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.

2 files changed, 22 insertions, 5 deletions

diff --git a/src/libstore/legacy-ssh-store.cc b/src/libstore/legacy-ssh-store.cc
index 45eed5707..5d006f6e2 100644
--- a/src/libstore/legacy-ssh-store.cc
+++ b/src/libstore/legacy-ssh-store.cc
@@ -82,9 +82,18 @@ struct LegacySSHStore : public virtual LegacySSHStoreConfig, public virtual Stor
             conn->to << SERVE_MAGIC_1 << SERVE_PROTOCOL_VERSION;
             conn->to.flush();
 
-            unsigned int magic = readInt(conn->from);
-            if (magic != SERVE_MAGIC_2)
-                throw Error("protocol mismatch with 'nix-store --serve' on '%s'", host);
+            StringSink saved;
+            try {
+                TeeSource tee(conn->from, saved);
+                unsigned int magic = readInt(tee);
+                if (magic != SERVE_MAGIC_2)
+                    throw Error("'nix-store --serve' protocol mismatch from '%s'", host);
+            } catch (SerialisationError & e) {
+                conn->sshConn->in.close();
+                auto msg = conn->from.drain();
+                throw Error("'nix-store --serve' protocol mismatch from '%s', got '%s'",
+                    host, chomp(*saved.s + msg));
+            }
             conn->remoteVersion = readInt(conn->from);
             if (GET_PROTOCOL_MAJOR(conn->remoteVersion) != 0x200)
                 throw Error("unsupported 'nix-store --serve' protocol version on '%s'", host);
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))