-16t64 \
   && rm -rf /var/lib/apt/lists/*
 
 # Set the locale
@@ -88,6 +112,9 @@ RUN chown nobody /app
 # set runner ENV
 ENV MIX_ENV="prod"
 
+# Copy wgrib2 binary from the build stage
+COPY --from=wgrib2-builder /usr/local/bin/wgrib2 /usr/local/bin/wgrib2
+
 # Only copy the final release from the build stage
 COPY --from=builder --chown=nobody:root /app/_build/${MIX_ENV}/rel/microwaveprop ./
 
diff --git a/lib/microwaveprop/weather/grib2/wgrib2.ex b/lib/microwaveprop/weather/grib2/wgrib2.ex
index 67e43b0d..63e457e9 100644
--- a/lib/microwaveprop/weather/grib2/wgrib2.ex
+++ b/lib/microwaveprop/weather/grib2/wgrib2.ex
@@ -9,7 +9,6 @@ defmodule Microwaveprop.Weather.Grib2.Wgrib2 do
 
   require Logger
 
-  @wgrib2_path System.find_executable("wgrib2")
   @undefined_value 9.999e20
 
   @doc """
@@ -29,9 +28,9 @@ defmodule Microwaveprop.Weather.Grib2.Wgrib2 do
   end
 
   @doc "Check if wgrib2 is available on the system."
-  def available? do
-    @wgrib2_path != nil
-  end
+  def available?, do: wgrib2_path() != nil
+
+  defp wgrib2_path, do: System.find_executable("wgrib2")
 
   defp extract_with_wgrib2(grib_binary, match_pattern, grid_spec) do
     %{
@@ -68,7 +67,7 @@ defmodule Microwaveprop.Weather.Grib2.Wgrib2 do
         "bin"
       ]
 
-      case System.cmd(@wgrib2_path, args, stderr_to_stdout: true) do
+      case System.cmd(wgrib2_path(), args, stderr_to_stdout: true) do
         {output, 0} ->
           # Parse message inventory from stdout to know which vars were extracted
           messages = parse_wgrib2_inventory(output)