Fix crash and session saving.

src/Snapshot.zig

@@ -23,7 +23,7 @@
 //!
 //! It lives in the *state* directory rather than beside `layouts.json` and
 //! `settings.json` in the config one. Those two are files a person writes;
-//! this one is written behind their back on every exit, and putting churn like
+//! this one is written behind their back every few seconds and on every exit, and putting churn like
 //! that in a config directory — which plenty of people keep in version control
 //! — would make every quit look like an edit. `$XDG_STATE_HOME` is where the
 //! spec puts exactly this: state that should carry across restarts but that
@@ -276,27 +276,37 @@ pub const SaveError = error{ OutOfMemory, WriteFailed };
 /// nothing about that state worth offering to restore — so the offer should not
 /// appear at the next launch, and the stale one it would otherwise still be
 /// holding should not either.
-pub fn save(self: *Snapshot) SaveError!void {
+///
+/// `written` is a hash of what the caller last had written, and is updated to
+/// this one's. A snapshot that would write the same bytes again is skipped: the
+/// window is photographed every few seconds while it is open, and nearly every
+/// one of those photographs is the same as the last.
+pub fn save(self: *Snapshot, written: *u64) SaveError!void {
     var path_buf: [max_path]u8 = undefined;
     const path = statePath(&path_buf) orelse return error.WriteFailed;
 
     if (!self.any()) {
         remove(path);
+        written.* = 0;
         return;
     }
 
+    const text = try self.serialize();
+    defer self.arena.child_allocator.free(text);
+
+    const hash = std.hash.Wyhash.hash(0, text);
+    if (hash == written.*) return;
+
     var dir_buf: [max_path]u8 = undefined;
     const dir = stateDir(&dir_buf) orelse return error.WriteFailed;
     if (glib.mkdirWithParents(dir.ptr, 0o700) != 0) return error.WriteFailed;
 
-    const text = try self.serialize();
-    defer self.arena.child_allocator.free(text);
-
     var err: ?*glib.Error = null;
     if (glib.fileSetContents(path.ptr, text.ptr, @intCast(text.len), &err) == 0) {
         if (err) |e| e.free();
         return error.WriteFailed;
     }
+    written.* = hash;
 }
 
 /// Delete the file. Failures are ignored on purpose: the common one is it not