1 files changed, 93 insertions, 0 deletions
diff --git a/lib/gitlab/chat/output.rb b/lib/gitlab/chat/output.rb
new file mode 100644
index 00000000000..411b1555a7d
--- /dev/null
+++ b/lib/gitlab/chat/output.rb
@@ -0,0 +1,93 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Chat
+    # Class for gathering and formatting the output of a `Ci::Build`.
+    class Output
+      attr_reader :build
+
+      MissingBuildSectionError = Class.new(StandardError)
+
+      # The primary trace section to look for.
+      PRIMARY_SECTION = 'chat_reply'
+
+      # The backup trace section in case the primary one could not be found.
+      FALLBACK_SECTION = 'build_script'
+
+      # build - The `Ci::Build` to obtain the output from.
+      def initialize(build)
+        @build = build
+      end
+
+      # Returns a `String` containing the output of the build.
+      #
+      # The output _does not_ include the command that was executed.
+      def to_s
+        offset, length = read_offset_and_length
+
+        trace.read do |stream|
+          stream.seek(offset)
+
+          output = stream
+            .stream
+            .read(length)
+            .force_encoding(Encoding.default_external)
+
+          without_executed_command_line(output)
+        end
+      end
+
+      # Returns the offset to seek to and the number of bytes to read relative
+      # to the offset.
+      def read_offset_and_length
+        section = find_build_trace_section(PRIMARY_SECTION) ||
+          find_build_trace_section(FALLBACK_SECTION)
+
+        unless section
+          raise(
+            MissingBuildSectionError,
+            "The build_script trace section could not be found for build #{build.id}"
+          )
+        end
+
+        length = section[:byte_end] - section[:byte_start]
+
+        [section[:byte_start], length]
+      end
+
+      # Removes the line containing the executed command from the build output.
+      #
+      # output - A `String` containing the output of a trace section.
+      def without_executed_command_line(output)
+        # If `output.split("\n")` produces an empty Array then the slicing that
+        # follows it will produce a nil. For example:
+        #
+        #     "\n".split("\n")        # => []
+        #     "\n".split("\n")[1..-1] # => nil
+        #
+        # To work around this we only "join" if we're given an Array.
+        if (converted = output.split("\n")[1..-1])
+          converted.join("\n")
+        else
+          ''
+        end
+      end
+
+      # Returns the trace section for the given name, or `nil` if the section
+      # could not be found.
+      #
+      # name - The name of the trace section to find.
+      def find_build_trace_section(name)
+        trace_sections.find { |s| s[:name] == name }
+      end
+
+      def trace_sections
+        @trace_sections ||= trace.extract_sections
+      end
+
+      def trace
+        @trace ||= build.trace
+      end
+    end
+  end
+end