Make regular copyPaths only copy again

The is new function parameter so just the build hook can opt into the
remote-side building.

src/build-remote/build-remote.cc

@@ -270,9 +270,11 @@ connected:
 
         auto substitute = settings.buildersUseSubstitutes ? Substitute : NoSubstitute;
 
+        auto copyStorePathImpl = sshStore2->isTrusting ? copyStorePathAdapter : copyOrBuildStorePath;
+
         {
             Activity act(*logger, lvlTalkative, actUnknown, fmt("copying dependencies to '%s'", storeUri));
-            copyPaths(store, sshStore2, store->parseStorePathSet(inputs), NoRepair, NoCheckSigs, substitute);
+            copyPaths(store, sshStore2, store->parseStorePathSet(inputs), NoRepair, NoCheckSigs, substitute, copyStorePathImpl);
         }
 
         uploadLock = -1;
@@ -285,7 +287,7 @@ connected:
             if (!result.success())
                 throw Error("build of '%s' on '%s' failed: %s", store->printStorePath(*drvPath), storeUri, result.errorMsg);
         } else {
-            copyPaths(store, sshStore2, {*drvPath}, NoRepair, NoCheckSigs, substitute);
+            copyPaths(store, sshStore2, {*drvPath}, NoRepair, NoCheckSigs, substitute, copyStorePathImpl);
             sshStore2->buildPaths({{*drvPath}});
         }
 
@@ -298,6 +300,7 @@ connected:
             Activity act(*logger, lvlTalkative, actUnknown, fmt("copying outputs from '%s'", storeUri));
             for (auto & i : missing)
                 store->locksHeld.insert(store->printStorePath(i)); /* FIXME: ugly */
+            /* No `copyStorePathImpl` because we always trust ourselves. */
             copyPaths(sshStore2, store, missing, NoRepair, NoCheckSigs, NoSubstitute);
         }
 

src/libstore/store-api.cc

@@ -819,8 +819,40 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
 }
 
 
+void copyStorePathAdapter(ref<Store> srcStore, ref<Store> dstStore,
+    const ValidPathInfo & info, RepairFlag repair, CheckSigsFlag checkSigs)
+{
+    copyStorePath(srcStore, dstStore, info.path, repair, checkSigs);
+}
+
+void copyOrBuildStorePath(ref<Store> srcStore, ref<Store> dstStore,
+    const ValidPathInfo & info, RepairFlag repair, CheckSigsFlag checkSigs)
+{
+    auto storePath = info.path;
+    if (dstStore->isTrusting || info.ca) {
+        copyStorePath(srcStore, dstStore, storePath, repair, checkSigs);
+    } else if (info.deriver && dstStore->storeDir == srcStore->storeDir) {
+        auto drvPath = *info.deriver;
+        auto outputMap = srcStore->queryDerivationOutputMap(drvPath);
+        auto p = std::find_if(outputMap.begin(), outputMap.end(), [&](auto & i) {
+            return i.second == storePath;
+        });
+        // drv file is always CA
+        srcStore->ensurePath(drvPath);
+        copyStorePath(srcStore, dstStore, drvPath, repair, checkSigs);
+        dstStore->buildPaths({{
+            drvPath,
+            p != outputMap.end() ? StringSet { p->first } : StringSet {},
+        }});
+    } else {
+        dstStore->ensurePath(storePath);
+    }
+}
+
+
 std::map<StorePath, StorePath> copyPaths(ref<Store> srcStore, ref<Store> dstStore, const StorePathSet & storePaths,
-    RepairFlag repair, CheckSigsFlag checkSigs, SubstituteFlag substitute)
+    RepairFlag repair, CheckSigsFlag checkSigs, SubstituteFlag substitute,
+    std::function<void(ref<Store>, ref<Store>, const ValidPathInfo &, RepairFlag, CheckSigsFlag)> copyStorePathImpl)
 {
     auto valid = dstStore->queryValidPaths(storePaths, substitute);
 
@@ -893,24 +925,7 @@ std::map<StorePath, StorePath> copyPaths(ref<Store> srcStore, ref<Store> dstStor
                 MaintainCount<decltype(nrRunning)> mc(nrRunning);
                 showProgress();
                 try {
-                    if (dstStore->isTrusting || info->ca) {
+                    copyStorePathImpl(srcStore, dstStore, *info, repair, checkSigs);
-                        copyStorePath(srcStore, dstStore, storePath, repair, checkSigs);
-                    } else if (info->deriver && dstStore->storeDir == srcStore->storeDir) {
-                        auto drvPath = *info->deriver;
-                        auto outputMap = srcStore->queryDerivationOutputMap(drvPath);
-                        auto p = std::find_if(outputMap.begin(), outputMap.end(), [&](auto & i) {
-                            return i.second == storePath;
-                        });
-                        // drv file is always CA
-                        srcStore->ensurePath(drvPath);
-                        copyStorePath(srcStore, dstStore, drvPath, repair, checkSigs);
-                        dstStore->buildPaths({{
-                            drvPath,
-                            p != outputMap.end() ? StringSet { p->first } : StringSet {},
-                        }});
-                    } else {
-                        dstStore->ensurePath(storePath);
-                    }
                 } catch (Error &e) {
                     nrFailed++;
                     if (!settings.keepGoing)

src/libstore/store-api.hh

@@ -738,17 +738,33 @@ void copyStorePath(ref<Store> srcStore, ref<Store> dstStore,
     const StorePath & storePath, RepairFlag repair = NoRepair, CheckSigsFlag checkSigs = CheckSigs);
 
 
+/* copyStorePath wrapped to be used with `copyPaths`. */
+void copyStorePathAdapter(ref<Store> srcStore, ref<Store> dstStore,
+    const ValidPathInfo & info, RepairFlag repair = NoRepair, CheckSigsFlag checkSigs = CheckSigs);
+
+/* The more liberal alternative to `copyStorePathAdapter`, useful for remote
+   stores that do not trust us. */
+void copyOrBuildStorePath(ref<Store> srcStore, ref<Store> dstStore,
+    const ValidPathInfo & info, RepairFlag repair = NoRepair, CheckSigsFlag checkSigs = CheckSigs);
+
 /* Copy store paths from one store to another. The paths may be copied
    in parallel. They are copied in a topologically sorted order (i.e.
    if A is a reference of B, then A is copied before B), but the set
    of store paths is not automatically closed; use copyClosure() for
    that. Returns a map of what each path was copied to the dstStore
-   as. */
+   as.
+
+   The `copyStorePathImpl` parameter allows doing something other than just
+   copying. For example, this is used with the build hook to allow the other
+   side to build dependencies we don't have permission to copy. This behavior
+   isn't just the default that way `nix copy` etc. still can be relied upon to
+   not build anything. */
 std::map<StorePath, StorePath> copyPaths(ref<Store> srcStore, ref<Store> dstStore,
     const StorePathSet & storePaths,
     RepairFlag repair = NoRepair,
     CheckSigsFlag checkSigs = CheckSigs,
-    SubstituteFlag substitute = NoSubstitute);
+    SubstituteFlag substitute = NoSubstitute,
+    std::function<void(ref<Store>, ref<Store>, const ValidPathInfo &, RepairFlag, CheckSigsFlag)> copyStorePathImpl = copyStorePathAdapter);
 
 
 /* Copy the closure of the specified paths from one store to another. */