From 4a6907755adef798fb24deeb93440f2b40b33b38 Mon Sep 17 00:00:00 2001
From: JavaZeroo <71128095+JavaZeroo@users.noreply.github.com>
Date: Fri, 30 Oct 2026 00:21:36 +0800
Subject: [PATCH] fix(shell): retry clipboard reads briefly before failing refs
 #38 #85

Hosted desktops can hold the clipboard for a few milliseconds; reads now retry five times with a short backoff on ERROR_ACCESS_DENIED, while writes keep the immediate failure the copy boundary contract asserts.
---
 experiments/shell/clipboard.hpp | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/experiments/shell/clipboard.hpp b/experiments/shell/clipboard.hpp
index bd44fc83..0e8205d3 100644
--- a/experiments/shell/clipboard.hpp
+++ b/experiments/shell/clipboard.hpp
@@ -13,8 +13,20 @@ class ClipboardError : public std::runtime_error {
 };
 class ClipboardAccess {
 public:
-    explicit ClipboardAccess(HWND owner) {
-        if (!OpenClipboard(owner)) throw ClipboardError("OpenClipboard", GetLastError());
+    // A hosted desktop can hold the clipboard for a few milliseconds at a time
+    // (Explorer or a neighbouring test). Reads retry briefly so contention is
+    // not reported as a hard refusal; writes keep the immediate failure because
+    // the copy-failure contract asserts the first OpenClipboard result.
+    explicit ClipboardAccess(HWND owner, bool retry = false) {
+        constexpr int attempts = 5;
+        for (int attempt = 0; attempt < attempts; ++attempt) {
+            if (OpenClipboard(owner)) return;
+            const DWORD error = GetLastError();
+            if (!retry || error != ERROR_ACCESS_DENIED || attempt + 1 == attempts)
+                throw ClipboardError("OpenClipboard", error);
+            Sleep(static_cast<DWORD>(20 * (attempt + 1)));
+        }
+        throw ClipboardError("OpenClipboard", GetLastError());
     }
     ~ClipboardAccess() { CloseClipboard(); }
     ClipboardAccess(const ClipboardAccess&) = delete;
@@ -22,7 +34,7 @@ class ClipboardAccess {
 inline std::u16string read_unicode_clipboard(HWND owner, size_t max_units = PastePolicy::hard_utf16_units) {
     static_assert(sizeof(wchar_t) == sizeof(char16_t), "Windows UTF-16 expected");
     if (max_units > PastePolicy::hard_utf16_units) throw std::length_error("clipboard limit exceeds hard bound");
-    ClipboardAccess clipboard(owner);
+    ClipboardAccess clipboard(owner, true);
     if (!IsClipboardFormatAvailable(CF_UNICODETEXT)) throw ClipboardError("Unicode text unavailable", ERROR_NOT_FOUND);
     HANDLE memory = GetClipboardData(CF_UNICODETEXT);
     if (!memory) throw ClipboardError("GetClipboardData", GetLastError());