From 5c5d398966a9f49e837638eac54ecb84e4a72575 Mon Sep 17 00:00:00 2001
From: Graham McIntire <graham@mcintire.me>
Date: Mon, 30 Mar 2026 09:21:27 -0500
Subject: [PATCH] Make GRIB2 section parser resilient to trailing padding and
 truncation

Instead of returning "malformed section" on trailing bytes or truncated
sections, attempt to return parsed results. This handles older HRRR
files (2019) that have padding bytes after the last section.
---
 lib/microwaveprop/weather/grib2/section.ex    |  16 ++-
 .../weather/grib2/section_test.exs            | 122 ++++++++++++++++++
 2 files changed, 133 insertions(+), 5 deletions(-)

diff --git a/lib/microwaveprop/weather/grib2/section.ex b/lib/microwaveprop/weather/grib2/section.ex
index bee48613..c3b1e0bc 100644
--- a/lib/microwaveprop/weather/grib2/section.ex
+++ b/lib/microwaveprop/weather/grib2/section.ex
@@ -55,18 +55,24 @@ defmodule Microwaveprop.Weather.Grib2.Section do
 
   defp parse_sections(<<length::32-big, section_num::8, rest::binary>>, acc) when length > 5 do
     body_length = length - 5
-    <<body::binary-size(body_length), remaining::binary>> = rest
 
-    acc = parse_section(section_num, body, acc)
-    parse_sections(remaining, acc)
+    if body_length <= byte_size(rest) do
+      <<body::binary-size(body_length), remaining::binary>> = rest
+      acc = parse_section(section_num, body, acc)
+      parse_sections(remaining, acc)
+    else
+      # Truncated section — return what we have
+      build_result(acc)
+    end
   end
 
   defp parse_sections(<<>>, acc) do
     build_result(acc)
   end
 
-  defp parse_sections(_other, _acc) do
-    {:error, "malformed section"}
+  # Trailing padding or short garbage — return what we have
+  defp parse_sections(_other, acc) do
+    build_result(acc)
   end
 
   defp parse_section(1, _body, acc) do
diff --git a/test/microwaveprop/weather/grib2/section_test.exs b/test/microwaveprop/weather/grib2/section_test.exs
index 4048402d..fadfe057 100644
--- a/test/microwaveprop/weather/grib2/section_test.exs
+++ b/test/microwaveprop/weather/grib2/section_test.exs
@@ -42,6 +42,20 @@ defmodule Microwaveprop.Weather.Grib2.SectionTest do
       indicator = <<"GRIB", 0::16, 0::8, 1::8, 100::64-big>>
       assert {:error, _} = Section.parse_message(indicator)
     end
+
+    test "parses message with trailing padding bytes instead of 7777" do
+      msg = build_synthetic_grib2_with_trailing_padding()
+      assert {:ok, parsed} = Section.parse_message(msg)
+      assert parsed.product.var == "TMP"
+      assert parsed.bitmap == :none
+    end
+
+    test "parses message with truncated trailing section" do
+      msg = build_synthetic_grib2_with_truncated_section()
+      assert {:ok, parsed} = Section.parse_message(msg)
+      assert parsed.product.var == "TMP"
+      assert parsed.bitmap == :none
+    end
   end
 
   describe "product identification" do
@@ -262,4 +276,112 @@ defmodule Microwaveprop.Weather.Grib2.SectionTest do
     length = 5 + byte_size(body)
     <<length::32-big, section_number::8, body::binary>>
   end
+
+  # Like build_synthetic_grib2 but with 3 padding bytes instead of "7777"
+  defp build_synthetic_grib2_with_trailing_padding do
+    body = build_sections_body() <> <<0, 0, 0>>
+    total_length = 16 + byte_size(body)
+    sec0 = <<"GRIB", 0::16, 0::8, 2::8, total_length::64-big>>
+    sec0 <> body
+  end
+
+  # Like build_synthetic_grib2 but total_length includes extra bytes after "7777"
+  # that look like a truncated section header
+  defp build_synthetic_grib2_with_truncated_section do
+    # All valid sections + "7777" + a truncated next section (length says 500 but only 10 bytes)
+    truncated = <<500::32-big, 4::8, 0, 0, 0, 0, 0>>
+    body = build_sections_body() <> "7777" <> truncated
+    total_length = 16 + byte_size(body)
+    sec0 = <<"GRIB", 0::16, 0::8, 2::8, total_length::64-big>>
+    sec0 <> body
+  end
+
+  defp build_sections_body do
+    sec1_body = <<
+      0::16-big,
+      0::16-big,
+      2::8,
+      1::8,
+      1::8,
+      2026::16-big,
+      3::8,
+      28::8,
+      18::8,
+      0::8,
+      0::8,
+      0::8,
+      1::8
+    >>
+
+    sec3_body = <<
+      0::8,
+      1_905_141::32-big,
+      0::8,
+      0::8,
+      30::16-big,
+      6::8,
+      0::8,
+      0::32-big,
+      0::8,
+      0::32-big,
+      0::8,
+      0::32-big,
+      1799::32-big,
+      1059::32-big,
+      21_138_123::32-big,
+      237_280_472::32-big,
+      0b00111000::8,
+      38_500_000::32-big,
+      262_500_000::32-big,
+      3_000_000::32-big,
+      3_000_000::32-big,
+      0::8,
+      64::8,
+      38_500_000::32-big,
+      38_500_000::32-big,
+      0::32-big,
+      0::32-big
+    >>
+
+    sec4_body = <<
+      0::16-big,
+      0::16-big,
+      0::8,
+      0::8,
+      2::8,
+      0::8,
+      0::8,
+      0::16-big,
+      0::8,
+      1::8,
+      0::32-big,
+      103::8,
+      0::8,
+      2::32-big,
+      255::8,
+      0::8,
+      0::32-big
+    >>
+
+    ref_val = <<0.0::float-32-big>>
+
+    sec5_body = <<
+      100::32-big,
+      0::16-big,
+      ref_val::binary,
+      0::16-big,
+      0::16-big,
+      16::8
+    >>
+
+    sec6_body = <<255::8>>
+    data_bytes = :binary.copy(<<42::16-big>>, 100)
+
+    section_wrap(1, sec1_body) <>
+      section_wrap(3, sec3_body) <>
+      section_wrap(4, sec4_body) <>
+      section_wrap(5, sec5_body) <>
+      section_wrap(6, sec6_body) <>
+      section_wrap(7, data_bytes)
+  end
 end