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

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;
+}
+
 };

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;
+};
+
 };

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())
 {
 }
 

src/libutil/experimental-features.cc

@@ -12,7 +12,19 @@ struct ExperimentalFeatureDetails
     std::string_view description;
 };
 
-constexpr std::array<ExperimentalFeatureDetails, 14> xpFeatureDetails = {{
+/**
+ * If two different PRs both add an experimental feature, and we just
+ * used a number for this, we *woudln't* get merge conflict and the
+ * counter will be incremented once instead of twice, causing a build
+ * failure.
+ *
+ * By instead defining this instead as 1 + the bottom experimental
+ * feature, we either have no issue at all if few features are not added
+ * at the end of the list, or a proper merge conflict if they are.
+ */
+constexpr size_t numXpFeatures = 1 + static_cast<size_t>(Xp::ReplAutomation);
+
+constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails = {{
     {
         .tag = Xp::CaDerivations,
         .name = "ca-derivations",
@@ -219,6 +231,13 @@ constexpr std::array<ExperimentalFeatureDetails, 14> xpFeatureDetails = {{
             Allow the use of the `read-only` parameter in [local store](@docroot@/command-ref/new-cli/nix3-help-stores.md#local-store) URIs.
         )",
     },
+    {
+        .tag = Xp::ReplAutomation,
+        .name = "repl-automation",
+        .description = R"(
+            Makes the repl not use readline/editline, print ENQ (U+0005) when ready for a command, and take commands followed by newline.
+        )",
+    },
 }};
 
 static_assert(

src/libutil/experimental-features.hh

@@ -31,6 +31,7 @@ enum struct ExperimentalFeature
     DynamicDerivations,
     ParseTomlTimestamps,
     ReadOnlyLocalStore,
+    ReplAutomation,
 };
 
 /**