Don't try to acquire a shared filed lock on Unix as it's not supported by NFS and CIFS

src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs

@@ -273,6 +273,8 @@ private static Version GetICUVersion()
 
         public static bool IsNet5CompatFileStreamEnabled => _net5CompatFileStream.Value;
 
+        public static bool IsFileLockingSupported => IsWindows || IsNet5CompatFileStreamEnabled;
+
         private static bool GetIsInContainer()
         {
             if (IsWindows)

src/libraries/System.IO.FileSystem/tests/File/ReadWriteAllBytes.cs

@@ -80,8 +80,8 @@ public void Overwrite()
             Assert.Equal(overwriteBytes, File.ReadAllBytes(path));
         }
 
-        [Fact]
         [ActiveIssue("https://github.com/dotnet/runtime/issues/40065", TestPlatforms.Browser)]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsFileLockingSupported))]
         public void OpenFile_ThrowsIOException()
         {
             string path = GetTestFilePath();

src/libraries/System.IO.FileSystem/tests/File/ReadWriteAllBytesAsync.cs

@@ -94,8 +94,8 @@ public async Task OverwriteAsync()
             Assert.Equal(overwriteBytes, await File.ReadAllBytesAsync(path));
         }
 
-        [Fact]
         [ActiveIssue("https://github.com/dotnet/runtime/issues/40065", TestPlatforms.Browser)]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsFileLockingSupported))]
         public async Task OpenFile_ThrowsIOExceptionAsync()
         {
             string path = GetTestFilePath();

src/libraries/System.IO.FileSystem/tests/File/ReadWriteAllLines.cs

@@ -77,8 +77,8 @@ public virtual void Overwrite()
             Assert.Equal(overwriteLines, Read(path));
         }
 
-        [Fact]
         [ActiveIssue("https://github.com/dotnet/runtime/issues/40065", TestPlatforms.Browser)]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsFileLockingSupported))]
         public void OpenFile_ThrowsIOException()
         {
             string path = GetTestFilePath();
@@ -267,8 +267,8 @@ public virtual void Overwrite()
             Assert.Equal(overwriteLines, Read(path));
         }
 
-        [Fact]
         [ActiveIssue("https://github.com/dotnet/runtime/issues/40065", TestPlatforms.Browser)]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsFileLockingSupported))]
         public void OpenFile_ThrowsIOException()
         {
             string path = GetTestFilePath();

src/libraries/System.IO.FileSystem/tests/File/ReadWriteAllLinesAsync.cs

@@ -75,8 +75,8 @@ public virtual async Task OverwriteAsync()
             Assert.Equal(overwriteLines, await ReadAsync(path));
         }
 
-        [Fact]
         [ActiveIssue("https://github.com/dotnet/runtime/issues/40065", TestPlatforms.Browser)]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsFileLockingSupported))]
         public async Task OpenFile_ThrowsIOExceptionAsync()
         {
             string path = GetTestFilePath();

src/libraries/System.IO.FileSystem/tests/File/ReadWriteAllText.cs

@@ -84,8 +84,8 @@ public virtual void Overwrite()
             Assert.Equal(overwriteLines, Read(path));
         }
 
-        [Fact]
         [ActiveIssue("https://github.com/dotnet/runtime/issues/40065", TestPlatforms.Browser)]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsFileLockingSupported))]
         public void OpenFile_ThrowsIOException()
         {
             string path = GetTestFilePath();

src/libraries/System.IO.FileSystem/tests/File/ReadWriteAllTextAsync.cs

@@ -83,8 +83,8 @@ public virtual async Task OverwriteAsync()
             Assert.Equal(overwriteLines, await ReadAsync(path));
         }
 
-        [Fact]
         [ActiveIssue("https://github.com/dotnet/runtime/issues/40065", TestPlatforms.Browser)]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsFileLockingSupported))]
         public async Task OpenFile_ThrowsIOExceptionAsync()
         {
             string path = GetTestFilePath();

src/libraries/System.IO.FileSystem/tests/FileStream/ctor_str_fm_fa_fs.write.cs

@@ -37,8 +37,8 @@ public void FileShareWriteExisting()
             }
         }
 
-        [Fact]
         [ActiveIssue("https://github.com/dotnet/runtime/issues/40065", TestPlatforms.Browser)]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsFileLockingSupported))]
         public void FileShareWithoutWriteThrows()
         {
             string fileName = GetTestFilePath();

src/libraries/System.IO.FileSystem/tests/Net5CompatTests/System.IO.FileSystem.Net5Compat.Tests.csproj

