libstore: add usage hints to repairPath on daemons

we can't change the protocol to allow daemons to do this, and we should
not try to guess what the `auto` store uri means depending on whether a
command was run by root or not due to copious side effects and not even
being able to tell whether the `auto` store uri was given explicitly or
not. while `auto` may *technically* allow this via its naming we should
resist the urge to add a hack and fix the underlying protocols instead,
especially since repairPath should be a rare, superuser only operation.

fixes #888

Change-Id: I1b53245db226199f827a89a237a2ab9907c3f766

lix/libstore/store-api.hh

@@ -27,6 +27,7 @@
 #include <atomic>
 #include <limits>
 #include <map>
+#include <optional>
 #include <type_traits>
 #include <unordered_map>
 #include <unordered_set>
@@ -927,11 +928,17 @@ protected:
      * \todo Using this should be a last resort. It is better to make
      * the method "virtual pure" and/or move it to a subclass.
      */
-    [[noreturn]] void unsupported(const std::string & op)
+    [[noreturn]]
+    void
+    unsupported(const std::string & op, const std::optional<std::string_view> info = std::nullopt)
     {
-        throw Unsupported("operation '%s' is not supported by store '%s'", op, getUri());
+        throw Unsupported(
+            "operation '%s' is not supported by store '%s'%s",
+            op,
+            getUri(),
+            Uncolored(info ? fmt(". %s", *info) : "")
+        );
     }
-
 };
 
 

lix/libstore/uds-remote-store.hh

@@ -49,6 +49,16 @@ public:
         return LocalFSStore::narFromPath(path, context);
     }
 
+    kj::Promise<Result<void>> repairPath(const StorePath & path) override
+    try {
+        unsupported(
+            "repairPath",
+            HintFmt("This command must be run as %s with %s", "root", "--store local").str()
+        );
+    } catch (...) {
+        return {result::current_exception()};
+    }
+
     /**
      * Implementation of `IndirectRootStore::addIndirectRoot()` which
      * delegates to the remote store.