Allow substituting paths when building remotely using `ssh-ng://`

Until now, it was not possible to substitute missing paths from e.g.
`https://cache.nixos.org` on a remote server when building on it using
the new `ssh-ng` protocol.

This is because every store implementation except legacy `ssh://`
ignores the substitution flag passed to `Store::queryValidPaths` while
the `legacy-ssh-store` substitutes the remote store using
`cmdQueryValidPaths` when the remote store is opened with `nix-store
--serve`.

This patch slightly modifies the daemon protocol to allow passing an
integer value suggesting whether to substitute missing paths during
`wopQueryValidPaths`. To implement this on the daemon-side, the
substitution logic from `nix-store --serve` has been moved into a
protected method named `Store::substitutePaths` which gets currently
called from `LocalStore::queryValidPaths` and `Store::queryValidPaths`
if `maybeSubstitute` is `true`.

Fixes #2770

1 files changed, 22 insertions, 0 deletions

diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index 83d3a1fa1..3129d6637 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -522,6 +522,28 @@ void Store::queryPathInfo(const StorePath & storePath,
 }
 
 
+void Store::substitutePaths(const StorePathSet & paths)
+{
+    std::vector<StorePathWithOutputs> paths2;
+    for (auto & path : paths)
+        if (!path.isDerivation())
+            paths2.push_back({path});
+    uint64_t downloadSize, narSize;
+    StorePathSet willBuild, willSubstitute, unknown;
+    queryMissing(paths2,
+        willBuild, willSubstitute, unknown, downloadSize, narSize);
+
+    if (!willSubstitute.empty())
+        try {
+            std::vector<StorePathWithOutputs> subs;
+            for (auto & p : willSubstitute) subs.push_back({p});
+            buildPaths(subs);
+        } catch (Error & e) {
+            logWarning(e.info());
+        }
+}
+
+
 StorePathSet Store::queryValidPaths(const StorePathSet & paths, SubstituteFlag maybeSubstitute)
 {
     struct State