Fix branching section evaluation when the parent section is empty (#560)

* Fix branching section evaluation when the parent section is empty

* Update changelog

CHANGELOG.md

@@ -17,11 +17,14 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 - Improved intellisense to handle structs and sigils ([#513](https://github.com/livebook-dev/livebook/pull/513))
 - Create new notebooks with an already focused code cell ([#527](https://github.com/livebook-dev/livebook/pull/527))
+- Switched base Docker image from alpine to debian-slim ([#552](https://github.com/livebook-dev/livebook/pull/552))
 
 ### Fixed
 
 - Improved Markdown and math integration by migrating to remark ([#495](https://github.com/livebook-dev/livebook/pull/495))
 - Improved the evaluator process to not consume user-submitted messages from inbox ([#502](https://github.com/livebook-dev/livebook/pull/502))
+- Improved sections panel UI to better handle numerous sections or long section names ([#534](https://github.com/livebook-dev/livebook/pull/534) and [#537](https://github.com/livebook-dev/livebook/pull/537))
+- Fixed branching section evaluation when the parent section is empty ([#560](https://github.com/livebook-dev/livebook/pull/560)
 
 ## [v0.2.3](https://github.com/livebook-dev/livebook/tree/v0.2.3) (2021-08-12)
 

lib/livebook/session.ex

@@ -1000,15 +1000,7 @@ defmodule Livebook.Session do
   """
   @spec find_prev_locator(Notebook.t(), Cell.t(), Section.t()) :: Runtime.locator()
   def find_prev_locator(notebook, cell, section) do
-    default =
-      case section.parent_id do
-        nil ->
-          {container_ref_for_section(section), nil}
-
-        parent_id ->
-          {:ok, parent} = Notebook.fetch_section(notebook, parent_id)
-          {container_ref_for_section(parent), nil}
-      end
+    default = {container_ref_for_section(section), nil}
 
     notebook
     |> Notebook.parent_cells_with_section(cell.id)

test/livebook/intellisense_test.exs

@@ -1154,6 +1154,7 @@ defmodule Livebook.IntellisenseTest do
       assert content =~ "## Examples"
     end
 
+    @tag :erl_docs
     test "returns full Erlang docs" do
       {binding, env} = eval(do: nil)
 

test/livebook/session_test.exs

@@ -2,6 +2,7 @@ defmodule Livebook.SessionTest do
   use ExUnit.Case, async: true
 
   alias Livebook.{Session, Delta, Runtime, Utils, Notebook, FileSystem}
+  alias Livebook.Notebook.{Section, Cell}
 
   # Note: queueing evaluation in most of the tests below
   # requires the runtime to synchronously start first,
@@ -549,6 +550,70 @@ defmodule Livebook.SessionTest do
     end
   end
 
+  describe "find_prev_locator/3" do
+    test "given cell in main flow returns previous Elixir cell" do
+      cell1 = %{Cell.new(:elixir) | id: "c1"}
+      cell2 = %{Cell.new(:markdown) | id: "c2"}
+      section1 = %{Section.new() | id: "s1", cells: [cell1, cell2]}
+
+      cell3 = %{Cell.new(:elixir) | id: "c3"}
+      section2 = %{Section.new() | id: "s2", cells: [cell3]}
+
+      notebook = %{Notebook.new() | sections: [section1, section2]}
+
+      assert {:main_flow, "c1"} = Session.find_prev_locator(notebook, cell3, section2)
+    end
+
+    test "given cell in branching section returns previous Elixir cell in that section" do
+      section1 = %{Section.new() | id: "s1"}
+
+      cell1 = %{Cell.new(:elixir) | id: "c1"}
+      cell2 = %{Cell.new(:markdown) | id: "c2"}
+      cell3 = %{Cell.new(:elixir) | id: "c3"}
+
+      section2 = %{
+        Section.new()
+        | id: "s2",
+          parent_id: "s1",
+          cells: [cell1, cell2, cell3]
+      }
+
+      notebook = %{Notebook.new() | sections: [section1, section2]}
+
+      assert {"s2", "c1"} = Session.find_prev_locator(notebook, cell3, section2)
+    end
+
+    test "given cell in main flow returns nil if there is no previous cell" do
+      cell1 = %{Cell.new(:markdown) | id: "c1"}
+      section1 = %{Section.new() | id: "s1", cells: [cell1]}
+
+      cell2 = %{Cell.new(:elixir) | id: "c2"}
+      section2 = %{Section.new() | id: "s2", cells: [cell2]}
+
+      notebook = %{Notebook.new() | sections: [section1, section2]}
+
+      assert {:main_flow, nil} = Session.find_prev_locator(notebook, cell2, section2)
+    end
+
+    test "given cell in branching section returns nil in that section if there is no previous cell" do
+      cell1 = %{Cell.new(:markdown) | id: "c1"}
+      section1 = %{Section.new() | id: "s1", cells: [cell1]}
+
+      cell2 = %{Cell.new(:elixir) | id: "c2"}
+
+      section2 = %{
+        Section.new()
+        | id: "s2",
+          parent_id: "s1",
+          cells: [cell2]
+      }
+
+      notebook = %{Notebook.new() | sections: [section1, section2]}
+
+      assert {"s2", nil} = Session.find_prev_locator(notebook, cell2, section2)
+    end
+  end
+
   defp start_session(opts \\ []) do
     session_id = Utils.random_id()
     {:ok, pid} = Session.start_link(Keyword.merge([id: session_id], opts))