fix: Avoid crash then gzip-decompressing empty body (#796)

When doing a HEAD-request with Accept-Encoding: gzip, some web servers set the
`content-encoding` to `gzip`, but naturally transfer an empty body.

Author: niklas

lib/tesla/middleware/compression.ex

@@ -84,6 +84,10 @@ defmodule Tesla.Middleware.Compression do
     Tesla.put_header(env, "content-encoding", Enum.join(unknown_codecs, ", "))
   end
 
+  defp decompress_body(_codecs, "" = body, acc) do
+    {body, acc}
+  end
+
   defp decompress_body([gzip | rest], body, acc) when gzip in ["gzip", "x-gzip"] do
     decompress_body(rest, :zlib.gunzip(body), acc)
   end

test/tesla/middleware/compression_test.exs

@@ -61,6 +61,9 @@ defmodule Tesla.Middleware.CompressionTest do
 
           "/response-identity" ->
             {200, [{"content-type", "text/plain"}, {"content-encoding", "identity"}], "unchanged"}
+
+          "/response-empty" ->
+            {200, [{"content-type", "text/plain"}, {"content-encoding", "gzip"}], ""}
         end
 
       {:ok, %{env | status: status, headers: headers, body: body}}
@@ -84,6 +87,12 @@ defmodule Tesla.Middleware.CompressionTest do
     assert env.headers == [{"content-type", "text/plain"}]
   end
 
+  test "return unchanged response for empty body (gzip)" do
+    assert {:ok, env} = CompressionResponseClient.get("/response-empty")
+    assert env.body == ""
+    assert env.headers == [{"content-type", "text/plain"}]
+  end
+
   defmodule CompressRequestDecompressResponseClient do
     use Tesla