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.

3 files changed, 62 insertions, 57 deletions

diff --git a/src/download-via-ssh/download-via-ssh.cc b/src/download-via-ssh/download-via-ssh.cc
index 6361e71e9..6cbcd9891 100644
--- a/src/download-via-ssh/download-via-ssh.cc
+++ b/src/download-via-ssh/download-via-ssh.cc
@@ -58,7 +58,7 @@ static std::pair<FdSink, FdSource> connect(const string & conn)
 
 static void substitute(std::pair<FdSink, FdSource> & pipes, Path storePath, Path destPath)
 {
-    writeInt(cmdSubstitute, pipes.first);
+    writeInt(cmdDumpStorePath, pipes.first);
     writeString(storePath, pipes.first);
     pipes.first.flush();
     restorePath(destPath, pipes.second);
@@ -68,20 +68,20 @@ static void substitute(std::pair<FdSink, FdSource> & pipes, Path storePath, Path
 
 static void query(std::pair<FdSink, FdSource> & pipes)
 {
-    writeInt(cmdQuery, pipes.first);
     for (string line; getline(std::cin, line);) {
         Strings tokenized = tokenizeString<Strings>(line);
         string cmd = tokenized.front();
         tokenized.pop_front();
         if (cmd == "have") {
-            writeInt(qCmdHave, pipes.first);
+            writeInt(cmdQueryValidPaths, pipes.first);
+            writeInt(0, pipes.first); // don't lock
             writeStrings(tokenized, pipes.first);
             pipes.first.flush();
             PathSet paths = readStrings<PathSet>(pipes.second);
             foreach (PathSet::iterator, i, paths)
                 std::cout << *i << std::endl;
         } else if (cmd == "info") {
-            writeInt(qCmdInfo, pipes.first);
+            writeInt(cmdQueryPathInfos, pipes.first);
             writeStrings(tokenized, pipes.first);
             pipes.first.flush();
             while (1) {
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index 5bcb82f32..849cb7e8a 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -869,8 +869,12 @@ static void opClearFailedPaths(Strings opFlags, Strings opArgs)
 /* Serve the nix store in a way usable by a restricted ssh user. */
 static void opServe(Strings opFlags, Strings opArgs)
 {
-    if (!opArgs.empty() || !opFlags.empty())
-        throw UsageError("no arguments or flags expected");
+    bool writeAllowed = false;
+    foreach (Strings::iterator, i, opFlags)
+        if (*i == "--write") writeAllowed = true;
+        else throw UsageError(format("unknown flag `%1%'") % *i);
+
+    if (!opArgs.empty()) throw UsageError("no arguments expected");
 
     FdSource in(STDIN_FILENO);
     FdSink out(STDOUT_FILENO);
@@ -883,50 +887,56 @@ static void opServe(Strings opFlags, Strings opArgs)
     out.flush();
     readInt(in); // Client version, unused for now
 
-    ServeCommand cmd = (ServeCommand) readInt(in);
-    switch (cmd) {
-        case cmdQuery:
-            while (true) {
-                QueryCommand qCmd;
-                try {
-                    qCmd = (QueryCommand) readInt(in);
-                } catch (EndOfFile & e) {
-                    break;
-                }
-                switch (qCmd) {
-                    case qCmdHave: {
-                        PathSet paths = readStorePaths<PathSet>(in);
-                        writeStrings(store->queryValidPaths(paths), out);
-                        break;
-                    }
-                    case qCmdInfo: {
-                        PathSet paths = readStorePaths<PathSet>(in);
-                        // !!! Maybe we want a queryPathInfos?
-                        foreach (PathSet::iterator, i, paths) {
-                            if (!store->isValidPath(*i))
-                                continue;
-                            ValidPathInfo info = store->queryPathInfo(*i);
-                            writeString(info.path, out);
-                            writeString(info.deriver, out);
-                            writeStrings(info.references, out);
-                            // !!! Maybe we want compression?
-                            writeLongLong(info.narSize, out); // downloadSize
-                            writeLongLong(info.narSize, out);
-                        }
-                        writeString("", out);
-                        break;
-                    }
-                    default:
-                        throw Error(format("unknown serve query `%1%'") % cmd);
+    while (true) {
+        ServeCommand cmd;
+        try {
+            cmd = (ServeCommand) readInt(in);
+        } catch (EndOfFile & e) {
+            break;
+        }
+
+        switch (cmd) {
+            case cmdQueryValidPaths: {
+                bool lock = readInt(in);
+                PathSet paths = readStorePaths<PathSet>(in);
+                if (lock && writeAllowed)
+                    for (auto & path : paths)
+                        store->addTempRoot(path);
+                writeStrings(store->queryValidPaths(paths), out);
+                out.flush();
+                break;
+            }
+            case cmdQueryPathInfos: {
+                PathSet paths = readStorePaths<PathSet>(in);
+                // !!! Maybe we want a queryPathInfos?
+                foreach (PathSet::iterator, i, paths) {
+                    if (!store->isValidPath(*i))
+                        continue;
+                    ValidPathInfo info = store->queryPathInfo(*i);
+                    writeString(info.path, out);
+                    writeString(info.deriver, out);
+                    writeStrings(info.references, out);
+                    // !!! Maybe we want compression?
+                    writeLongLong(info.narSize, out); // downloadSize
+                    writeLongLong(info.narSize, out);
                 }
+                writeString("", out);
                 out.flush();
+                break;
             }
-            break;
-        case cmdSubstitute:
-            dumpPath(readStorePath(in), out);
-            break;
-        default:
-            throw Error(format("unknown serve command `%1%'") % cmd);
+            case cmdDumpStorePath:
+                dumpPath(readStorePath(in), out);
+                out.flush();
+                break;
+            case cmdImportPaths:
+                if (!writeAllowed) throw Error("importing paths not allowed");
+                store->importPaths(false, in);
+                writeInt(1, out); // indicate success
+                out.flush();
+                break;
+            default:
+                throw Error(format("unknown serve command %1%") % cmd);
+        }
     }
 }
 
diff --git a/src/nix-store/serve-protocol.hh b/src/nix-store/serve-protocol.hh
index 69277bc1b..07ff4f7a7 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;
-
 }