addMultipleToStore(): Move pathsToCopy

This allows RemoteStore::addMultipleToStore() to free the Source
objects early (and in particular the associated sinkToSource()
buffers). This should fix NixOS#7359. For example, memory consumption of

  nix copy --derivation --to ssh-ng://localhost?remote-store=/tmp/nix --derivation --no-check-sigs \
    /nix/store/4p9xmfgnvclqpii8pxqcwcvl9bxqy2xf-nixos-system-...drv

went from 353 MB to 74 MB.

src/libstore/remote-store.cc

@@ -534,21 +534,24 @@ void RemoteStore::addToStore(const ValidPathInfo & info, Source & source,
 
 
 void RemoteStore::addMultipleToStore(
-    PathsSource & pathsToCopy,
+    PathsSource && pathsToCopy,
     Activity & act,
     RepairFlag repair,
     CheckSigsFlag checkSigs)
 {
     auto source = sinkToSource([&](Sink & sink) {
         sink << pathsToCopy.size();
-        for (auto & [pathInfo, pathSource] : pathsToCopy) {
+        std::reverse(pathsToCopy.begin(), pathsToCopy.end());
+        while (!pathsToCopy.empty()) {
+            auto & [pathInfo, pathSource] = pathsToCopy.back();
             WorkerProto::Serialise<ValidPathInfo>::write(*this,
                  WorkerProto::WriteConn {
                      .to = sink,
                      .version = 16,
                  },
                  pathInfo);
             pathSource->drainInto(sink);
+            pathsToCopy.pop_back();
         }
     });
 

src/libstore/remote-store.hh

@@ -102,7 +102,7 @@ public:
         CheckSigsFlag checkSigs) override;
 
     void addMultipleToStore(
-        PathsSource & pathsToCopy,
+        PathsSource && pathsToCopy,
         Activity & act,
         RepairFlag repair,
         CheckSigsFlag checkSigs) override;

src/libstore/store-api.cc

@@ -223,7 +223,7 @@ StorePath Store::addToStore(
 }
 
 void Store::addMultipleToStore(
-    PathsSource & pathsToCopy,
+    PathsSource && pathsToCopy,
     Activity & act,
     RepairFlag repair,
     CheckSigsFlag checkSigs)
@@ -1138,7 +1138,7 @@ std::map<StorePath, StorePath> copyPaths(
         pathsToCopy.push_back(std::pair{infoForDst, std::move(source)});
     }
 
-    dstStore.addMultipleToStore(pathsToCopy, act, repair, checkSigs);
+    dstStore.addMultipleToStore(std::move(pathsToCopy), act, repair, checkSigs);
 
     return pathsMap;
 }

src/libstore/store-api.hh

@@ -425,7 +425,7 @@ public:
         CheckSigsFlag checkSigs = CheckSigs);
 
     virtual void addMultipleToStore(
-        PathsSource & pathsToCopy,
+        PathsSource && pathsToCopy,
         Activity & act,
         RepairFlag repair = NoRepair,
         CheckSigsFlag checkSigs = CheckSigs);