feat: Add HttpBinding#probable_event? (#64)

Signed-off-by: Daniel Azuma <[email protected]>

Author: dazuma

lib/cloud_events/http_binding.rb

@@ -107,11 +107,33 @@ def register_formatter_methods formatter,
     #
     attr_accessor :default_encoder_name
 
+    ##
+    # Analyze a Rack environment hash and determine whether it is _probably_
+    # a CloudEvent. This is done by examining headers only, and does not read
+    # or parse the request body. The result is a best guess: false negatives or
+    # false positives are possible for edge cases, but the logic should
+    # generally detect canonically-formatted events.
+    #
+    # @param env [Hash] The Rack environment.
+    # @return [boolean] Whether the request is likely a CloudEvent.
+    #
+    def probable_event? env
+      return true if env["HTTP_CE_SPECVERSION"]
+      content_type = ContentType.new env["CONTENT_TYPE"].to_s
+      content_type.media_type == "application" &&
+        ["cloudevents", "cloudevents-batch"].include?(content_type.subtype_base)
+    end
+
     ##
     # Decode an event from the given Rack environment hash. Following the
     # CloudEvents spec, this chooses a handler based on the Content-Type of
     # the request.
     #
+    # Note that this method will read the body (i.e. `rack.input`) stream.
+    # If you need to access the body after calling this method, you will need
+    # to rewind the stream. To determine whether the request is a CloudEvent
+    # without reading the body, use {#probable_event?}.
+    #
     # @param env [Hash] The Rack environment.
     # @param allow_opaque [boolean] If true, returns opaque event objects if
     #     the input is not in a recognized format. If false, raises
@@ -203,6 +225,7 @@ def decode_rack_env env, **format_args
       content_type_string = env["CONTENT_TYPE"]
       content_type = ContentType.new content_type_string if content_type_string
       content = read_with_charset env["rack.input"], content_type&.charset
+      env["rack.input"].rewind rescue nil
       decode_binary_content(content, content_type, env, legacy_data_decode: true) ||
         decode_structured_content(content, content_type, false, **format_args)
     end

test/test_http_binding.rb

@@ -458,4 +458,32 @@
     assert_equal({ "Content-Type" => "application/cloudevents-batch+json" }, headers)
     assert_equal my_json_batch_encoded, body
   end
+
+  it "detects a probable binary event" do
+    env = {
+      "HTTP_CE_SPECVERSION" => "1.0"
+    }
+    assert http_binding.probable_event? env
+  end
+
+  it "detects a probable structured event" do
+    env = {
+      "CONTENT_TYPE" => "application/cloudevents+myformat"
+    }
+    assert http_binding.probable_event? env
+  end
+
+  it "detects a probable batch event" do
+    env = {
+      "CONTENT_TYPE" => "application/cloudevents-batch+myformat"
+    }
+    assert http_binding.probable_event? env
+  end
+
+  it "detects when something is unlikely an event" do
+    env = {
+      "CONTENT_TYPE" => "application/json"
+    }
+    refute http_binding.probable_event? env
+  end
 end