Fix Unix file descriptor assertion

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/SafeFileHandle/GetFileType.Unix.cs

@@ -1,15 +1,17 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using Microsoft.DotNet.RemoteExecutor;
 using Microsoft.DotNet.XUnitExtensions;
 using Microsoft.Win32.SafeHandles;
+using System.Runtime.InteropServices;
 using System.Threading.Tasks;
 using Xunit;
 
 namespace System.IO.Tests
 {
     [PlatformSpecific(TestPlatforms.AnyUnix)]
-    public class SafeFileHandle_GetFileType_Unix : FileSystemTest
+    public partial class SafeFileHandle_GetFileType_Unix : FileSystemTest
     {
         [Fact]
         public void GetFileType_Directory()
@@ -94,5 +96,53 @@ public void GetFileType_BlockDevice()
                 throw new SkipTestException("Insufficient privileges to open block device");
             }
         }
+
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        [PlatformSpecific(TestPlatforms.AnyUnix)]
+        public void WriteCanUseFileDescriptorAboveCurrentLimit()
+        {
+            RemoteExecutor.Invoke(() =>
+            {
+                const ulong MaximumFileDescriptorLimit = 4_096;
+
+                Assert.Equal(0, Interop.Sys.GetRLimit(Interop.Sys.RlimitResources.RLIMIT_NOFILE, out Interop.Sys.RLimit limits));
+
+                limits.CurrentLimit = Math.Min(limits.CurrentLimit, MaximumFileDescriptorLimit);
+                Assert.InRange(limits.CurrentLimit, 2UL, (ulong)int.MaxValue);
+                Assert.Equal(0, Interop.Sys.SetRLimit(Interop.Sys.RlimitResources.RLIMIT_NOFILE, ref limits));
+
+                int maxAllowedFileDescriptor = checked((int)limits.CurrentLimit - 1);
+                string filePath = Path.GetTempFileName();
+
+                try
+                {
+                    using (SafeFileHandle fileHandle = File.OpenHandle(filePath, FileMode.Open, FileAccess.ReadWrite))
+                    {
+                        Assert.Equal(maxAllowedFileDescriptor, dup2((int)fileHandle.DangerousGetHandle(), maxAllowedFileDescriptor));
+
+                        limits.CurrentLimit--;
+                        Assert.Equal(0, Interop.Sys.SetRLimit(Interop.Sys.RlimitResources.RLIMIT_NOFILE, ref limits));
+
+                        using (SafeFileHandle duplicatedHandle = new(maxAllowedFileDescriptor, ownsHandle: true))
+                        {
+                            RandomAccess.Write(duplicatedHandle, new byte[] { 1 }, fileOffset: 0);
+                        }
+
+                        byte[] buffer = new byte[1];
+                        Assert.Equal(1, RandomAccess.Read(fileHandle, buffer, fileOffset: 0));
+                        Assert.Equal(new byte[] { 1 }, buffer);
+                    }
+                }
+                finally
+                {
+                    File.Delete(filePath);
+                }
+
+                return RemoteExecutor.SuccessExitCode;
+            }).Dispose();
+        }
+
+        [LibraryImport("libc", SetLastError = true)]
+        private static partial int dup2(int oldFileDescriptor, int newFileDescriptor);
     }
 }

src/libraries/System.Runtime/tests/System.IO.FileSystem.Tests/System.IO.FileSystem.Tests.csproj

@@ -103,6 +103,8 @@
   <ItemGroup Condition="'$(TargetPlatformIdentifier)' == 'unix'">
     <Compile Include="FileSystemTest.Unix.cs" />
     <Compile Include="$(CommonPath)Interop\Unix\Interop.Libraries.cs" Link="Interop\Unix\Interop.Libraries.cs" />
+    <Compile Include="$(CommonPath)Interop\Unix\System.Native\Interop.ResourceLimits.cs"
+             Link="Common\Interop\Unix\Interop.ResourceLimits.cs" />
     <Compile Include="FileStream\ctor_options.Unix.cs" />
     <Compile Include="Directory\CreateDirectory_UnixFileMode.Unix.cs" />
     <Compile Include="SafeFileHandle\GetFileType.Unix.cs" />

src/native/libs/Common/pal_utilities.h

@@ -79,8 +79,9 @@ inline static int ToFileDescriptorUnchecked(intptr_t fd)
 inline static int ToFileDescriptor(intptr_t fd)
 {
 #ifndef TARGET_WASI // the valid range of file descriptors is probably INT32_MIN <= fd && fd <= INT32_MAX, the negative handles are valid for console.
-    assert(0 <= fd && fd < sysconf(_SC_OPEN_MAX));
+    assert(0 <= fd);
 #endif
+    assert(fd <= INT_MAX);
 
     return ToFileDescriptorUnchecked(fd);
 }