Legacy vs non-legacy to_string/parse for DerivedPath

As requested by @roberth, it is good to call out the specific instances
we care about, which is `!` for the RPC protocols, and `^` for humans.

This doesn't take advantage of parametricity as much, but since the
human and computer interfaces are good to decouple anyways (we don't
care if they drift further apart over time in the slightest) some
separation and slight duplication is fine.

Also, unit test both round trips.

src/libcmd/installable-derived-path.cc

@@ -47,7 +47,7 @@ InstallableDerivedPath InstallableDerivedPath::parse(
                 };
                 warn(
                     "The interpretation of store paths arguments ending in `.drv` recently changed. If this command is now failing try again with '%s'",
-                    oldDerivedPath.to_string(*store, '^'));
+                    oldDerivedPath.to_string(*store));
             };
             return DerivedPath::Opaque {
                 .path = std::move(storePath),

src/libstore/derived-path.cc

@@ -59,17 +59,32 @@ std::string DerivedPath::Opaque::to_string(const Store & store) const
     return store.printStorePath(path);
 }
 
-std::string DerivedPath::Built::to_string(const Store & store, char separator) const
+std::string DerivedPath::Built::to_string(const Store & store) const
 {
     return store.printStorePath(drvPath)
-        + separator
+        + '^'
         + outputs.to_string();
 }
 
-std::string DerivedPath::to_string(const Store & store, char separator) const
+std::string DerivedPath::Built::to_string_legacy(const Store & store) const
+{
+    return store.printStorePath(drvPath)
+        + '!'
+        + outputs.to_string();
+}
+
+std::string DerivedPath::to_string(const Store & store) const
 {
     return std::visit(overloaded {
-        [&](const DerivedPath::Built & req) { return req.to_string(store, separator); },
+        [&](const DerivedPath::Built & req) { return req.to_string(store); },
+        [&](const DerivedPath::Opaque & req) { return req.to_string(store); },
+    }, this->raw());
+}
+
+std::string DerivedPath::to_string_legacy(const Store & store) const
+{
+    return std::visit(overloaded {
+        [&](const DerivedPath::Built & req) { return req.to_string_legacy(store); },
         [&](const DerivedPath::Opaque & req) { return req.to_string(store); },
     }, this->raw());
 }
@@ -88,14 +103,24 @@ DerivedPath::Built DerivedPath::Built::parse(const Store & store, std::string_vi
     };
 }
 
-DerivedPath DerivedPath::parse(const Store & store, std::string_view s)
+static inline DerivedPath parseWith(const Store & store, std::string_view s, std::string_view separator)
 {
-    size_t n = s.find("!");
+    size_t n = s.find(separator);
     return n == s.npos
         ? (DerivedPath) DerivedPath::Opaque::parse(store, s)
         : (DerivedPath) DerivedPath::Built::parse(store, s.substr(0, n), s.substr(n + 1));
 }
 
+DerivedPath DerivedPath::parse(const Store & store, std::string_view s)
+{
+	return parseWith(store, s, "^");
+}
+
+DerivedPath DerivedPath::parseLegacy(const Store & store, std::string_view s)
+{
+	return parseWith(store, s, "!");
+}
+
 RealisedPath::Set BuiltPath::toRealisedPaths(Store & store) const
 {
     RealisedPath::Set res;

src/libstore/derived-path.hh

@@ -48,8 +48,18 @@ struct DerivedPathBuilt {
     StorePath drvPath;
     OutputsSpec outputs;
 
-    std::string to_string(const Store & store, char separator = '!') const;
-    static DerivedPathBuilt parse(const Store & store, std::string_view, std::string_view);
+    /**
+     * Uses `^` as the separator
+     */
+    std::string to_string(const Store & store) const;
+    /**
+     * Uses `!` as the separator
+     */
+    std::string to_string_legacy(const Store & store) const;
+    /**
+     * The caller splits on the separator, so it works for both variants.
+     */
+    static DerivedPathBuilt parse(const Store & store, std::string_view drvPath, std::string_view outputs);
     nlohmann::json toJSON(ref<Store> store) const;
 
     GENERATE_CMP(DerivedPathBuilt, me->drvPath, me->outputs);
@@ -81,8 +91,22 @@ struct DerivedPath : _DerivedPathRaw {
         return static_cast<const Raw &>(*this);
     }
 
-    std::string to_string(const Store & store, char separator = '!') const;
+    /**
+     * Uses `^` as the separator
+     */
+    std::string to_string(const Store & store) const;
+    /**
+     * Uses `!` as the separator
+     */
+    std::string to_string_legacy(const Store & store) const;
+    /**
+     * Uses `^` as the separator
+     */
     static DerivedPath parse(const Store & store, std::string_view);
+    /**
+     * Uses `!` as the separator
+     */
+    static DerivedPath parseLegacy(const Store & store, std::string_view);
 };
 
 /**

src/libstore/remote-store.cc

@@ -90,12 +90,12 @@ void write(const Store & store, Sink & out, const ContentAddress & ca)
 DerivedPath read(const Store & store, Source & from, Phantom<DerivedPath> _)
 {
     auto s = readString(from);
-    return DerivedPath::parse(store, s);
+    return DerivedPath::parseLegacy(store, s);
 }
 
 void write(const Store & store, Sink & out, const DerivedPath & req)
 {
-    out << req.to_string(store);
+    out << req.to_string_legacy(store);
 }
 
 

src/libstore/tests/derived-path.cc

@@ -51,6 +51,14 @@ TEST_F(DerivedPathTest, force_init)
 {
 }
 
+RC_GTEST_FIXTURE_PROP(
+    DerivedPathTest,
+    prop_legacy_round_rip,
+    (const DerivedPath & o))
+{
+    RC_ASSERT(o == DerivedPath::parseLegacy(*store, o.to_string_legacy(*store)));
+}
+
 RC_GTEST_FIXTURE_PROP(
     DerivedPathTest,
     prop_round_rip,