Actually run compdb asynchronously

Turns out vim.schedule() only defers calling the function until main
event loop has some free cycles but can still block the UI, just a bit
later. This patch replaces the external command invoked in the callback
passed into vim.schedule() with vim.fn.jobstart() and moves the other
code into job callbacks.

lua/build.lua

@@ -19,22 +19,25 @@ function build.set_dir(dirname)
     echo('directory does not exist: ' .. build_dir, 'Error')
     return
   end
-  if vim.fn.filereadable(build_dir .. '/compile_commands.json') == 0 then
-    echo('compile_commands.json not found in: ' .. build_dir, 'Error')
-    return
-  end
 
   build.dir = build_dir
 
-  -- Invoke async function to avoid blocking the UI
-  vim.schedule(function()
   echo('Processing compile_commands.json with compdb ...', 'DiagnosticInfo')
-
-    -- Post-process compile_commands.json with compdb
   local compile_commands = current_dir .. '/compile_commands.json'
-    local output = vim.fn.systemlist(
-      'compdb -p ' .. build.dir .. ' list > ' .. compile_commands)
+  -- Post-process compile_commands.json with compdb as an async job to avoid
+  -- blocking the user interface
+  vim.fn.jobstart(
+    'compdb -p ' .. build.dir .. ' list > ' .. compile_commands, {
 
+      -- Restart clangd language server
+      -- TODO: Configure cmake language server?
+      on_exit = function()
+        vim.cmd [[ LspRestart clangd ]]
+        echo('Build directory selected: ' .. dirname, 'DiagnosticInfo')
+      end,
+
+      -- Display any error messages to the user
+      on_stderr = function(_, output, _)
         -- Remove any lines containing a compdb warning
         local error = {}
         local warning = 'WARNING:compdb'
@@ -49,14 +52,11 @@ function build.set_dir(dirname)
           echo(vim.fn.join(error, '\n'), 'Error')
           return
         end
+      end,
 
-    -- TODO: Configure cmake language server?
-
-    -- Restart clangd language server
-    vim.cmd [[ LspRestart clangd ]]
-
-    echo('Build directory selected: ' .. dirname, 'DiagnosticInfo')
-  end)
+      stderr_buffered = true,
+    }
+  )
 end
 
 function build.select_dir(callback)