[Tests] Add VirtualMemory and PhysicalVirtualMemory edge case tests (#266)

VirtualMemory: 4 new tests covering unmapped address returns false,
zero-length operations, page-boundary access, and cross-gap failures.

PhysicalVirtualMemory: 5 new tests covering lazy commit on demand,
reserve-only GetPointer, unmapped GetPointer returns null, free-list
first-fit reuse, and coalescing both neighbours on middle-range free.

Total: 9 new tests, 18 passed (8 VirtualMemory + 5 PhysicalVirtualMemory
+ 5 GuestMemoryAllocator). No production code changes.

tests/SharpEmu.Libs.Tests/Memory/PhysicalVirtualMemoryTests.cs

@@ -0,0 +1,229 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+using SharpEmu.Core.Memory;
+using SharpEmu.Core.Loader;
+using SharpEmu.HLE.Host;
+using Xunit;
+
+namespace SharpEmu.Libs.Tests.Memory;
+
+// PhysicalVirtualMemory is the host-backed (identity-mapped) implementation.
+// Reserve-only regions (> 4 GiB, non-executable) defer commit until first
+// access; TryAllocateGuestMemory serves a first-fit free-list with coalescing.
+// These tests pin that behaviour through fake IHostMemory implementations.
+public sealed class PhysicalVirtualMemoryTests
+{
+    // 1. Lazy commit: a reserve-only region has its pages committed on demand
+    //    when read; freshly committed pages read as zero.
+    [Fact]
+    public void LazyReadCommitsPageOnDemandAndReadsZero()
+    {
+        using var host = new LazyZeroedHostMemory();
+        using var memory = new PhysicalVirtualMemory(host);
+
+        // > 4 GiB, non-executable -> reserve-only with lazy commit.
+        var address = memory.AllocateAt(0, (4UL << 30) + 0x1000, executable: false);
+        Assert.NotEqual(0UL, address);
+
+        // Discard the priming commits AllocateAt issues up front; we want to
+        // observe the on-demand commit triggered by the read itself.
+        host.CommitCalls.Clear();
+
+        var buffer = new byte[1];
+        Assert.True(memory.TryRead(address, buffer));
+        Assert.Equal(0, buffer[0]);
+
+        // The touched page (page-aligned to `address`) was committed on demand.
+        var page = address & ~0xFFFUL;
+        Assert.Equal([(page, 0x1000UL, HostPageProtection.ReadWrite)], host.CommitCalls);
+    }
+
+    // 2. Reserve-only region: GetPointer commits the page before returning it,
+    //    so callers receive a valid (non-null) pointer. An unmapped address yields null.
+    [Fact]
+    public unsafe void GetPointerOnReserveOnlyRegionCommitsAndReturnsValidPointer()
+    {
+        using var host = new LazyZeroedHostMemory();
+        using var memory = new PhysicalVirtualMemory(host);
+
+        var address = memory.AllocateAt(0, (4UL << 30) + 0x1000, executable: false);
+        host.CommitCalls.Clear();
+
+        var pointer = memory.GetPointer(address + 0x123);
+        Assert.NotEqual(0UL, (ulong)pointer);
+        Assert.Equal(address + 0x123, (ulong)pointer);
+
+        var page = (address + 0x123) & ~0xFFFUL;
+        Assert.Equal([(page, 0x1000UL, HostPageProtection.ReadWrite)], host.CommitCalls);
+    }
+
+    [Fact]
+    public unsafe void GetPointerOnUnmappedAddressReturnsNull()
+    {
+        using var host = new LazyZeroedHostMemory();
+        using var memory = new PhysicalVirtualMemory(host);
+
+        Assert.Equal(0UL, (ulong)memory.GetPointer(0x0001_0000));
+    }
+
+    // 3. Free-list reuse: a freed range is served back by first-fit allocation,
+    //    preferring the lowest fitting free range over the larger trailing span.
+    [Fact]
+    public void FreedRangeIsReusedByFirstFitAllocation()
+    {
+        using var memory = new PhysicalVirtualMemory(new FakeHostMemory());
+
+        Assert.True(memory.TryAllocateGuestMemory(0x4000, 0x1000, out var first));
+        Assert.True(memory.TryAllocateGuestMemory(0x4000, 0x1000, out var second));
+        Assert.NotEqual(first, second);
+        Assert.True(memory.TryFreeGuestMemory(first));
+
+        // A smaller allocation must reuse first's freed slot (lowest fitting range),
+        // not the larger trailing free range.
+        Assert.True(memory.TryAllocateGuestMemory(0x2000, 0x1000, out var reused));
+        Assert.Equal(first, reused);
+    }
+
+    // 4. Coalescing: freeing the middle of three adjacent ranges merges both the
+    //    left and right free neighbours in a single TryFreeGuestMemory call,
+    //    restoring the full span for subsequent first-fit reuse.
+    [Fact]
+    public void FreeingMiddleRangeCoalescesBothNeighbours()
+    {
+        using var memory = new PhysicalVirtualMemory(new FakeHostMemory());
+
+        // Three adjacent 0x1000 allocations: offsets 0x1000, 0x2000, 0x3000.
+        Assert.True(memory.TryAllocateGuestMemory(0x1000, 0x1000, out var first));
+        Assert.True(memory.TryAllocateGuestMemory(0x1000, 0x1000, out var second));
