Add "export" to Nix

This adds a ‘nix export’ command which hooks into nix-bundle. It can
be used in a similar way as nix-bundle, with the benefit of hooking
into the new “app” functionality. For instance,

$ nix export nixpkgs#jq
$ ./jq --help
jq - commandline JSON processor [version 1.6]
...
$ scp jq machine-without-nix:
$ ssh machine-without-nix ./jq
jq - commandline JSON processor [version 1.6]
...

Note that nix-bundle currently requires Linux to run. Other exporters
might not have that requirement.

“exporters” are meant to be reusable, so that, other repos can
implement their own bundling.

Fixes #3705

src/nix/export.cc

@@ -0,0 +1,126 @@
+#include "command.hh"
+#include "common-args.hh"
+#include "shared.hh"
+#include "store-api.hh"
+#include "fs-accessor.hh"
+
+using namespace nix;
+
+struct CmdExport : InstallableCommand
+{
+    std::string exporter = "github:matthewbauer/nix-bundle";
+    Path outLink;
+
+    CmdExport()
+    {
+        addFlag({
+            .longName = "exporter",
+            .description = "use custom exporter",
+            .labels = {"flake-url"},
+            .handler = {&exporter},
+            .completer = {[&](size_t, std::string_view prefix) {
+                completeFlakeRef(getStore(), prefix);
+            }}
+        });
+
+        addFlag({
+            .longName = "out-link",
+            .shortName = 'o',
+            .description = "path of the symlink to the build result",
+            .labels = {"path"},
+            .handler = {&outLink},
+            .completer = completePath
+        });
+    }
+
+    std::string description() override
+    {
+        return "export an application out of the Nix store";
+    }
+
+    Examples examples() override
+    {
+        return {
+            Example{
+                "To export Hello:",
+                "nix export hello"
+            },
+        };
+    }
+
+    Strings getDefaultFlakeAttrPaths() override
+    {
+        Strings res{"defaultApp." + settings.thisSystem.get()};
+        for (auto & s : SourceExprCommand::getDefaultFlakeAttrPaths())
+            res.push_back(s);
+        return res;
+    }
+
+    Strings getDefaultFlakeAttrPathPrefixes() override
+    {
+        Strings res{"apps." + settings.thisSystem.get() + ".", "packages"};
+        for (auto & s : SourceExprCommand::getDefaultFlakeAttrPathPrefixes())
+            res.push_back(s);
+        return res;
+    }
+
+    void run(ref<Store> store) override
+    {
+        auto evalState = getEvalState();
+
+        auto app = installable->toApp(*evalState);
+        store->buildPaths(app.context);
+
+        auto [exporterFlakeRef, exporterName] = parseFlakeRefWithFragment(exporter, absPath("."));
+        const flake::LockFlags lockFlags{ .writeLockFile = false };
+        auto exporter = InstallableFlake(
+            evalState, std::move(exporterFlakeRef),
+            Strings{exporterName == "" ? ("defaultExporter." + settings.thisSystem.get()) : exporterName},
+            Strings({"exporters." + settings.thisSystem.get() + "."}), lockFlags);
+
+        Value * arg = evalState->allocValue();
+        evalState->mkAttrs(*arg, 1);
+
+        PathSet context;
+        for (auto & i : app.context)
+            context.insert("=" + store->printStorePath(i.path));
+        mkString(*evalState->allocAttr(*arg, evalState->symbols.create("program")), app.program, context);
+
+        auto vRes = evalState->allocValue();
+        evalState->callFunction(*exporter.toValue(*evalState).first, *arg, *vRes, noPos);
+
+        if (!evalState->isDerivation(*vRes))
+            throw Error("the exporter '%s' does not produce a derivation", exporter.what());
+
+        Bindings::iterator i = vRes->attrs->find(evalState->sDrvPath);
+        if (i == vRes->attrs->end())
+            throw Error("the exporter '%s' does not produce a derivation", exporter.what());
+
+        PathSet context2;
+        StorePath drvPath = store->parseStorePath(evalState->coerceToPath(*i->pos, *i->value, context2));
+
+        i = vRes->attrs->find(evalState->sOutPath);
+        if (i == vRes->attrs->end())
+            throw Error("the exporter '%s' does not produce a derivation", exporter.what());
+
+        StorePath outPath = store->parseStorePath(evalState->coerceToPath(*i->pos, *i->value, context2));
+
+        store->buildPaths({{drvPath}});
+
+        auto accessor = store->getFSAccessor();
+        auto outPathS = store->printStorePath(outPath);
+        if (accessor->stat(outPathS).type != FSAccessor::tRegular)
+            throw Error("'%s' is not a file; an exporter must only create a single file", outPathS);
+
+        auto info = store->queryPathInfo(outPath);
+        if (!info->references.empty())
+            throw Error("'%s' has references; an exporter must not leave any references", outPathS);
+
+        if (outLink == "")
+            outLink = baseNameOf(app.program);
+
+        store.dynamic_pointer_cast<LocalFSStore>()->addPermRoot(outPath, absPath(outLink), true);
+    }
+};
+
+static auto r2 = registerCommand<CmdExport>("export");

src/nix/flake.cc

@@ -368,6 +368,21 @@ struct CmdFlakeCheck : FlakeCommand
             }
         };
 
+        auto checkExporter = [&](const std::string & attrPath, Value & v, const Pos & pos) {
+            try {
+                state->forceValue(v, pos);
+                if (v.type != tLambda)
+                    throw Error("exporter must be a function");
+                if (!v.lambda.fun->formals ||
+                    v.lambda.fun->formals->argNames.find(state->symbols.create("program")) == v.lambda.fun->formals->argNames.end() ||
+                    v.lambda.fun->formals->argNames.find(state->symbols.create("args")) == v.lambda.fun->formals->argNames.end())
+                    throw Error("exporter must take formal arguments 'program' and 'args'");
+            } catch (Error & e) {
+                e.addTrace(pos, hintfmt("while checking the template '%s'", attrPath));
+                throw;
+            }
+        };
+
         {
             Activity act(*logger, lvlInfo, actUnknown, "evaluating flake");
 
@@ -490,6 +505,16 @@ struct CmdFlakeCheck : FlakeCommand
                                     *attr.value, *attr.pos);
                         }
 
+                        else if (name == "defaultExporter")
+                            checkExporter(name, vOutput, pos);
+
+                        else if (name == "exporters") {
+                            state->forceAttrs(vOutput, pos);
+                            for (auto & attr : *vOutput.attrs)
+                                checkExporter(fmt("%s.%s", name, attr.name),
+                                    *attr.value, *attr.pos);
+                        }
+
                         else
                             warn("unknown flake output '%s'", name);