From d09855ae7b347aaaebb4e9dc5a78ecef6d8fd62e Mon Sep 17 00:00:00 2001
From: Mikolaj Boc <mikolaj.boc@qt.io>
Date: Fri, 21 Apr 2023 13:42:57 +0200
Subject: [PATCH] Copy memory to buffer when saving files on wasm with
 threading on
MIME-Version: 1.0
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wasm heap uses a shared array buffer, which cannot be fed to
FileSystemWritableFileStream.write due to security limitations.
Heap memory has to be copied to a temporary buffer for the operation to
succeed.
This is only done if __EMSCRIPTEN_SHARED_MEMORY__ is on to optimize the
non-threading path.

Fixes: QTBUG-112881
Pick-to: 6.5
Change-Id: I0d117a8703caf4c17abc67b30df5248a53406d5f
Reviewed-by: Aleksandr Reviakin <aleksandr.reviakin@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
---
 .../platform/wasm/qwasmlocalfileaccess.cpp    | 32 ++++++++++++++++---
 1 file changed, 28 insertions(+), 4 deletions(-)

diff --git a/src/gui/platform/wasm/qwasmlocalfileaccess.cpp b/src/gui/platform/wasm/qwasmlocalfileaccess.cpp
index 762d5e1a40..80e32d629d 100644
--- a/src/gui/platform/wasm/qwasmlocalfileaccess.cpp
+++ b/src/gui/platform/wasm/qwasmlocalfileaccess.cpp
@@ -258,6 +258,11 @@ void saveDataToFileInChunks(emscripten::val fileHandle, const QByteArray &data)
                 std::function<void(val result)> continuation;
             };
 
+            static constexpr size_t desiredChunkSize = 1024u;
+#if defined(__EMSCRIPTEN_SHARED_MEMORY__)
+            qstdweb::Uint8Array chunkArray(desiredChunkSize);
+#endif
+
             auto state = std::make_shared<State>();
             state->written = 0u;
             state->continuation = [=](val) mutable {
@@ -267,11 +272,30 @@ void saveDataToFileInChunks(emscripten::val fileHandle, const QByteArray &data)
                     state.reset();
                     return;
                 }
-                static constexpr size_t desiredChunkSize = 1024u;
+
                 const auto currentChunkSize = std::min(remaining, desiredChunkSize);
-                Promise::make(writable, QStringLiteral("write"), {
-                    .thenFunc = state->continuation,
-                }, val(typed_memory_view(currentChunkSize, data.constData() + state->written)));
+
+#if defined(__EMSCRIPTEN_SHARED_MEMORY__)
+                // If shared memory is used, WebAssembly.Memory is instantiated with the 'shared'
+                // option on. Passing a typed_memory_view to SharedArrayBuffer to
+                // FileSystemWritableFileStream.write is disallowed by security policies, so we
+                // need to make a copy of the data to a chunk array buffer.
+                Promise::make(
+                    writable, QStringLiteral("write"),
+                    {
+                        .thenFunc = state->continuation,
+                    },
+                    chunkArray.copyFrom(data.constData() + state->written, currentChunkSize)
+                        .val()
+                        .call<emscripten::val>("subarray", emscripten::val(0),
+                                               emscripten::val(currentChunkSize)));
+#else
+                Promise::make(writable, QStringLiteral("write"),
+                    {
+                        .thenFunc = state->continuation,
+                    },
+                    val(typed_memory_view(currentChunkSize, data.constData() + state->written)));
+#endif
                 state->written += currentChunkSize;
             };