+        Assert.True(memory.TryAllocateGuestMemory(0x1000, 0x1000, out var third));
+
+        // Free the outer ranges first, leaving two separate free ranges.
+        Assert.True(memory.TryFreeGuestMemory(first));
+        Assert.True(memory.TryFreeGuestMemory(third));
+
+        // Freeing the middle range must coalesce both neighbours at once.
+        Assert.True(memory.TryFreeGuestMemory(second));
+
+        // The whole arena is now one coalesced free range; a full-arena allocation
+        // reuses first's base address.
+        Assert.True(memory.TryAllocateGuestMemory(0x000F_F000, 0x1000, out var coalesced));
+        Assert.Equal(first, coalesced);
+    }
+
+    /// <summary>
+    /// Host memory backed by a single real, zero-initialised page. Reserve/Allocate
+    /// report the page-aligned buffer address so lazy-commit read paths can actually
+    /// dereference the returned pointer. Query always reports Reserved, so
+    /// EnsureRangeCommitted issues a Commit on first access.
+    /// </summary>
+    private sealed unsafe class LazyZeroedHostMemory : IHostMemory, IDisposable
+    {
+        private readonly void* _allocation;
+        private readonly ulong _address;
+        private bool _freed;
+
+        public LazyZeroedHostMemory()
+        {
+            _allocation = System.Runtime.InteropServices.NativeMemory.AllocZeroed(0x2000);
+            _address = ((ulong)_allocation + 0xFFF) & ~0xFFFUL;
+        }
+
+        public List<(ulong Address, ulong Size, HostPageProtection Protection)> CommitCalls { get; } = [];
+
+        public ulong Allocate(ulong desiredAddress, ulong size, HostPageProtection protection) => _address;
+
+        public ulong Reserve(ulong desiredAddress, ulong size, HostPageProtection protection) => _address;
+
+        public bool Commit(ulong address, ulong size, HostPageProtection protection)
+        {
+            CommitCalls.Add((address, size, protection));
+            return true;
+        }
+
+        public bool Free(ulong address)
+        {
+            // The real buffer is released in Dispose; keep Free a no-op so
+            // PhysicalVirtualMemory.Clear does not double-free it.
+            return true;
+        }
+
+        public bool Protect(ulong address, ulong size, HostPageProtection protection, out uint rawOldProtection)
+        {
+            rawOldProtection = 0;
+            return true;
+        }
+
+        public bool ProtectRaw(ulong address, ulong size, uint rawProtection, out uint rawOldProtection)
+        {
+            rawOldProtection = 0;
+            return true;
+        }
+
+        public bool Query(ulong address, out HostRegionInfo info)
+        {
+            var pageAddress = address & ~0xFFFUL;
+            info = new HostRegionInfo(
+                pageAddress,
+                pageAddress,
+                0x1000,
+                HostRegionState.Reserved,
+                0,
+                HostPageProtection.NoAccess,
+                0,
+                0);
+            return true;
+        }
+
+        public void FlushInstructionCache(ulong address, ulong size)
+        {
+        }
+
+        public void Dispose()
+        {
+            if (!_freed)
+            {
+                System.Runtime.InteropServices.NativeMemory.Free(_allocation);
+                _freed = true;
+            }
+        }
+    }
+
+    // Minimal host memory for free-list tests: Allocate honours the desired
+    // address (or a fallback), everything else succeeds as a no-op. The guest
+    // allocation arena never dereferences, so no real backing is required.
+    private sealed class FakeHostMemory : IHostMemory
+    {
+        public ulong Allocate(ulong desiredAddress, ulong size, HostPageProtection protection) =>
+            desiredAddress != 0 ? desiredAddress : 0x00007000_0000_0000;
+
+        public ulong Reserve(ulong desiredAddress, ulong size, HostPageProtection protection) =>
+            Allocate(desiredAddress, size, protection);
+
+        public bool Commit(ulong address, ulong size, HostPageProtection protection) => true;
+
+        public bool Free(ulong address) => true;
+
+        public bool Protect(ulong address, ulong size, HostPageProtection protection, out uint rawOldProtection)
+        {
+            rawOldProtection = 0;
+            return true;
+        }
+
+        public bool ProtectRaw(ulong address, ulong size, uint rawProtection, out uint rawOldProtection)
+        {
+            rawOldProtection = 0;
+            return true;
+        }
+
+        public bool Query(ulong address, out HostRegionInfo info)
+        {
+            info = default;
+            return false;
+        }
+
+        public void FlushInstructionCache(ulong address, ulong size)
+        {
+        }
+    }
+}

