nix-store: make --dump-db take a list of paths to dump

Inside a derivation, exportReferencesGraph already provides a way to
dump the Nix database for a specific closure. On the command line,
--dump-db gave us the same information, but only for the entire Nix
database at once.

With this change, one can now pass a list of paths to --dump-db to get
the Nix database dumped for just those paths. (The user is responsible
for ensuring this is a closure, like for --export).

Among other things, this is useful for deploying a closure to a new
host without using --import/--export; one can use tar to transfer the
store paths, and --dump-db/--load-db to transfer the validity
information. This is useful if the new host doesn't actually have Nix
yet, and the closure that is being deployed itself contains Nix.

1 files changed, 10 insertions, 5 deletions

diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index a9ad14762..33138baff 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -485,11 +485,16 @@ static void opReadLog(Strings opFlags, Strings opArgs)
 static void opDumpDB(Strings opFlags, Strings opArgs)
 {
     if (!opFlags.empty()) throw UsageError("unknown flag");
-    if (!opArgs.empty())
-        throw UsageError("no arguments expected");
-    PathSet validPaths = store->queryAllValidPaths();
-    for (auto & i : validPaths)
-        cout << store->makeValidityRegistration({i}, true, true);
+    if (!opArgs.empty()) {
+        for (auto & i : opArgs)
+            i = store->followLinksToStorePath(i);
+        for (auto & i : opArgs)
+            cout << store->makeValidityRegistration({i}, true, true);
+    } else {
+        PathSet validPaths = store->queryAllValidPaths();
+        for (auto & i : validPaths)
+            cout << store->makeValidityRegistration({i}, true, true);
+    }
 }