Fix length caching

Author: stephentoub

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/OSFileStreamStrategy.cs

@@ -17,7 +17,7 @@ internal abstract class OSFileStreamStrategy : FileStreamStrategy
 
         protected long _filePosition;
         private long _appendStart; // When appending, prevent overwriting file.
-        private long _length = -1; // When the file is locked for writes (_share <= FileShare.Read) cache file length in-memory, negative means that hasn't been fetched.
+        private long _length = -1; // When the file is locked for writes on Windows (_share <= FileShare.Read) cache file length in-memory, negative means that hasn't been fetched.
         private bool _exposedHandle; // created from handle, or SafeFileHandle was used and the handle got exposed
 
         internal OSFileStreamStrategy(SafeFileHandle handle, FileAccess access, FileShare share)
@@ -84,7 +84,7 @@ public unsafe sealed override long Length
         {
             get
             {
-                if (!OperatingSystem.IsWindows() || _share > FileShare.Read || _exposedHandle)
+                if (!LengthCachingSupported)
                 {
                     return RandomAccess.GetFileLength(_fileHandle);
                 }
@@ -105,7 +105,7 @@ protected void UpdateLengthOnChangePosition()
         {
             // Do not update the cached length if the file is not locked
             // or if the length hasn't been fetched.
-            if (!OperatingSystem.IsWindows() || _share > FileShare.Read || _length < 0 || _exposedHandle)
+            if (!LengthCachingSupported || _length < 0)
             {
                 Debug.Assert(_length < 0);
                 return;
@@ -117,6 +117,8 @@ protected void UpdateLengthOnChangePosition()
             }
         }
 
+        private bool LengthCachingSupported => OperatingSystem.IsWindows() && _share <= FileShare.Read && !_exposedHandle;
+
         /// <summary>Gets or sets the position within the current stream</summary>
         public sealed override long Position
         {
@@ -235,7 +237,10 @@ protected unsafe void SetLengthCore(long value)
             Debug.Assert(value >= 0, "value >= 0");
 
             FileStreamHelpers.SetFileLength(_fileHandle, value);
-            _length = value;
+            if (LengthCachingSupported)
+            {
+                _length = value;
+            }
 
             if (_filePosition > value)
             {