Download HRRR ranges sequentially to prevent memory accumulation

The parallel download was holding all ~530MB of range responses in
memory before writing to disk. Now each range is fetched and written
one at a time, so only one chunk is in memory at a time.

lib/microwaveprop/weather/hrrr_client.ex

@@ -347,46 +347,29 @@ defmodule Microwaveprop.Weather.HrrrClient do
 
   Returns `:ok` or `{:error, reason}`.
   """
-  def download_grib_ranges_to_file(_url, [], _dest_path), do: {:ok, <<>>}
+  def download_grib_ranges_to_file(_url, [], _dest_path), do: :ok
 
   def download_grib_ranges_to_file(url, ranges, dest_path) do
-    merged = merge_ranges(ranges)
+    merged = merge_ranges(ranges) |> Enum.sort_by(&elem(&1, 0))
 
-    results =
+    file = File.open!(dest_path, [:write, :binary])
-      merged
-      |> Task.async_stream(
-        fn {start, stop} ->
-          case Req.get(url, [{:headers, [{"Range", "bytes=#{start}-#{stop}"}]} | req_options()]) do
-            {:ok, %{status: 206, body: body}} -> {:ok, start, body}
-            {:ok, %{status: status}} -> {:error, "HRRR grib HTTP #{status}"}
-            {:error, reason} -> {:error, reason}
-          end
-        end,
-        max_concurrency: 4,
-        timeout: 120_000
-      )
-      |> Enum.map(fn
-        {:ok, result} -> result
-        {:exit, reason} -> {:error, reason}
-      end)
 
-    case Enum.find(results, &match?({:error, _}, &1)) do
+    try do
-      {:error, reason} ->
+      Enum.reduce_while(merged, :ok, fn {start, stop}, :ok ->
-        {:error, reason}
+        case Req.get(url, [{:headers, [{"Range", "bytes=#{start}-#{stop}"}]} | req_options()]) do
-
+          {:ok, %{status: 206, body: body}} ->
-      nil ->
-        sorted = Enum.sort_by(results, fn {:ok, offset, _} -> offset end)
-        file = File.open!(dest_path, [:write, :binary])
-
-        try do
-          Enum.each(sorted, fn {:ok, _offset, body} ->
             IO.binwrite(file, body)
-          end)
+            {:cont, :ok}
-        after
-          File.close(file)
-        end
 
-        :ok
+          {:ok, %{status: status}} ->
+            {:halt, {:error, "HRRR grib HTTP #{status}"}}
+
+          {:error, reason} ->
+            {:halt, {:error, reason}}
+        end
+      end)
+    after
+      File.close(file)
     end
   end