nix-copy-closure: Fix race condition

There is a long-standing race condition when copying a closure to a
remote machine, particularly affecting build-remote.pl: the client
first asks the remote machine which paths it already has, then copies
over the missing paths. If the garbage collector kicks in on the
remote machine between the first and second step, the already-present
paths may be deleted. The missing paths may then refer to deleted
paths, causing nix-copy-closure to fail. The client now performs both
steps using a single remote Nix call (using ‘nix-store --serve’),
locking all paths in the closure while querying.

I changed the --serve protocol a bit (getting rid of QueryCommand), so
this breaks the SSH substituter from older versions. But it was marked
experimental anyway.

Fixes #141.

1 files changed, 5 insertions, 10 deletions

diff --git a/src/nix-store/serve-protocol.hh b/src/nix-store/serve-protocol.hh
index 69277bc1b99b..07ff4f7a7cc4 100644
--- a/src/nix-store/serve-protocol.hh
+++ b/src/nix-store/serve-protocol.hh
@@ -2,23 +2,18 @@
 
 namespace nix {
 
-
 #define SERVE_MAGIC_1 0x390c9deb
 #define SERVE_MAGIC_2 0x5452eecb
 
-#define SERVE_PROTOCOL_VERSION 0x101
+#define SERVE_PROTOCOL_VERSION 0x200
 #define GET_PROTOCOL_MAJOR(x) ((x) & 0xff00)
 #define GET_PROTOCOL_MINOR(x) ((x) & 0x00ff)
 
-
 typedef enum {
-    cmdQuery = 0,
-    cmdSubstitute = 1,
+    cmdQueryValidPaths = 1,
+    cmdQueryPathInfos = 2,
+    cmdDumpStorePath = 3,
+    cmdImportPaths = 4,
 } ServeCommand;
 
-typedef enum {
-    qCmdHave = 0,
-    qCmdInfo = 1,
-} QueryCommand;
-
 }