Fix define_derived_metadata so that it supports cascades. (#2630)

For example:

```
RSpec.configure do |c|
  c.define_derived_metadata(:elasticsearch) do |meta|
    meta[:vcr] = true
  end

  c.define_derived_metadata(:vcr) do |meta|
    meta[:retries] = 2
  end
end
```

With this configuration, an example or group tagged with `:elasticsearch`
should get tagged with `:vcr` as well, which in turn should add
`retries: 2` metadata to the example or group. Before this change,
this did not work properly, because we did look to see if additional
metadata blocks should apply after applying them once.

Author: myronmarston

Changelog.md

@@ -19,6 +19,8 @@ Bug Fixes:
 * When defining `let` methods that overwrite an existing method, prevent
   a warning being issued by removing the old definition. (Jon Rowe, #2593)
 * Prevent warning on Ruby 2.6.0-rc1 (Keiji Yoshimi, #2582)
+* Fix `config.define_derived_metadata` so that it supports cascades.
+  (Myron Marston, #2630).
 
 ### 3.8.0 / 2018-08-04
 [Full Changelog](http://github.com/rspec/rspec-core/compare/v3.7.1...v3.8.0)

lib/rspec/core/configuration.rb

@@ -1855,9 +1855,28 @@ def when_first_matching_example_defined(*filters)
 
       # @private
       def apply_derived_metadata_to(metadata)
-        @derived_metadata_blocks.items_for(metadata).each do |block|
-          block.call(metadata)
+        already_run_blocks = Set.new
+
+        # We loop and attempt to re-apply metadata blocks to support cascades
+        # (e.g. where a derived bit of metadata triggers the application of
+        # another piece of derived metadata, etc)
+        #
+        # We limit our looping to 200 times as a way to detect infinitely recursing derived metadata blocks.
+        # It's hard to imagine a valid use case for a derived metadata cascade greater than 200 iterations.
+        200.times do
+          return if @derived_metadata_blocks.items_for(metadata).all? do |block|
+            already_run_blocks.include?(block).tap do |skip_block|
+              block.call(metadata) unless skip_block
+              already_run_blocks << block
+            end
+          end
         end
+
+        # If we got here, then `@derived_metadata_blocks.items_for(metadata).all?` never returned
+        # `true` above and we treat this as an attempt to recurse infinitely. It's better to fail
+        # with a clear # error than hang indefinitely, which is what would happen if we didn't limit
+        # the looping above.
+        raise SystemStackError, "Attempted to recursively derive metadata indefinitely."
       end
 
       # Defines a `before` hook. See {Hooks#before} for full docs.

spec/rspec/core/configuration_spec.rb

@@ -1819,6 +1819,42 @@ def exclude?(line)
         expect(group.metadata).to include(:b1_desc => "bar (block 1)", :b2_desc => "bar (block 1) (block 2)")
       end
 
+      it 'supports cascades of derived metadata, but avoids re-running derived metadata blocks that have already been applied' do
+        RSpec.configure do |c|
+          c.define_derived_metadata(:foo1) { |m| m[:foo2] = (m[:foo2] || 0) + 1 }
+          c.define_derived_metadata(:foo2) { |m| m[:foo3] = (m[:foo3] || 0) + 1 }
+          c.define_derived_metadata(:foo3) { |m| m[:foo1] += 1 }
+        end
+
+        group = RSpec.describe("bar", :foo1 => 0)
+        expect(group.metadata).to include(:foo1 => 1, :foo2 => 1, :foo3 => 1)
+
+        ex = RSpec.describe("My group").example("foo", :foo1 => 0)
+        expect(ex.metadata).to include(:foo1 => 1, :foo2 => 1, :foo3 => 1)
+      end
+
+      it 'does not allow a derived metadata cascade to recurse infinitely' do
+        RSpec.configure do |c|
+          counter = 1
+          derive_next_metadata = lambda do |outer_meta|
+            tag = :"foo#{counter += 1}"
+            outer_meta[tag] = true
+
+            c.define_derived_metadata(tag) do |inner_meta|
+              derive_next_metadata.call(inner_meta)
+            end
+          end
+
+          c.define_derived_metadata(:foo1) do |meta|
+            derive_next_metadata.call(meta)
+          end
+        end
+
+        expect {
+          RSpec.describe("group", :foo1)
+        }.to raise_error(SystemStackError)
+      end
+
       it "derives metadata before the group or example blocks are eval'd so their logic can depend on the derived metadata" do
         RSpec.configure do |c|
           c.define_derived_metadata(:foo) do |metadata|