Factor out EvalCache::forceDerivation()

src/libexpr/eval-cache.cc

@@ -2,6 +2,7 @@
 #include "sqlite.hh"
 #include "eval.hh"
 #include "eval-inline.hh"
+#include "store-api.hh"
 
 namespace nix::eval_cache {
 
@@ -502,4 +503,19 @@ bool AttrCursor::isDerivation()
     return aType && aType->getString() == "derivation";
 }
 
+StorePath AttrCursor::forceDerivation()
+{
+    auto aDrvPath = getAttr(root->state.sDrvPath);
+    auto drvPath = root->state.store->parseStorePath(aDrvPath->getString());
+    if (!root->state.store->isValidPath(drvPath) && !settings.readOnlyMode) {
+        /* The eval cache contains 'drvPath', but the actual path has
+           been garbage-collected. So force it to be regenerated. */
+        aDrvPath->forceValue();
+        if (!root->state.store->isValidPath(drvPath))
+            throw Error("don't know how to recreate store derivation '%s'!",
+                root->state.store->printStorePath(drvPath));
+    }
+    return drvPath;
+}
+
 }

src/libexpr/eval-cache.hh

@@ -101,6 +101,9 @@ public:
     bool isDerivation();
 
     Value & forceValue();
+
+    /* Force creation of the .drv file in the Nix store. */
+    StorePath forceDerivation();
 };
 
 }

src/nix/installables.cc

@@ -444,17 +444,7 @@ std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableF
         if (!attr->isDerivation())
             throw Error("flake output attribute '%s' is not a derivation", attrPath);
 
-        auto aDrvPath = attr->getAttr(state->sDrvPath);
+        auto drvPath = attr->forceDerivation();
-        auto drvPath = state->store->parseStorePath(aDrvPath->getString());
-        if (!state->store->isValidPath(drvPath) && !settings.readOnlyMode) {
-            /* The eval cache contains 'drvPath', but the actual path
-               has been garbage-collected. So force it to be
-               regenerated. */
-            aDrvPath->forceValue();
-            if (!state->store->isValidPath(drvPath))
-                throw Error("don't know how to recreate store derivation '%s'!",
-                    state->store->printStorePath(drvPath));
-        }
 
         auto drvInfo = DerivationInfo{
             std::move(drvPath),