nix run: Use packages/legacyPackages as fallback if there is no app definition

'nix run' will try to run $out/bin/<name>, where <name> is the
derivation name (excluding the version). This often works well:

  $ nix run nixpkgs#hello
  Hello, world!

  $ nix run nix -- --version
  nix (Nix) 2.4pre20200626_adf2fbb

  $ nix run patchelf -- --version
  patchelf 0.11.20200623.e61654b

  $ nix run nixpkgs#firefox -- --version
  Mozilla Firefox 77.0.1

  $ nix run nixpkgs#gimp -- --version
  GNU Image Manipulation Program version 2.10.14

though not always:

  $ nix run nixpkgs#git
  error: unable to execute '/nix/store/kp7wp760l4gryq9s36x481b2x4rfklcy-git-2.25.4/bin/git-minimal': No such file or directory

1 files changed, 32 insertions, 14 deletions

diff --git a/src/nix/app.cc b/src/nix/app.cc
index 749dea02b..3935297cf 100644
--- a/src/nix/app.cc
+++ b/src/nix/app.cc
@@ -1,28 +1,46 @@
 #include "installables.hh"
 #include "store-api.hh"
 #include "eval-inline.hh"
+#include "eval-cache.hh"
+#include "names.hh"
 
 namespace nix {
 
-App::App(EvalState & state, Value & vApp)
+App Installable::toApp(EvalState & state)
 {
-    state.forceAttrs(vApp);
+    auto [cursor, attrPath] = getCursor(state, true);
 
-    auto aType = vApp.attrs->need(state.sType);
-    if (state.forceStringNoCtx(*aType.value, *aType.pos) != "app")
-        throw Error("value does not have type 'app', at %s", *aType.pos);
+    auto type = cursor->getAttr("type")->getString();
 
-    auto aProgram = vApp.attrs->need(state.symbols.create("program"));
-    program = state.forceString(*aProgram.value, context, *aProgram.pos);
+    if (type == "app") {
+        auto [program, context] = cursor->getAttr("program")->getStringWithContext();
 
-    // FIXME: check that 'program' is in the closure of 'context'.
-    if (!state.store->isInStore(program))
-        throw Error("app program '%s' is not in the Nix store", program);
-}
+        if (!state.store->isInStore(program))
+            throw Error("app program '%s' is not in the Nix store", program);
 
-App Installable::toApp(EvalState & state)
-{
-    return App(state, *toValue(state).first);
+        std::vector<StorePathWithOutputs> context2;
+        for (auto & [path, name] : context)
+            context2.push_back({state.store->parseStorePath(path), {name}});
+
+        return App {
+            .context = std::move(context2),
+            .program = program,
+        };
+    }
+
+    else if (type == "derivation") {
+        auto drvPath = cursor->forceDerivation();
+        auto outPath = cursor->getAttr(state.sOutPath)->getString();
+        auto outputName = cursor->getAttr(state.sOutputName)->getString();
+        auto name = cursor->getAttr(state.sName)->getString();
+        return App {
+            .context = { { drvPath, {outputName} } },
+            .program = outPath + "/bin/" + DrvName(name).name,
+        };
+    }
+
+    else
+        throw Error("attribute '%s' has unsupported type '%s'", attrPath, type);
 }
 
 }