Mount dials its peer at once and fails if none answers; a peer out of reach is said as `dial failed: <reason>`

Mount took any well-formed dial and nothing was found out until the
first look through it, which then said `look: look: dial`. Mount now
probes the peer (version, attach, its root) with the turn out, and fails
its write, mounting nothing, when none answers; the dial errors read as
words, `no answer`, `timed out`, `hung up`, and a look through a peer
gone since names its path: `look: /n/peer/f: dial failed: no answer`.

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

1 files changed, 10 insertions, 0 deletions

diff --git a/src/9p_io.zig b/src/9p_io.zig
index bda1ad36..2130db27 100644
--- a/src/9p_io.zig
+++ b/src/9p_io.zig
@@ -1761,6 +1761,16 @@ pub const Client = struct {
         return fetchBytes(gpa, sock, names[0..n], &remote, null, display_path, @min(max_bytes, limits.max_file_bytes));
     }
 
+    /// Whether a peer answers at `dial`: a version and attach, and its root
+    /// read. Any answer from the peer, an error of its own included, is one.
+    pub fn probe(gpa: std.mem.Allocator, dial: []const u8) !void {
+        const got = readLimit(gpa, dial, "", "", 0) catch |err| switch (err) {
+            error.IsDirectory, error.NotFound, error.Remote, error.FileTooLarge => return,
+            else => return err,
+        };
+        gpa.free(got);
+    }
+
     pub fn write(gpa: std.mem.Allocator, dial: []const u8, path: []const u8, bytes: []const u8) !void {
         if (comptime !supported) return error.Unsupported;
         var names: [max_depth][]const u8 = undefined;