Raise a catchable error on malformed Huffman-coded input

lib/hpax/huffman.ex

@@ -85,6 +85,14 @@ defmodule HPAX.Huffman do
     end
   end
 
+  # Anything left over here is 8 or more bits that don't match any Huffman code (the clauses
+  # above only match a complete code, the empty binary, or up to 7 bits of valid EOS padding).
+  # This can only happen with a malformed/malicious encoding, since a real encoder never
+  # produces output with more than 7 trailing bits that aren't a complete code.
+  def decode(<<_rest::bitstring>>) do
+    throw({:hpax, {:protocol_error, :invalid_huffman_encoding}})
+  end
+
   ## Helpers
 
   @compile {:inline, take_significant_bits: 3}

test/hpax/huffman_test.exs

@@ -19,4 +19,17 @@ defmodule HPAX.HuffmanTest do
       assert Huffman.decode(encoded) == :huffman.decode(encoded)
     end
   end
+
+  describe "decode/1 with malformed input" do
+    test "raises a catchable :hpax error instead of a FunctionClauseError when the trailing bits don't correspond to any complete code" do
+      # 8+ bits of all 1s is a valid prefix of the 30-bit EOS code but is neither a complete
+      # code nor <= 7 bits of valid EOS padding, so it can't come from a real encoder
+      for n_bits <- [8, 9, 15, 16, 23, 29] do
+        invalid = for _ <- 1..n_bits, into: <<>>, do: <<1::1>>
+
+        assert catch_throw(Huffman.decode(invalid)) ==
+                 {:hpax, {:protocol_error, :invalid_huffman_encoding}}
+      end
+    end
+  end
 end