Allow HTTPoison 3.0 as optional dependency

.changesets/allow-httpoison-3.md

@@ -0,0 +1,9 @@
+---
+bump: patch
+type: change
+integrations: all
+---
+
+Allow HTTPoison 3.0 to be used as an optional dependency. Applications
+on HTTPoison 3.0 can now use AppSignal's HTTPoison instrumentation
+without a version conflict.

.dialyzer_ignore.exs

@@ -0,0 +1,11 @@
+# HTTPoison 3.0's `HTTPoison.Base` macro generates a `stream_next/1` that calls
+# `:hackney.stream_next/1`. hackney 4.0 narrowed that function's typespec, so
+# dialyzer treats the error clause as dead code and flags the generated spec and
+# callback. The code is HTTPoison's, injected into `Appsignal.HTTPoison` through
+# `use`, so we ignore these rather than work around a third-party type change.
+[
+  {"deps/httpoison/lib/httpoison/base.ex", :callback_arg_type_mismatch},
+  {"deps/httpoison/lib/httpoison/base.ex", :callback_type_mismatch},
+  {"deps/httpoison/lib/httpoison/base.ex", :pattern_match_cov},
+  {"lib/appsignal/httpoison.ex", :invalid_contract}
+]

mix.exs

@@ -54,7 +54,8 @@ defmodule Appsignal.Mixfile do
       ],
       dialyzer: [
         plt_file: {:no_warn, "priv/plts/dialyzer.plt"},
-        plt_add_apps: [:mix]
+        plt_add_apps: [:mix],
+        ignore_warnings: ".dialyzer_ignore.exs"
       ]
     ]
   end
@@ -135,6 +136,14 @@ defmodule Appsignal.Mixfile do
         false -> "~> 0.4 or ~> 1.0"
       end
 
+    # httpoison 3.0 depends on hackney 4.0, which pulls in quic and requires
+    # OTP 26 or later. Cap httpoison at 2.x on older OTP releases.
+    httpoison_version =
+      case otp_version < "26" do
+        true -> "~> 2.0"
+        false -> "~> 2.0 or ~> 3.0"
+      end
+
     mime_dependency =
       case Version.compare(system_version, "1.10.0") do
         :lt -> [{:mime, "~> 1.0", only: [:test, :test_no_nif]}]
@@ -195,7 +204,7 @@ defmodule Appsignal.Mixfile do
       {:credo, credo_version, only: [:test, :dev], runtime: false},
       {:dialyxir, "~> 1.4", only: [:dev, :test], runtime: false},
       {:telemetry, telemetry_version},
-      {:httpoison, "~> 2.0", optional: true}
+      {:httpoison, httpoison_version, optional: true}
     ] ++ mime_dependency ++ logger_backends_dependency ++ hpax_dependency
   end
 end