nix/daemon: wrap kj stdio forwarding errors nicely

not doing this has lix crash if stdio forwarding fails for odd reasons,
such as ssh failing to set up a connection due to transient dns errors.

cf #1148

Change-Id: I543b7444236e69d3c13b47fb6c6eb37c65b96763

lix/nix/daemon.cc

@@ -34,7 +34,9 @@
 #include <cstdint>
 #include <cstring>
 
+#include <exception>
 #include <kj/async.h>
+#include <kj/exception.h>
 #include <string>
 #include <unistd.h>
 #include <signal.h>
@@ -536,13 +538,20 @@ static void forwardStdioConnection(AsyncIoRoot & aio, RemoteStore & store)
 
     aio.blockOn(connSocket->pumpTo(*asyncStdout)
                     .then([](auto) -> Result<void> {
-                        return {
-                            std::make_exception_ptr(EndOfFile("unexpected EOF from daemon socket"))
-                        };
+                        return {std::make_exception_ptr(EndOfFile("unexpected EOF from daemon socket"))};
                     })
                     .exclusiveJoin(asyncStdin->pumpTo(*connSocket).then([](auto) -> Result<void> {
                         return result::success();
-                    })));
+                    }))
+                    .catch_([](kj::Exception && e) -> Result<void> {
+                        if (e.getType() == kj::Exception::Type::DISCONNECTED) {
+                            return std::make_exception_ptr(Error("peer disconnected unexpectedly"));
+                        } else {
+                            return std::make_exception_ptr(
+                                Error("forwarding daemon connection: %s", e.getDescription().cStr())
+                            );
+                        }
+                    }));
 }
 
 /**

tests/functional2/cli/test_daemon.py

@@ -0,0 +1,26 @@
+from pathlib import Path
+from testlib.fixtures.nix import Nix
+from testlib.fixtures.file_helper import with_files
+from testlib.utils import get_global_asset_pack
+
+
+@with_files(get_global_asset_pack("simple-drv"))
+def test_stdio_forward_failure(nix: Nix, files: Path):
+    result = (
+        nix.nix_build(
+            [
+                f"{files}/simple.nix",
+                "--builders",
+                # build-remote handles the first ssh-ng in-process, but we want to test how nix-daemon
+                # behaves in error cases. we need to explicitly set remote-program for the main remote
+                # connection since we may choose the wrong binary otherwise. the inner ssh-ng also has
+                # a remote-program set to cause predictable failures (rather than using ssh impurely).
+                f"ssh-ng://localhost?remote-program={nix.env.dirs.nix_bin_dir}/nix-daemon&remote-store=ssh-ng://localhost?remote-program=false",
+                "-j0",
+            ]
+        )
+        .run()
+        .expect(1)
+    )
+    assert "stream ended unexpectedly" in result.stderr_plain
+    assert "Lix crashed" not in result.stderr_plain