Creating a MemoryMappedFile with fixed capacity throws ArgumentException, fatal crash during attempt to write

[NightOwl888]

Steps to Reproduce

Run the following NUnit tests on Xamarin.Android using Visual Studio 2019 on Windows with the Android emulator.

Note the tests are in a .NET Standard 2.0 library that is referenced by a Xamarin.Android project. I tried a couple of different emulator configurations x86 and x86_64 with similar results. All other tests are running, most passing.

        [Test]
        public void TestMemoryMappedXamarin()
        {
            var tmp = new FileInfo(Path.GetTempFileName());
            try
            {
                MemoryMappedViewAccessor accessor;

                using (var fileStream = new FileStream(tmp.FullName, FileMode.Open))
                // Throws ArgumentException on capacity parameter (with no message)
                using (var f = MemoryMappedFile.CreateFromFile(fileStream, mapName: null, capacity: 100L, 
                    access: MemoryMappedFileAccess.ReadWrite,
                    HandleInheritability.None, leaveOpen: false))
                {
                    accessor = f.CreateViewAccessor(offset: 0L, size: 100L, access: MemoryMappedFileAccess.ReadWrite);
                }

                Store(accessor, 1, 1);
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
        }

        [Test]
        public void TestMemoryMappedXamarin2()
        {
            var tmp = new FileInfo(Path.GetTempFileName());
            try
            {
                MemoryMappedViewAccessor accessor;

                using (var fileStream = new FileStream(tmp.FullName, FileMode.Open))
                // Capacity 0 doesn't throw, but then the MemoryMappedFile is the wrong size
                using (var f = MemoryMappedFile.CreateFromFile(fileStream, mapName: null, capacity: 0L, 
                    access: MemoryMappedFileAccess.ReadWrite,
                    HandleInheritability.None, leaveOpen: false))
                {
                    accessor = f.CreateViewAccessor(0L, 100L, MemoryMappedFileAccess.ReadWrite);
                }

                // Fatal crash when trying to write to the accessor
                Store(accessor, 1, 1);
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
        }

        [Test]
        public void TestMemoryMappedXamarin3()
        {
            var tmp = new FileInfo(Path.GetTempFileName());
            try
            {
                MemoryMappedViewAccessor accessor;

                // Setting the length of the FileStream and capacity to 0 should cause the MemoryMappedFile to inherit 
                // the size of the stream.
                // In theory, this should be the same behavior as TestMemoryMappedXamarin() with alternate syntax,
                // but it makes no difference in terms of writing to the view accessor.
                using (var fileStream = new FileStream(path: tmp.FullName, mode: FileMode.Open, 
                    access: FileAccess.ReadWrite, share: FileShare.Read, bufferSize: 100))
                using (var f = MemoryMappedFile.CreateFromFile(fileStream, mapName: null, capacity: 0L, 
                    access: MemoryMappedFileAccess.ReadWrite,
                    HandleInheritability.None, leaveOpen: false))
                {
                    accessor = f.CreateViewAccessor(0L, 100L, MemoryMappedFileAccess.ReadWrite);
                }

                // Fatal crash when trying to write to the accessor
                Store(accessor, 1, 1);
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
        }

        private void Store(MemoryMappedViewAccessor accessor, int index, int value)
        {
            int baseOffset = index;

            // Write big endian int32
            for (int i = 3; i >= 0; i--)
            {
                accessor.Write(baseOffset + i, (byte)(value & 0xFF));
                value >>= 8;
            }
        }

Expected Behavior

1. When calling MemoryMappedFile.CreateFromFile(), it should be possible to set a fixed capacity.
2. Writing to a writable view accessor should either succeed or throw a managed exception.

Actual Behavior

1. When creating a MemoryMappedFile with a fixed capacity, an ArgumentException is thrown. There is no message in the exception, so it is not clear why the exception is being thrown, only that it refers to the capacity parameter. On .NET Framework 4.8, .NET Core 3.1, and .NET 5.0 there is no exception in this case.
2. When writing to an open view accesor, a fatal crash occurs. This exact same code works on .NET Framework 4.8, .NET Core 3.1, and .NET 5.0 when running as either x86 or x64.

Version Information

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Microsoft Visual Studio Community 2019
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Log File

No log available via adb command, but here is the output from the debug session when calling TestMemoryMappedXamarin3():

crash.txt

[jpobst]

@grendello Is this a Mono thing? If so, can you please route it to the correct place/people?

[grendello]

This appears to be a segfault:

=================================================================
	Native Crash Reporting
=================================================================
Got a SEGV while executing native code. This usually indicates
a fatal error in the mono runtime or one of the native libraries 
used by your application.
=================================================================

No native Android stacktrace (see debuggerd output).

