add automated usage mode to the repl

This is definitely not a stable thing, but it does feel slightly crimes
to put it as an experimental feature. Shrug, up for bikeshedding.

Change-Id: I6ef176e3dee6fb1cac9c0a7a60d553a2c63ea728

3 files changed, 38 insertions, 1 deletions

diff --git a/src/libcmd/repl-interacter.cc b/src/libcmd/repl-interacter.cc
index 18c1cad0f..829383add 100644
--- a/src/libcmd/repl-interacter.cc
+++ b/src/libcmd/repl-interacter.cc
@@ -1,4 +1,6 @@
 #include <cstdio>
+#include <iostream>
+#include <string>
 
 #ifdef READLINE
 #include <readline/history.h>
@@ -183,4 +185,24 @@ ReadlineLikeInteracter::~ReadlineLikeInteracter()
     write_history(historyFile.c_str());
 }
 
+AutomationInteracter::Guard AutomationInteracter::init(detail::ReplCompleterMixin *)
+{
+    return Guard([] {});
+}
+
+// ASCII ENQ character
+constexpr const char * automationPrompt = "\x05";
+
+bool AutomationInteracter::getLine(std::string & input, ReplPromptType promptType)
+{
+    std::cout << std::unitbuf;
+    std::cout << automationPrompt;
+    if (!std::getline(std::cin, input)) {
+        // reset failure bits on EOF
+        std::cin.clear();
+        return false;
+    }
+    return true;
+}
+
 };
diff --git a/src/libcmd/repl-interacter.hh b/src/libcmd/repl-interacter.hh
index cc70efd07..c31b1a1e6 100644
--- a/src/libcmd/repl-interacter.hh
+++ b/src/libcmd/repl-interacter.hh
@@ -45,4 +45,13 @@ public:
     virtual ~ReadlineLikeInteracter() override;
 };
 
+class AutomationInteracter : public virtual ReplInteracter
+{
+public:
+    AutomationInteracter() = default;
+    virtual Guard init(detail::ReplCompleterMixin * repl) override;
+    virtual bool getLine(std::string & input, ReplPromptType promptType) override;
+    virtual ~AutomationInteracter() override = default;
+};
+
 };
diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index 5809ab285..9e5599152 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -121,6 +121,12 @@ std::string removeWhitespace(std::string s)
     return s;
 }
 
+static box_ptr<ReplInteracter> makeInteracter() {
+    if (experimentalFeatureSettings.isEnabled(Xp::ReplAutomation))
+        return make_box_ptr<AutomationInteracter>();
+    else
+        return make_box_ptr<ReadlineLikeInteracter>(getDataDir() + "/nix/repl-history");
+}
 
 NixRepl::NixRepl(const SearchPath & searchPath, nix::ref<Store> store, ref<EvalState> state,
             std::function<NixRepl::AnnotatedValues()> getValues)
@@ -128,7 +134,7 @@ NixRepl::NixRepl(const SearchPath & searchPath, nix::ref<Store> store, ref<EvalS
     , debugTraceIndex(0)
     , getValues(getValues)
     , staticEnv(new StaticEnv(nullptr, state->staticBaseEnv.get()))
-    , interacter(make_box_ptr<ReadlineLikeInteracter>(getDataDir() + "/nix/repl-history"))
+    , interacter(makeInteracter())
 {
 }