A failed Dump or a Restore of no dump fails its write, before any warning

A Dump into a DumpDir it could not write, or a Restore of a file that is
not a dump, answered a ctl write with rc 0 and only a message, and the
Restore warned about unsaved panes before it ever looked at the file. The
hosts' dump write now fails the waiting 9P write with EIO (the late failure
a Save uses), naming the path and the reason in the log, and Restore parses
the file before its unsaved-panes refusal, failing with `not a pardes
dump`.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 9 insertions, 0 deletions

diff --git a/src/dump.zig b/src/dump.zig
index ef442ddb..5ebb3c8d 100644
--- a/src/dump.zig
+++ b/src/dump.zig
@@ -305,6 +305,15 @@ pub const Parsed = struct {
     }
 };
 
+/// Whether `bytes` are a dump a Restore could load, saying nothing: a
+/// Restore asks before it warns about unsaved text or replaces anything.
+pub fn check(gpa: std.mem.Allocator, bytes: []const u8) !void {
+    var arena: std.heap.ArenaAllocator = .init(gpa);
+    defer arena.deinit();
+    const source = try arena.allocator().dupeZ(u8, bytes);
+    try validate(try std.zon.parse.fromSliceAlloc(State, arena.allocator(), source, null, .{ .free_on_error = false }));
+}
+
 pub fn readZon(gpa: std.mem.Allocator, bytes: []const u8, label: []const u8) !Parsed {
     var parsed: Parsed = .{ .value = undefined, .arena = .init(gpa) };
     errdefer parsed.deinit();