tests/SharpEmu.Libs.Tests/Memory/VirtualMemoryTests.cs

@@ -84,4 +84,74 @@ public sealed class VirtualMemoryTests
         Assert.False(memory.TryRead(0x3000, unchanged));
         Assert.True(memory.TryWrite(0x3000, [9]));
     }
+
+    [Fact]
+    public void TryReadAndTryWriteOnUnmappedAddressReturnFalse()
+    {
+        var memory = new VirtualMemory();
+        memory.Map(0x1000, 0x100, 0, [], ProgramHeaderFlags.Read | ProgramHeaderFlags.Write);
+
+        // Addresses with no covering region fail for both reads and writes.
+        Span<byte> value = stackalloc byte[1];
+        Assert.False(memory.TryRead(0x2000, value));
+        Assert.False(memory.TryWrite(0x2000, value));
+
+        // With no regions mapped at all, every access fails.
+        var empty = new VirtualMemory();
+        Assert.False(empty.TryRead(0x1000, value));
+        Assert.False(empty.TryWrite(0x1000, value));
+    }
+
+    // Zero-length operations succeed on a mapped address and touch nothing, but
+    // still require a mapped address.
+    [Fact]
+    public void ZeroLengthOperationsOnMappedAddressReturnTrue()
+    {
+        var memory = new VirtualMemory();
+        memory.Map(0x1000, 0x100, 0, [0x01], ProgramHeaderFlags.Read | ProgramHeaderFlags.Write);
+
+        Assert.True(memory.TryRead(0x1000, []));
+        Assert.True(memory.TryWrite(0x1000, []));
+
+        Span<byte> value = stackalloc byte[1];
+        Assert.True(memory.TryRead(0x1000, value));
+        Assert.Equal(0x01, value[0]);
+
+        Assert.False(memory.TryRead(0x2000, []));
+        Assert.False(memory.TryWrite(0x2000, []));
+    }
+
+    // A page-aligned region must be accessible at both offset 0 and the final byte.
+    [Fact]
+    public void MappingAtPageBoundarySupportsOffsetZeroAndLastByte()
+    {
+        const ulong pageSize = 0x1000;
+        var memory = new VirtualMemory();
+        memory.Map(pageSize, pageSize, 0, [], ProgramHeaderFlags.Read | ProgramHeaderFlags.Write);
+
+        Assert.True(memory.TryWrite(pageSize, [0x5A]));
+        Assert.True(memory.TryWrite(pageSize + pageSize - 1, [0xA5]));
+
+        Span<byte> head = stackalloc byte[1];
+        Span<byte> tail = stackalloc byte[1];
+        Assert.True(memory.TryRead(pageSize, head));
+        Assert.True(memory.TryRead(pageSize + pageSize - 1, tail));
+        Assert.Equal(0x5A, head[0]);
+        Assert.Equal(0xA5, tail[0]);
+    }
+
+    // A read/write that starts in one region but extends into the unmapped gap
+    // between two non-adjacent regions must fail across the entire range.
+    [Fact]
+    public void ReadWriteAcrossGapBetweenNonAdjacentRegionsReturnsFalse()
+    {
+        const ulong pageSize = 0x1000;
+        var memory = new VirtualMemory();
+        const ProgramHeaderFlags protection = ProgramHeaderFlags.Read | ProgramHeaderFlags.Write;
+        memory.Map(pageSize, pageSize, 0, [], protection);
+        memory.Map(pageSize * 3, pageSize, 0, [], protection);
+
+        Assert.False(memory.TryRead(pageSize, new byte[(int)pageSize * 2]));
+        Assert.False(memory.TryWrite(pageSize, new byte[(int)pageSize * 2]));
+    }
 }