Enable socket-only daemon operations without filesystem access

Allows the Nix daemon to serve store paths purely over Unix domain
sockets without requiring the client to have filesystem access to
the store directory. This can be useful for VM setups where the host
serves paths to the guest via socket.

Tests verify socket-only operations work for copying, substitution,
and remote building (tested on Linux), with both local and binary cache stores.

doc/manual/rl-next/socket-only-daemon.md

@@ -0,0 +1,10 @@
+---
+synopsis: "Nix daemon can serve store paths over sockets without filesystem access"
+---
+
+The Nix daemon can now serve store paths purely over Unix domain sockets without
+requiring the client to have filesystem access to the store directory. This can be
+useful for VM setups where the host serves store paths to the guest via socket,
+with the guest having no direct access to the host's `/nix/store`.
+
+This works for copying paths, substitution, and building.

src/libstore/daemon.cc

@@ -896,7 +896,7 @@ static void performOp(
         auto path = WorkerProto::Serialise<StorePath>::read(*store, rconn);
         logger->startWork();
         logger->stopWork();
-        dumpPath(store->toRealPath(path), conn.to);
+        store->narFromPath(path, conn.to);
         break;
     }
 

src/libstore/include/nix/store/uds-remote-store.hh

@@ -68,7 +68,7 @@ struct UDSRemoteStore : virtual IndirectRootStore, virtual RemoteStore
 
     void narFromPath(const StorePath & path, Sink & sink) override
     {
-        Store::narFromPath(path, sink);
+        RemoteStore::narFromPath(path, sink);
     }
 
     /**

tests/functional/meson.build

@@ -84,6 +84,8 @@ suites = [
       'referrers.sh',
       'optimise-store.sh',
       'substitute-with-invalid-ca.sh',
+      'socket-only-daemon.sh',
+      'socket-only-daemon-binary-cache.sh',
       'signing.sh',
       'hash-convert.sh',
       'hash-path.sh',

tests/functional/socket-only-daemon-binary-cache.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+
+# Run the socket-only-daemon test with a binary cache backend
+# This tests that
+# - The daemon can serve a binary cache store; not just a local store
+# - Client's store operations do not reach into the file system secretly,
+#   because the files don't exist in the places where a local store would
+#   put them. (We have NARs instead)
+
+daemon_backing_store_is_binary_cache=1
+source socket-only-daemon.sh

