Report which hotcell-server version a cell runs in its describe response

CHANGELOG.md

@@ -24,6 +24,7 @@ Some actions that application developers should consider taking when upgrading f
 #### Added
 
 * `hot_cell/health_operations` defines `health.echo` and `health.reopen`, the round trips an application calls to prove it can use a cell's work socket. A cell serves them only if it requires the file.
+* The `hotcell.describe` response now includes `server_version`. This value is the version of the `hotcell-server` gem that the cell runs. You do not need a shell in the container to find it. (#21)
 
 #### Improved
 

hotcell-server/lib/hot_cell/control.rb

@@ -40,8 +40,12 @@ def answer(line, running:, queued:)
 
     # Static, and called once per registered cell at app boot. It is the cheapest way to catch a client
     # whose own timeout is below what this cell may take, and it is what `bin/hotcell describe` reads.
+    #
+    # `v` is the wire format and changes only when the format does; `server_version` says which release of
+    # hotcell-server is answering.
     def describe
-      { v: PROTOCOL_VERSION, operations: Registry.names, groups: groups, **@configuration.to_h }
+      { v: PROTOCOL_VERSION, server_version: Server::VERSION, operations: Registry.names, groups: groups,
+        **@configuration.to_h }
     end
 
     private

hotcell-server/test/control_test.rb

@@ -16,6 +16,12 @@ def test_describe_reports_what_the_cell_carries_and_how_long_it_may_take
     end
   end
 
+  def test_describe_reports_which_build_of_the_cell_is_running
+    TestCell.boot do |cell|
+      assert_equal HotCell::Server::VERSION, assert_ok(cell.control("hotcell.describe")).result[:server_version]
+    end
+  end
+
   # Counts lag responses, and the wait is the assertion rather than a workaround. The worker writes the
   # response and the supervisor increments the counter when it later reads that worker's idle report — two
   # processes — so a caller can be holding its answer before the count exists. Anything reading these for