fix: this fixes the exception when using unknown atoms as compressor options (#294)

Author: zookzook

lib/mongo/compressor.ex

@@ -4,13 +4,39 @@ defmodule Mongo.Compressor do
   @zlib_compressor_id 2
   @zstd_module Enum.find([:zstd, :ezstd], &Code.ensure_loaded?/1)
 
+  if @zstd_module do
+    @support_compressors ["zstd", "zlib"]
+  else
+    @support_compressors ["zlib"]
+  end
+
   if @zstd_module do
     @zstd_compressor_id 3
   end
 
+  def map_compressors(nil) do
+    []
+  end
+
+  def map_compressors(compressors) do
+    compressors
+    |> Enum.filter(fn compressor -> compressor in @support_compressors end)
+    |> Enum.map(fn compressor -> compressor_to_atom(compressor) end)
+  end
+
+  def compressor_to_atom("zstd") do
+    :zstd
+  end
+
+  def compressor_to_atom("zlib") do
+    :zlib
+  end
+
   def zstd_available?, do: not is_nil(@zstd_module)
 
-  def compressors, do: if(zstd_available?(), do: ["zstd", "zlib"], else: ["zlib"])
+  def compressors() do
+    @support_compressors
+  end
 
   def compress(binary, :zlib) do
     {@zlib_compressor_id, :zlib.compress(binary)}

lib/mongo/server_description.ex

@@ -10,7 +10,6 @@ defmodule Mongo.ServerDescription do
   @type type :: :standalone | :mongos | :possible_primary | :rs_primary | :rs_secondary | :rs_arbiter | :rs_other | :rs_ghost | :unknown
 
   @type compressor_types :: :zlib | :zstd
-  @support_compressors Compressor.compressors()
 
   @type t :: %{
           address: String.t() | nil,
@@ -110,7 +109,7 @@ defmodule Mongo.ServerDescription do
       max_bson_object_size: hello_response["maxBsonObjectSize"] || 16_777_216,
       max_message_size_bytes: hello_response["maxMessageSizeBytes"] || 48_000_000,
       max_write_batch_size: hello_response["maxWriteBatchSize"] || 100_000,
-      compression: map_compressors(hello_response["compression"]),
+      compression: Compressor.map_compressors(hello_response["compression"]),
       read_only: hello_response["readOnly"] || false,
       logical_session_timeout: hello_response["logicalSessionTimeoutMinutes"] || 30,
       supports_retryable_writes: supports_retryable_writes,
@@ -143,24 +142,14 @@ defmodule Mongo.ServerDescription do
       max_bson_object_size: hello_response["maxBsonObjectSize"] || 16_777_216,
       max_message_size_bytes: hello_response["maxMessageSizeBytes"] || 48_000_000,
       max_write_batch_size: hello_response["maxWriteBatchSize"] || 100_000,
-      compression: map_compressors(hello_response["compression"]),
+      compression: Mongo.Compressor.map_compressors(hello_response["compression"]),
       read_only: hello_response["readOnly"] || false,
       logical_session_timeout: hello_response["logicalSessionTimeoutMinutes"] || 30,
       supports_retryable_writes: server_type != :standalone && max_wire_version >= @retryable_wire_version && hello_response["logicalSessionTimeoutMinutes"] != nil,
       replica?: replica?(server_type)
     }
   end
 
-  defp map_compressors(nil) do
-    []
-  end
-
-  defp map_compressors(compressors) do
-    compressors
-    |> Enum.filter(fn compressor -> compressor in @support_compressors end)
-    |> Enum.map(fn compressor -> String.to_existing_atom(compressor) end)
-  end
-
   # see https://github.com/mongodb/specifications/blob/master/source/server-discovery-and-monitoring/server-discovery-and-monitoring.rst#type
   defp determine_server_type(%{"ok" => n}) when n != 1, do: :unknown
   defp determine_server_type(%{"msg" => "isdbgrid"}), do: :mongos
@@ -178,10 +167,6 @@ defmodule Mongo.ServerDescription do
 
   defp determine_server_type(_), do: :standalone
 
-  def support_compressors() do
-    Enum.map(@support_compressors, &String.to_existing_atom/1)
-  end
-
   defp replica?(server_type) do
     server_type in [:rs_primary, :rs_secondary, :rs_arbiter, :rs_other, :rs_ghost]
   end

test/mongo/compression_test.exs

@@ -0,0 +1,13 @@
+defmodule Mongo.CompressorTest do
+  use ExUnit.Case
+
+  alias Mongo.Compressor
+
+  test "the map_compressors should filter unsupported compressors" do
+    assert [:zstd, :zlib] = Compressor.map_compressors(["snappy", "zstd", "zlib"])
+  end
+
+  test "the map_compressors should return [] if no compressor is supported]" do
+    assert [] = Compressor.map_compressors(["snappy"])
+  end
+end