=================================================================
	Basic Fault Address Reporting
=================================================================
Memory around native instruction pointer (0xcaf97872):0xcaf97862  03 c1 8b 4d 0c 8b 55 10 8b d1 03 c1 0f b6 4d 14  ...M..U.......M.
0xcaf97872  88 08 8b 4d 
f4 8d 64 24 00 90 90 90 c7 45 e8 00  ...M..d$.....E..
0xcaf97882  00 00 00 83 ec 0c e8 14 00 00 00 83 c4 0c 83 7d  ...............}
0xcaf97892  e8 00 74 06 90 e8 58 d6 85 1c e9 69 00 00 00 89  ..t...X....i....

=================================================================
	Managed Stacktrace:
=================================================================
	  at System.IO.UnmanagedMemoryAccessor:Write <0x000e2>
	  at J2N.IO.MemoryMappedFiles.TestMemoryMappedViewByteBuffer:Store <0x000bf>
	  at J2N.IO.MemoryMappedFiles.TestMemoryMappedViewByteBuffer:TestMemoryMappedXamarin3 <0x0033f>
	  at System.Object:runtime_invoke_void__this__ <0x00104>
	  at <unknown> <0xffffffff>
	  at System.Reflection.RuntimeMethodInfo:InternalInvoke <0x00012>
	  at System.Reflection.RuntimeMethodInfo:Invoke <0x0033f>
	  at System.Reflection.MethodBase:Invoke <0x0007b>
	  at NUnit.Framework.Internal.Reflect:InvokeMethod <0x000ab>
	  at NUnit.Framework.Internal.MethodWrapper:Invoke <0x00067>
	  at NUnit.Framework.Internal.Commands.
TestMethodCommand:RunNonAsyncTestMethod <0x000b1>
	  at NUnit.Framework.Internal.Commands.TestMethodCommand:RunTestMethod <0x000f3>
	  at NUnit.Framework.Internal.Commands.TestMethodCommand:Execute <0x0004b>
	  at NUnit.Framework.Internal.Commands.BeforeAndAfterTestCommand:Execute <0x001d5>
	  at NUnit.Framework.Internal.Execution.SimpleWorkItem:PerformWork <0x0008a>
	  at NUnit.Framework.Internal.Execution.WorkItem:RunOnCurrentThread <0x002d3>
	  at NUnit.Framework.Internal.Execution.WorkItem:Execute <0x00377>
	  at NUnit.Framework.Internal.Execution.ParallelWorkItemDispatcher:Dispatch <0x00182>
	  at NUnit.Framework.Internal.Execution.ParallelWorkItemDispatcher:Dispatch <0x00077>
	  at NUnit.Framework.Internal.Execution.CompositeWorkItem:RunChildren <0x00536>
	  at NUnit.Framework.Internal.Execution.CompositeWorkItem:PerformWork <0x0036b>
	  at NUnit.Framework.Internal.Execution.WorkItem:RunOnCurrentThread <0x002d3>
	  at NUnit.Framework.Internal.Execution.WorkItem:Execute <0x00377>
	  at NUnit.Framework.In
ternal.Execution.TestWorker:TestWorkerThreadProc <0x002eb>
	  at System.Threading.ThreadHelper:ThreadStart_Context <0x000de>
	  at System.Threading.ExecutionContext:RunInternal <0x0035f>
	  at System.Threading.ExecutionContext:Run <0x0005b>
	  at System.Threading.ExecutionContext:Run <0x00147>
	  at System.Threading.ThreadHelper:ThreadStart <0x0007f>
	  at System.Object:runtime_invoke_void__this__ <0x00104>
=================================================================
12-15 12:16:52.603 F/libc    ( 8123): Fatal signal 7 (SIGBUS), code 2, fault addr 0xf00af004 in tid 8158 (NonParallelWork), pid 8123 (N.Tests.Android)

Considering the last frame, it's most likely a Mono runtime error (UnmanagedMemoryAccessor comes from reference sources but it uses runtime helpers in Mono)

@NightOwl888 would you mind recording and attaching a full bug report on this crash? To do so, run the tests and when they crash, run adb bugreport - it should give the Mono team more info on the crash (hopefully), thanks!

[NightOwl888]

@NightOwl888 would you mind recording and attaching a full bug report on this crash? To do so, run the tests and when they crash, run adb bugreport - it should give the Mono team more info on the crash (hopefully), thanks!

I did a factory reset on the emulator, then ran TestMemoryMappedXamarin3() and then adb bugreport after it crashed. Attached is the result.

Let me know if you require anything else. Thanks.

bugreport-sdk_gphone_x86_64-QSR1.200715.002-2021-01-10-22-14-10.zip

[jpobst]

With support for Classic Xamarin.Android ended May 1st, 2024, this issue is likely no longer relevant.

If this still persists in .NET 8+, please open a new issue with updated information based on net8.0-android or greater. Please include a link to this issue for context.