Rewrite REPL test parser

- Use a recursive descent parser so that it's easy to extend.
- Add `@args` to enable customizing command-line arguments
- Add `@should-start` to enable `nix repl` tests that error before
  entering the REPL
- Make sure to read all stdout output before comparing. This catches
  some extra output we were tossing out before!

Change-Id: I5522555df4c313024ab15cd10f9f04e7293bda3a

1 files changed, 20 insertions, 2 deletions

diff --git a/tests/functional/repl_characterization/test-session.hh b/tests/functional/repl_characterization/test-session.hh
index 19636640b..2552542fb 100644
--- a/tests/functional/repl_characterization/test-session.hh
+++ b/tests/functional/repl_characterization/test-session.hh
@@ -1,7 +1,9 @@
 #pragma once
 ///@file
 
+#include <functional>
 #include <sched.h>
+#include <span>
 #include <string>
 
 #include "util.hh"
@@ -22,8 +24,7 @@ struct RunningProcess
 class ReplOutputParser
 {
 public:
-    ReplOutputParser(std::string prompt)
-        : prompt(prompt)
+    ReplOutputParser(std::string prompt) : prompt(prompt)
     {
         assert(!prompt.empty());
     }
@@ -60,10 +61,27 @@ struct TestSession
     {
     }
 
+    /** Waits for the prompt and then returns if a prompt was found */
     bool waitForPrompt();
 
+    /** Feeds a line of input into the command */
     void runCommand(std::string command);
 
+    /** Closes the session, closing standard input and waiting for standard
+     * output to close, capturing any remaining output. */
     void close();
+
+private:
+    /** Waits until the command closes its output */
+    void wait();
+
+    enum class ReadOutThenCallbackResult { Stop, Continue };
+    using ReadOutThenCallback = std::function<ReadOutThenCallbackResult(std::span<char>)>;
+    /** Reads some chunks of output, calling the callback provided for each
+     * chunk and stopping if it returns Stop.
+     *
+     * @returns false if EOF, true if the callback requested we stop first.
+     * */
+    bool readOutThen(ReadOutThenCallback cb);
 };
 };