Add a nix store import and nix store export commands

Fix https://github.com/NixOS/nix/issues/9038

The only thing that the issue didn't precise is how we should call the
current (`nix-store --export`) format[^1]. I've settled to `binary` by lack
of something better.

[^1]: The issue mentions `--format tar`, but this is whishful thinking
for the future, the current format isn't `tar`-based at all.

doc/manual/rl-next/nix-store-import-export.md

@@ -0,0 +1,14 @@
+synopsis: Add `nix store import` and `nix store export`
+prs: #9474
+issues: #9038
+description: {
+
+Added a pair of new commands: [`nix store import`] and [`nix store export`].
+
+[`nix store export`]: @docroot@/command-ref/new-cli/nix3-store-export.md
+[`nix store import`]: @docroot@/command-ref/new-cli/nix3-store-import.md
+
+}
+
+
+

src/nix/store-export.md

@@ -0,0 +1,15 @@
+R""(
+
+# Examples
+
+* Export the closure of a given installable and re-import it in another machine
+
+  ```console
+  $ nix store export --recursive --format binary nixpkgs#hello > hello-closure.tar
+  $ ssh user@otherHost nix store import < hello-closure.tar
+  ```
+
+# Description
+
+This command generates an archive containing the serialisation of *installable*, as well as all the metadata required so that it can be imported with [`nix store import`](@docroot@/command-ref/new-cli/nix3-store-import.md).
+)""

src/nix/store-import.cc

@@ -0,0 +1,88 @@
+#include "command.hh"
+#include "store-api.hh"
+
+using namespace nix;
+
+struct MixImportExport : virtual Args
+{
+    enum struct ArchiveFormat
+    {
+        Binary,
+    };
+
+    std::optional<ArchiveFormat> format = std::nullopt;
+    MixImportExport()
+    {
+        addFlag(
+            {.longName = "format",
+             .description = R"(
+                Format of the archive.
+                The only supported format is `binary`, which corresponds to the format used by [`nix-store --export`](@docroot@/command-ref/nix-store/export.md).
+            )",
+             .labels = {"format"},
+             .handler = {[&](std::string_view arg) {
+                 if (arg == "binary")
+                     format = ArchiveFormat::Binary;
+                 else
+                     throw Error("Unknown archive format: %s", arg);
+             }}});
+    }
+
+};
+
+struct CmdStoreExport : StorePathsCommand, MixImportExport
+{
+    std::string description() override
+    {
+        return "Export the given store path(s) in a way that can be imported by `nix store import`.";
+    }
+
+    std::string doc() override
+    {
+        return
+            #include "store-export.md"
+        ;
+    }
+
+    void run(ref<Store> store, StorePaths && storePaths) override
+    {
+
+        // We don't use the format parameter now, but we still want to enforce it
+        // being specified to not block us on backwards-compatibility.
+        if (!format)
+            throw UsageError("`--format` must be specified");
+
+        StorePathSet pathsAsSet;
+        pathsAsSet.insert(storePaths.begin(), storePaths.end());
+        FdSink sink(STDOUT_FILENO);
+        store->exportPaths(pathsAsSet, sink);
+        sink.flush();
+    }
+};
+
+struct CmdStoreImport : StoreCommand, MixImportExport
+{
+    std::string description() override
+    {
+        return "Import the given store path(s) from a file created by `nix store export`.";
+    }
+    std::string doc() override
+    {
+        return
+            #include "store-import.md"
+        ;
+    }
+
+    void run(ref<Store> store) override
+    {
+        FdSource source(STDIN_FILENO);
+        auto paths = store->importPaths(source, NoCheckSigs);
+
+        for (auto & path : paths)
+            logger->cout("%s", store->printStorePath(path));
+    }
+
+};
+
+static auto rStoreExport = registerCommand2<CmdStoreExport>({"store", "export"});
+static auto rStoreImport = registerCommand2<CmdStoreImport>({"store", "import"});

src/nix/store-import.md

@@ -0,0 +1,16 @@
+R""(
+
+# Examples
+
+* Import a closure that has been exported from another machine
+
+  ```console
+  $ ssh user@otherHost nix store export --recursive --format binary nixpkgs#hello > hello-closure.tar
+  $ nix store import < hello-closure.tar
+  ```
+
+# Description
+
+This command reads an archive of store paths, as produced by [`nix store export`](@docroot@/command-ref/new-cli/nix3-store-export.md), and adds it to the store.
+
+)""

tests/functional/local.mk

@@ -121,6 +121,7 @@ nix_tests = \
   flakes/show.sh \
   impure-derivations.sh \
   path-from-hash-part.sh \
+  store-import-export.sh \
   path-info.sh \
   toString-path.sh \
   read-only-store.sh \

tests/functional/store-import-export.sh

@@ -0,0 +1,24 @@
+source config.sh
+
+clearStore
+
+BUILT_STORE_PATHS=$(nix build -f ./dependencies.nix input1_drv input2_drv --no-link --print-out-paths | sort)
+
+# Make sure that we require the `--format` argument.
+expect 1 nix store export --recursive $BUILT_STORE_PATHS > "$TEST_ROOT/store-export" 2> /dev/null || \
+    fail "nix store export should require the --format argument"
+nix store export --format binary --recursive $BUILT_STORE_PATHS > "$TEST_ROOT/store-export"
+
+clearStore
+IMPORTED_STORE_PATHS=$(nix store import < "$TEST_ROOT/store-export" | sort)
+
+# Make sure that the paths we built previously are still valid.
+for BUILT_STORE_PATH in $BUILT_STORE_PATHS; do
+    nix path-info "$BUILT_STORE_PATH" || \
+        fail "path $BUILT_STORE_PATH should have been imported but isn't valid"
+done
+# Make sure that all the imported paths are valid.
+for IMPORTED_STORE_PATH in $IMPORTED_STORE_PATHS; do
+    nix path-info "$IMPORTED_STORE_PATH" ||
+        fail "path $BUILT_STORE_PATH should have been imported but isn't valid"
+done