fix(libc): leave resident runs under an open host write out of MapAlias

demote() fences a resident run read-only, copies it into the section and
replaces its pages with views. A host write between BeginHostWrite and
EndHostWrite runs outside the mapping lock, so one in flight on the run
could fail on the fence (an APR file read raised ios_base::failure) or land
in the private pages after the copy and be lost. MapAlias now returns null
without demoting when any page of a run it would demote has an open host
write; the AGC driver then imports the range without an alias, as before
this change. A blocking host write such as a socket receive must not stall
the import, so MapAlias does not wait.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/libs/prx/libc/include/WindowsMappings.hpp

@@ -256,6 +256,14 @@ public:
             return std::runtime_error(text);
         };
         if (address % pageBytes != 0 || bytes % pageBytes != 0 || bytes == 0) throw refuse("the range is not made of whole shared pages");
+        for (std::size_t done = 0; done < bytes; done += pageBytes) {
+            const auto found = views.find(address + done);
+            if (found == views.end() || !found->second.resident) continue;
+            const auto run = std::prev(residentRuns.upper_bound(address + done));
+            for (auto page = views.find(run->first); page != views.end() && page->first < run->second.end; ++page) {
+                if (page->second.hostWrites != 0) return nullptr;
+            }
+        }
         for (std::size_t done = 0; done < bytes; done += pageBytes) {
             const auto found = views.find(address + done);
             if (found != views.end() && found->second.resident) demote(address + done);

core/libs/tests/GuestMemory.cpp

@@ -1189,6 +1189,28 @@ static void CheckAliasAcrossDirectAllocations() {
     Require(sceKernelReleaseDirectMemory(first, page * 3) == 0);
 }
 
+static void CheckAliasLeavesResidentRunsUnderAHostWrite() {
+    constexpr std::size_t page = 0x4000;
+    std::int64_t phys = 0;
+    Require(sceKernelAllocateDirectMemory(0, 0x7fffffffffll, page * 2, 0, 0, &phys) == 0);
+    void* mapped = nullptr;
+    Require(sceKernelMapDirectMemory(&mapped, page * 2, 3, 0, phys, 0) == 0);
+    auto* bytes = static_cast<volatile unsigned char*>(mapped);
+    bytes[5] = 0x11;
+    {
+        const GuestArena::HostWrite open(static_cast<unsigned char*>(mapped) + page, page);
+        Require(open.Open());
+        Require(GuestArena::GuestArenaMapAlias_nid_postfix(reinterpret_cast<std::uintptr_t>(mapped), page * 2) == nullptr);
+        bytes[page + 5] = 0x22;
+    }
+    auto* alias = static_cast<volatile unsigned char*>(GuestArena::GuestArenaMapAlias_nid_postfix(reinterpret_cast<std::uintptr_t>(mapped), page * 2));
+    Require(alias != nullptr && alias[5] == 0x11 && alias[page + 5] == 0x22);
+    GuestArena::GuestArenaUnmapAlias_nid_postfix(const_cast<unsigned char*>(alias));
+    Require(bytes[5] == 0x11 && bytes[page + 5] == 0x22);
+    Require(sceKernelMunmap(mapped, page * 2) == 0);
+    Require(sceKernelReleaseDirectMemory(phys, page * 2) == 0);
+}
+
 static void CheckAliasRefusesPagesThatAreNotSharedViews() {
     constexpr std::size_t page = 0x4000;
     std::int64_t phys = 0;
@@ -2574,6 +2596,7 @@ int main(int argc, char** argv) {
     CheckAliasAcrossDirectAllocations();
     CheckAliasOfReorderedSharedPages();
     CheckAliasRefusesPagesThatAreNotSharedViews();
+    CheckAliasLeavesResidentRunsUnderAHostWrite();
 #endif
     CheckUnhintedKernelMappingsLandInTheWindow();
 #if defined(__linux__)