SSHStore / LegacySSHStore: Show a better error message if the remote is "nologin"

Instead of

  error: serialised integer 7161674624452356180 is too large for type 'j'

we now get

  error: 'nix-store --serve' protocol mismatch from 'sshtest@localhost', got 'This account is currently not available.'

Fixes https://github.com/NixOS/nixpkgs/issues/37287.

1 files changed, 10 insertions, 2 deletions

diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 73f590e7b..c71eb4631 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -162,8 +162,16 @@ void RemoteStore::initConnection(Connection & conn)
     try {
         conn.to << WORKER_MAGIC_1;
         conn.to.flush();
-        unsigned int magic = readInt(conn.from);
-        if (magic != WORKER_MAGIC_2) throw Error("protocol mismatch");
+        StringSink saved;
+        try {
+            TeeSource tee(conn.from, saved);
+            unsigned int magic = readInt(tee);
+            if (magic != WORKER_MAGIC_2)
+                throw Error("protocol mismatch");
+        } catch (SerialisationError & e) {
+            auto msg = conn.from.drain();
+            throw Error("protocol mismatch, got '%s'", chomp(*saved.s + msg));
+        }
 
         conn.from >> conn.daemonVersion;
         if (GET_PROTOCOL_MAJOR(conn.daemonVersion) != GET_PROTOCOL_MAJOR(PROTOCOL_VERSION))