tests/functional/socket-only-daemon.sh

@@ -0,0 +1,149 @@
+#!/usr/bin/env bash
+
+# Test that a daemon can serve store paths purely over the socket,
+# without requiring filesystem access to the store directory.
+# This is important for VM setups where the host serves paths to
+# the guest via socket, but the store directory is not shared.
+#
+# Can be called with daemon_backing_store_is_binary_cache=1 to test with a binary cache
+# instead of a regular local store. See socket-only-daemon-binary-cache.sh.
+
+source common.sh
+
+needLocalStore "This test requires starting a separate daemon"
+
+# Create state and cache locations
+# Note: We use the same NIX_STORE_DIR (logical store path) as the test environment so paths are compatible
+remote_cache_dir="$TEST_ROOT/remote-cache-$RANDOM"
+remote_state_dir="$TEST_ROOT/remote-state-$RANDOM"
+remote_real_store_dir="$TEST_ROOT/remote-real-store-$RANDOM"
+remote_socket="$TEST_ROOT/remote-socket-initial"
+moved_socket="$TEST_ROOT/moved-socket"
+
+# Set up the remote store URI based on store type
+if [[ "${daemon_backing_store_is_binary_cache:-0}" == "1" ]]; then
+    echo "Using binary cache store"
+    mkdir -p "$remote_cache_dir"
+    remote_full_store_uri="file://$remote_cache_dir"
+else
+    echo "Using local store with different physical location"
+    mkdir -p "$remote_state_dir"
+    mkdir -p "$remote_real_store_dir"
+    remote_full_store_uri="local?store=$NIX_STORE_DIR&real=$remote_real_store_dir&state=$remote_state_dir"
+fi
+
+# Create a test derivation file
+cat > "$TEST_ROOT/test-derivation.nix" <<EOF
+with import ${config_nix};
+mkDerivation {
+    name = "socket-test-path";
+    buildCommand = "echo hello-from-remote > \$out";
+}
+EOF
+
+# Build a test path in our local store
+out=$(nix-build --no-out-link "$TEST_ROOT/test-derivation.nix")
+
+echo "Built path: $out"
+
+# Copy the path to the remote store
+nix copy --to "$remote_full_store_uri" --no-check-sigs "$out"
+
+# Verify it exists in the remote store
+if [[ "${daemon_backing_store_is_binary_cache:-0}" == "1" ]]; then
+    cache_hash=$(basename "$out" | cut -d- -f1)
+    [[ -f "$remote_cache_dir/$cache_hash.narinfo" ]] || fail "Path not in binary cache"
+else
+    [[ -f "$out" ]] || fail "Path not in remote store"
+fi
+
+# Start a daemon for the remote store
+rm -f "$remote_socket"
+NIX_DAEMON_SOCKET_PATH="$remote_socket" \
+    nix --extra-experimental-features 'nix-command' daemon --store "$remote_full_store_uri" &
+remote_daemon_pid=$!
+
+# Ensure daemon is cleaned up on exit
+cleanup_daemon() {
+    if [[ -n "${remote_daemon_pid:-}" ]]; then
+        kill "$remote_daemon_pid" 2>/dev/null || true
+        wait "$remote_daemon_pid" 2>/dev/null || true
+    fi
+}
+trap cleanup_daemon EXIT
+
+# Wait for socket to appear
+for ((i = 0; i < 60; i++)); do
+    if [[ -S "$remote_socket" ]]; then
+        daemon_started=1
+        break
+    fi
+    if ! kill -0 "$remote_daemon_pid"; then
+        fail "Remote daemon died unexpectedly"
+    fi
+    sleep 0.1
+done
+[[ -n "${daemon_started:-}" ]] || fail "Remote daemon didn't start"
+
+echo "Remote daemon started with PID $remote_daemon_pid"
+
+# Move the socket to a different location to prevent any path-based
+# assumptions from accidentally working (mildly paranoid, mildly effective;
+# ideally we'd use a namespace, but that level of complexity is not actually
+# needed)
+mv "$remote_socket" "$moved_socket"
+
+echo "Socket moved to: $moved_socket"
+
+# Clear our local store so we need to substitute
+clearStore
+
+# Try to copy the path from the daemon via the moved socket
+# NOTE: We do NOT pass the store location to the client - only the socket!
+# The daemon must be able to serve paths knowing only what's in its own configuration.
+nix copy --from "unix://$moved_socket" --no-require-sigs "$out"
+
+# Verify the content
+[[ -f "$out" ]] || fail "Output path doesn't exist"
+[[ "$(cat "$out")" == "hello-from-remote" ]] || fail "Output content is wrong"
+
+echo "Socket-only copy test PASSED"
+
+# Clear the store again to test substituters mechanism
+clearStore
+
+# First verify that --max-jobs 0 without substituters fails (test our assumption)
+if nix-build --max-jobs 0 --no-out-link "$TEST_ROOT/test-derivation.nix" 2>/dev/null; then
+    fail "Building with --max-jobs 0 should have failed without substituters"
+fi
+
+echo "Confirmed: --max-jobs 0 without substituters fails as expected"
+
+# Now test using the socket as a substituter with --max-jobs 0 (no building allowed)
+# This ensures the substituter mechanism works, not just nix copy
+nix-build --max-jobs 0 \
+    --option substituters "unix://$moved_socket" \
+    --option require-sigs false \
+    --no-out-link \
+    "$TEST_ROOT/test-derivation.nix"
+
+echo "Socket-only substituter test PASSED"
+
+# Test builders mechanism (only on Linux with daemon backed by local store)
+# - Builders need sandboxing with namespace support to mount the correct store path
+# - Binary cache stores can't build, only serve files
+if [[ $(uname) == Linux && "${daemon_backing_store_is_binary_cache:-0}" == "0" ]]; then
+    # Clear the store again to test builders mechanism
+    clearStore
+
+    # Test using the socket as a remote builder
+    # This ensures the builders mechanism can also use socket-only connections
+    nix-build \
+        --option builders "unix://$moved_socket" \
+        --option require-sigs false \
+        --max-jobs 0 \
+        --no-out-link \
+        "$TEST_ROOT/test-derivation.nix"
+
+    echo "Socket-only builder test PASSED"
+fi