Create nix derivation add command

Also refine `nix derivation show`'s docs very slightly.

src/nix/derivation-add.cc

@@ -0,0 +1,45 @@
+// FIXME: rename to 'nix plan add' or 'nix derivation add'?
+
+#include "command.hh"
+#include "common-args.hh"
+#include "store-api.hh"
+#include "archive.hh"
+#include "derivations.hh"
+#include <nlohmann/json.hpp>
+
+using namespace nix;
+using json = nlohmann::json;
+
+struct CmdAddDerivation : MixDryRun, StoreCommand
+{
+    std::string description() override
+    {
+        return "Add a store derivation";
+    }
+
+    std::string doc() override
+    {
+        return
+          #include "derivation-add.md"
+          ;
+    }
+
+    Category category() override { return catUtility; }
+
+    void run(ref<Store> store) override
+    {
+        auto json = nlohmann::json::parse(drainFD(STDIN_FILENO));
+
+        auto drv = Derivation::fromJSON(*store, json);
+
+        auto drvPath = writeDerivation(*store, drv, NoRepair, /* read only */ dryRun);
+
+        drv.checkInvariants(*store, drvPath);
+
+        writeDerivation(*store, drv, NoRepair, dryRun);
+
+        logger->cout("%s", store->printStorePath(drvPath));
+    }
+};
+
+static auto rCmdAddDerivation = registerCommand2<CmdAddDerivation>({"derivation", "add"});

src/nix/derivation-add.md

@@ -0,0 +1,18 @@
+R""(
+
+# Description
+
+This command reads from standard input a JSON representation of a
+[store derivation] to which an [*installable*](./nix.md#installables) evaluates.
+
+Store derivations are used internally by Nix. They are store paths with
+extension `.drv` that represent the build-time dependency graph to which
+a Nix expression evaluates.
+
+[store derivation]: ../../glossary.md#gloss-store-derivation
+
+The JSON format is documented under the [`derivation show`] command.
+
+[`derivation show`]: ./nix3-derivation-show.md
+
+)""

src/nix/derivation-show.md

@@ -39,10 +39,11 @@ R""(
 # Description
 
 This command prints on standard output a JSON representation of the
-[store derivation]s to which [*installables*](./nix.md#installables) evaluate. Store derivations
-are used internally by Nix. They are store paths with extension `.drv`
-that represent the build-time dependency graph to which a Nix
-expression evaluates.
+[store derivation]s to which [*installables*](./nix.md#installables) evaluate.
+
+Store derivations are used internally by Nix. They are store paths with
+extension `.drv` that represent the build-time dependency graph to which
+a Nix expression evaluates.
 
 By default, this command only shows top-level derivations, but with
 `--recursive`, it also shows their dependencies.

tests/ca/derivation-json.sh

@@ -0,0 +1,26 @@
+source common.sh
+
+export NIX_TESTS_CA_BY_DEFAULT=1
+
+drvPath=$(nix-instantiate ../simple.nix)
+
+nix derivation show $drvPath | jq .[] > $TEST_HOME/simple.json
+
+drvPath2=$(nix derivation add < $TEST_HOME/simple.json)
+
+[[ "$drvPath" = "$drvPath2" ]]
+
+# Content-addressed derivations can be renamed.
+jq '.name = "foo"' < $TEST_HOME/simple.json > $TEST_HOME/foo.json
+drvPath3=$(nix derivation add --dry-run < $TEST_HOME/foo.json)
+# With --dry-run nothing is actually written
+[[ ! -e "$drvPath3" ]]
+
+# Without --dry-run it is actually written
+drvPath4=$(nix derivation add < $TEST_HOME/foo.json)
+[[ "$drvPath4" = "$drvPath3" ]]
+[[ -e "$drvPath3" ]]
+
+# The modified derivation read back as JSON matches
+nix derivation show $drvPath3 | jq .[] > $TEST_HOME/foo-read.json
+diff $TEST_HOME/foo.json $TEST_HOME/foo-read.json

tests/derivation-json.sh

@@ -0,0 +1,12 @@
+source common.sh
+
+drvPath=$(nix-instantiate simple.nix)
+
+nix derivation show $drvPath | jq .[] > $TEST_HOME/simple.json
+
+drvPath2=$(nix derivation add < $TEST_HOME/simple.json)
+
+[[ "$drvPath" = "$drvPath2" ]]
+
+# Input addressed derivations cannot be renamed.
+jq '.name = "foo"' < $TEST_HOME/simple.json | expectStderr 1 nix derivation add | grepQuiet "has incorrect output"

tests/local.mk

@@ -101,6 +101,8 @@ nix_tests = \
   eval-store.sh \
   why-depends.sh \
   ca/why-depends.sh \
+  derivation-json.sh \
+  ca/derivation-json.sh \
   import-derivation.sh \
   ca/import-derivation.sh \
   nix_path.sh \