Make adding paths via nix-store --serve run in constant memory

It adds a new operation, cmdAddToStoreNar, that does the same thing as
the corresponding nix-daemon operation, i.e. call addToStore(). This
replaces cmdImportPaths, which has the major issue that it sends the
NAR first and the store path second, thus requiring us to store the
incoming NAR either in memory or on disk until we decide what to do
with it.

For example, this reduces the memory usage of

  $ nix copy --to 'ssh://localhost?remote-store=/tmp/nix' /nix/store/95cwv4q54dc6giaqv6q6p4r02ia2km35-blender-2.79

from 267 MiB to 12 MiB.

Probably fixes #1988.

src/libstore/legacy-ssh-store.cc

@@ -28,6 +28,7 @@ struct LegacySSHStore : public Store
         FdSink to;
         FdSource from;
         int remoteVersion;
+        bool good = true;
     };
 
     std::string host;
@@ -42,7 +43,7 @@ struct LegacySSHStore : public Store
         , connections(make_ref<Pool<Connection>>(
             std::max(1, (int) maxConnections),
             [this]() { return openConnection(); },
-            [](const ref<Connection> & r) { return true; }
+            [](const ref<Connection> & r) { return r->good; }
             ))
         , master(
             host,
@@ -58,7 +59,7 @@ struct LegacySSHStore : public Store
     {
         auto conn = make_ref<Connection>();
         conn->sshConn = master.startCommand(
-            fmt("%s --serve --write", remoteProgram)
+            fmt("command time %s --serve --write", remoteProgram)
             + (remoteStore.get() == "" ? "" : " --store " + shellEscape(remoteStore.get())));
         conn->to = FdSink(conn->sshConn->in.get());
         conn->from = FdSource(conn->sshConn->out.get());
@@ -130,10 +131,38 @@ struct LegacySSHStore : public Store
 
         auto conn(connections->get());
 
+        if (GET_PROTOCOL_MINOR(conn->remoteVersion) >= 4) {
+
+            conn->to
+                << cmdAddToStoreNar
+                << info.path
+                << info.deriver
+                << info.narHash.to_string(Base16, false)
+                << info.references
+                << info.registrationTime
+                << info.narSize
+                << info.ultimate
+                << info.sigs
+                << info.ca;
+            try {
+                copyNAR(source, conn->to);
+            } catch (...) {
+                conn->good = false;
+                throw;
+            }
+            conn->to.flush();
+
+        } else {
+
             conn->to
                 << cmdImportPaths
                 << 1;
+            try {
                 copyNAR(source, conn->to);
+            } catch (...) {
+                conn->good = false;
+                throw;
+            }
             conn->to
                 << exportMagic
                 << info.path
@@ -143,6 +172,8 @@ struct LegacySSHStore : public Store
                 << 0;
             conn->to.flush();
 
+        }
+
         if (readInt(conn->from) != 1)
             throw Error("failed to add path '%s' to remote host '%s', info.path, host");
     }

src/libstore/serve-protocol.hh

@@ -5,7 +5,7 @@ namespace nix {
 #define SERVE_MAGIC_1 0x390c9deb
 #define SERVE_MAGIC_2 0x5452eecb
 
-#define SERVE_PROTOCOL_VERSION 0x204
+#define SERVE_PROTOCOL_VERSION 0x205
 #define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x) & 0x00ff)
 
@@ -18,6 +18,7 @@ typedef enum {
     cmdBuildPaths = 6,
     cmdQueryClosure = 7,
     cmdBuildDerivation = 8,
+    cmdAddToStoreNar = 9,
 } ServeCommand;
 
 }

src/nix-daemon/nix-daemon.cc

@@ -707,6 +707,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
 
         logger->startWork();
 
+        // FIXME: race if addToStore doesn't read source?
         store.cast<Store>()->addToStore(info, *source, (RepairFlag) repair,
             dontCheckSigs ? NoCheckSigs : CheckSigs, nullptr);
 

src/nix-store/nix-store.cc

@@ -924,6 +924,28 @@ static void opServe(Strings opFlags, Strings opArgs)
                 break;
             }
 
+            case cmdAddToStoreNar: {
+                if (!writeAllowed) throw Error("importing paths is not allowed");
+
+                ValidPathInfo info;
+                info.path = readStorePath(*store, in);
+                in >> info.deriver;
+                if (!info.deriver.empty())
+                    store->assertStorePath(info.deriver);
+                info.narHash = Hash(readString(in), htSHA256);
+                info.references = readStorePaths<PathSet>(*store, in);
+                in >> info.registrationTime >> info.narSize >> info.ultimate;
+                info.sigs = readStrings<StringSet>(in);
+                in >> info.ca;
+
+                // FIXME: race if addToStore doesn't read source?
+                store->addToStore(info, in, NoRepair, NoCheckSigs);
+
+                out << 1; // indicate success
+
+                break;
+            }
+
             default:
                 throw Error(format("unknown serve command %1%") % cmd);
         }