nix copy: Add --out-link

src/libcmd/command.cc

@@ -334,4 +334,29 @@ void MixEnvironment::setEnviron()
     }
 }
 
+void createOutLinks(
+    const std::filesystem::path & outLink,
+    const BuiltPaths & buildables,
+    LocalFSStore & store)
+{
+    for (const auto & [_i, buildable] : enumerate(buildables)) {
+        auto i = _i;
+        std::visit(overloaded {
+            [&](const BuiltPath::Opaque & bo) {
+                auto symlink = outLink;
+                if (i) symlink += fmt("-%d", i);
+                store.addPermRoot(bo.path, absPath(symlink.string()));
+            },
+            [&](const BuiltPath::Built & bfd) {
+                for (auto & output : bfd.outputs) {
+                    auto symlink = outLink;
+                    if (i) symlink += fmt("-%d", i);
+                    if (output.first != "out") symlink += fmt("-%s", output.first);
+                    store.addPermRoot(output.second, absPath(symlink.string()));
+                }
+            },
+        }, buildable.raw());
+    }
+}
+
 }

src/libcmd/command.hh

@@ -18,6 +18,7 @@ extern char * * savedArgv;
 class EvalState;
 struct Pos;
 class Store;
+class LocalFSStore;
 
 static constexpr Command::Category catHelp = -1;
 static constexpr Command::Category catSecondary = 100;
@@ -367,4 +368,13 @@ void printClosureDiff(
     const StorePath & afterPath,
     std::string_view indent);
 
+/**
+ * Create symlinks prefixed by `outLink` to the store paths in
+ * `buildables`.
+ */
+void createOutLinks(
+    const std::filesystem::path & outLink,
+    const BuiltPaths & buildables,
+    LocalFSStore & store);
+
 }

src/libcmd/installables.cc

@@ -917,4 +917,12 @@ void BuiltPathsCommand::applyDefaultInstallables(std::vector<std::string> & rawI
         rawInstallables.push_back(".");
 }
 
+BuiltPaths toBuiltPaths(const std::vector<BuiltPathWithResult> & builtPathsWithResult)
+{
+    BuiltPaths res;
+    for (auto & i : builtPathsWithResult)
+        res.push_back(i.path);
+    return res;
+}
+
 }

src/libcmd/installables.hh

@@ -86,6 +86,8 @@ struct BuiltPathWithResult
     std::optional<BuildResult> result;
 };
 
+BuiltPaths toBuiltPaths(const std::vector<BuiltPathWithResult> & builtPathsWithResult);
+
 /**
  * Shorthand, for less typing and helping us keep the choice of
  * collection in sync.

src/nix/build.cc

@@ -42,29 +42,6 @@ static nlohmann::json builtPathsWithResultToJSON(const std::vector<BuiltPathWith
     return res;
 }
 
-// TODO deduplicate with other code also setting such out links.
-static void createOutLinks(const std::filesystem::path& outLink, const std::vector<BuiltPathWithResult>& buildables, LocalFSStore& store2)
-{
-    for (const auto & [_i, buildable] : enumerate(buildables)) {
-        auto i = _i;
-        std::visit(overloaded {
-            [&](const BuiltPath::Opaque & bo) {
-                auto symlink = outLink;
-                if (i) symlink += fmt("-%d", i);
-                store2.addPermRoot(bo.path, absPath(symlink.string()));
-            },
-            [&](const BuiltPath::Built & bfd) {
-                for (auto & output : bfd.outputs) {
-                    auto symlink = outLink;
-                    if (i) symlink += fmt("-%d", i);
-                    if (output.first != "out") symlink += fmt("-%s", output.first);
-                    store2.addPermRoot(output.second, absPath(symlink.string()));
-                }
-            },
-        }, buildable.path.raw());
-    }
-}
-
 struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
 {
     Path outLink = "result";
@@ -140,7 +117,7 @@ struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
 
         if (outLink != "")
             if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>())
-                createOutLinks(outLink, buildables, *store2);
+                createOutLinks(outLink, toBuiltPaths(buildables), *store2);
 
         if (printOutputPaths) {
             stopProgressBar();

src/nix/copy.cc

@@ -1,18 +1,29 @@
 #include "command.hh"
 #include "shared.hh"
 #include "store-api.hh"
+#include "local-fs-store.hh"
 
 using namespace nix;
 
 struct CmdCopy : virtual CopyCommand, virtual BuiltPathsCommand, MixProfile
 {
+    std::optional<std::filesystem::path> outLink;
     CheckSigsFlag checkSigs = CheckSigs;
 
     SubstituteFlag substitute = NoSubstitute;
 
     CmdCopy()
         : BuiltPathsCommand(true)
     {
+        addFlag({
+            .longName = "out-link",
+            .shortName = 'o',
+            .description = "Create symlinks prefixed with *path* to the top-level store paths fetched from the source store.",
+            .labels = {"path"},
+            .handler = {&outLink},
+            .completer = completePath
+        });
+
         addFlag({
             .longName = "no-check-sigs",
             .description = "Do not require that paths are signed by trusted keys.",
@@ -58,6 +69,13 @@ struct CmdCopy : virtual CopyCommand, virtual BuiltPathsCommand, MixProfile
             *srcStore, *dstStore, stuffToCopy, NoRepair, checkSigs, substitute);
 
         updateProfile(rootPaths);
+
+        if (outLink) {
+            if (auto store2 = dstStore.dynamic_pointer_cast<LocalFSStore>())
+                createOutLinks(*outLink, rootPaths, *store2);
+            else
+                throw Error("'--out-link' is not supported for this Nix store");
+        }
     }
 };
 

tests/functional/zstd.sh

@@ -18,9 +18,10 @@ HASH=$(nix hash path $outPath)
 clearStore
 clearCacheCache
 
-nix copy --from $cacheURI $outPath --no-check-sigs --profile $TEST_ROOT/profile
+nix copy --from $cacheURI $outPath --no-check-sigs --profile $TEST_ROOT/profile --out-link $TEST_ROOT/result
 
 [[ -e $TEST_ROOT/profile ]]
+[[ -e $TEST_ROOT/result ]]
 
 if ls $cacheDir/nar/*.zst &> /dev/null; then
     echo "files do exist"