@@ -2,8 +2,7 @@
   <PropertyGroup>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <IncludeRemoteExecutor>true</IncludeRemoteExecutor>
-    <!-- Windows is currently the only OS for which we provide a new strategy, so we test the Net5Compat only for Windows -->
-    <TargetFrameworks>$(NetCoreAppCurrent)-windows</TargetFrameworks>
+    <TargetFrameworks>$(NetCoreAppCurrent)-windows;$(NetCoreAppCurrent)-Unix;</TargetFrameworks>
 
     <WasmXHarnessMonoArgs>--working-dir=/test-dir</WasmXHarnessMonoArgs>
   </PropertyGroup>

src/libraries/System.Private.CoreLib/src/Microsoft/Win32/SafeHandles/SafeFileHandle.Unix.cs

@@ -13,6 +13,7 @@ public sealed partial class SafeFileHandle : SafeHandleZeroOrMinusOneIsInvalid
         // not using bool? as it's not thread safe
         private volatile NullableBool _canSeek = NullableBool.Undefined;
         private bool _deleteOnClose;
+        private bool _releaseLock;
 
         public SafeFileHandle() : this(ownsHandle: true)
         {
@@ -117,7 +118,11 @@ protected override bool ReleaseHandle()
             // which could prevent subsequent usage of the file until this process dies.  To avoid that, we proactively
             // try to release the lock before we close the handle. (If it's not locked, there's no behavioral
             // problem trying to unlock it.)
-            Interop.Sys.FLock(handle, Interop.Sys.LockOperations.LOCK_UN); // ignore any errors
+            if (_releaseLock)
+            {
+                Interop.Sys.FLock(handle, Interop.Sys.LockOperations.LOCK_UN); // ignore any errors
+                _releaseLock = false;
+            }
 
             // If DeleteOnClose was requested when constructed, delete the file now.
             // (Unix doesn't directly support DeleteOnClose, so we mimic it here.)
@@ -249,20 +254,27 @@ private void Init(string path, FileMode mode, FileAccess access, FileShare share
             IsAsync = (options & FileOptions.Asynchronous) != 0;
             _deleteOnClose = (options & FileOptions.DeleteOnClose) != 0;
 
-            // Lock the file if requested via FileShare.  This is only advisory locking. FileShare.None implies an exclusive
-            // lock on the file and all other modes use a shared lock.  While this is not as granular as Windows, not mandatory,
-            // and not atomic with file opening, it's better than nothing.
-            Interop.Sys.LockOperations lockOperation = (share == FileShare.None) ? Interop.Sys.LockOperations.LOCK_EX : Interop.Sys.LockOperations.LOCK_SH;
-            if (Interop.Sys.FLock(this, lockOperation | Interop.Sys.LockOperations.LOCK_NB) < 0)
+            if (FileStreamHelpers.UseNet5CompatStrategy || share == FileShare.None)
             {
-                // The only error we care about is EWOULDBLOCK, which indicates that the file is currently locked by someone
-                // else and we would block trying to access it.  Other errors, such as ENOTSUP (locking isn't supported) or
-                // EACCES (the file system doesn't allow us to lock), will only hamper FileStream's usage without providing value,
-                // given again that this is only advisory / best-effort.
-                Interop.ErrorInfo errorInfo = Interop.Sys.GetLastErrorInfo();
-                if (errorInfo.Error == Interop.Error.EWOULDBLOCK)
+                // Lock the file if requested via FileShare.  This is only advisory locking. FileShare.None implies an exclusive
+                // lock on the file and all other modes use a shared lock.  While this is not as granular as Windows, not mandatory,
+                // and not atomic with file opening, it's better than nothing.
+                Interop.Sys.LockOperations lockOperation = (share == FileShare.None) ? Interop.Sys.LockOperations.LOCK_EX : Interop.Sys.LockOperations.LOCK_SH;
+                if (Interop.Sys.FLock(this, lockOperation | Interop.Sys.LockOperations.LOCK_NB) < 0)
+                {
+                    // The only error we care about is EWOULDBLOCK, which indicates that the file is currently locked by someone
+                    // else and we would block trying to access it.  Other errors, such as ENOTSUP (locking isn't supported) or
+                    // EACCES (the file system doesn't allow us to lock), will only hamper FileStream's usage without providing value,
+                    // given again that this is only advisory / best-effort.
+                    Interop.ErrorInfo errorInfo = Interop.Sys.GetLastErrorInfo();
+                    if (errorInfo.Error == Interop.Error.EWOULDBLOCK)
+                    {
+                        throw Interop.GetExceptionForIoErrno(errorInfo, path, isDirectory: false);
+                    }
+                }
+                else
                 {
-                    throw Interop.GetExceptionForIoErrno(errorInfo, path, isDirectory: false);
+                    _releaseLock = true;
                 }
             }