Add StoreReference::render

This will be needed for the next step.

Also allows us to write round trip tests.

tests/unit/libstore/data/store-reference/auto.txt

@@ -0,0 +1 @@
+auto

tests/unit/libstore/data/store-reference/auto_param.txt

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+auto?root=/foo/bar/baz

tests/unit/libstore/data/store-reference/local_1.txt

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+local://?root=/foo/bar/baz

tests/unit/libstore/data/store-reference/local_2.txt

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+local:///foo/bar/baz?trusted=true

tests/unit/libstore/data/store-reference/local_shorthand_1.txt

@@ -0,0 +1 @@
+local?root=/foo/bar/baz

tests/unit/libstore/data/store-reference/local_shorthand_2.txt

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+/foo/bar/baz?trusted=true

tests/unit/libstore/data/store-reference/ssh.txt

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+ssh://localhost

tests/unit/libstore/data/store-reference/unix.txt

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+unix://?max-connections=7&trusted=true

tests/unit/libstore/data/store-reference/unix_shorthand.txt

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+daemon?max-connections=7&trusted=true

tests/unit/libstore/store-reference.cc

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+#include <nlohmann/json.hpp>
+#include <gtest/gtest.h>
+
+#include "file-system.hh"
+#include "store-reference.hh"
+
+#include "tests/characterization.hh"
+#include "tests/libstore.hh"
+
+namespace nix {
+
+using nlohmann::json;
+
+class StoreReferenceTest : public CharacterizationTest, public LibStoreTest
+{
+    Path unitTestData = getUnitTestData() + "/store-reference";
+
+    Path goldenMaster(PathView testStem) const override
+    {
+        return unitTestData + "/" + testStem + ".txt";
+    }
+};
+
+#define URI_TEST_READ(STEM, OBJ)                         \
+  TEST_F(StoreReferenceTest, PathInfo_##STEM##_from_uri) \
+  {                                                      \
+    readTest(#STEM, ([&](const auto & encoded) {         \
+           StoreReference expected = OBJ;                \
+           auto got = StoreReference::parse(encoded);    \
+           ASSERT_EQ(got, expected);                     \
+             }));                                        \
+  }
+
+#define URI_TEST_WRITE(STEM, OBJ)                                                           \
+  TEST_F(StoreReferenceTest, PathInfo_##STEM##_to_uri)                                      \
+  {                                                                                         \
+    writeTest(                                                                              \
+        #STEM,                                                                              \
+        [&]() -> StoreReference { return OBJ; },                                            \
+        [](const auto & file) { return StoreReference::parse(readFile(file)); },            \
+        [](const auto & file, const auto & got) { return writeFile(file, got.render()); }); \
+  }
+
+#define URI_TEST(STEM, OBJ) \
+  URI_TEST_READ(STEM, OBJ)  \
+  URI_TEST_WRITE(STEM, OBJ)
+
+URI_TEST(
+    auto,
+    (StoreReference{
+        .variant = StoreReference::Auto{},
+        .params = {},
+    }))
+
+URI_TEST(
+    auto_param,
+    (StoreReference{
+        .variant = StoreReference::Auto{},
+        .params =
+            {
+                {"root", "/foo/bar/baz"},
+            },
+    }))
+
+static StoreReference localExample_1{
+    .variant =
+        StoreReference::Specified{
+            .scheme = "local",
+        },
+    .params =
+        {
+            {"root", "/foo/bar/baz"},
+        },
+};
+
+static StoreReference localExample_2{
+    .variant =
+        StoreReference::Specified{
+            .scheme = "local",
+            .authority = "/foo/bar/baz",
+        },
+    .params =
+        {
+            {"trusted", "true"},
+        },
+};
+
+URI_TEST(local_1, localExample_1)
+
+URI_TEST(local_2, localExample_2)
+
+URI_TEST_READ(local_shorthand_1, localExample_1)
+
+URI_TEST_READ(local_shorthand_2, localExample_2)
+
+static StoreReference unixExample{
+    .variant =
+        StoreReference::Specified{
+            .scheme = "unix",
+        },
+    .params =
+        {
+            {"max-connections", "7"},
+            {"trusted", "true"},
+        },
+};
+
+URI_TEST(unix, unixExample)
+
+URI_TEST_READ(unix_shorthand, unixExample)
+
+URI_TEST(
+    ssh,
+    (StoreReference{
+        .variant =
+            StoreReference::Specified{
+                .scheme = "ssh",
+                .authority = "localhost",
+            },
+        .params = {},
+    }))
+
+}