ensure file does not exist after allocation fails

src/libraries/System.IO.FileSystem/tests/FileStream/FileStreamConformanceTests.cs

@@ -62,7 +62,7 @@ public async Task FileOffsetIsPreservedWhenFileStreamIsCreatedFromSafeFileHandle
 
             using FileStream createdFromHandle = new FileStream(stream.SafeFileHandle, FileAccess.Write);
 
-            Assert.Equal(buffer.Length, stream.Position); 
+            Assert.Equal(buffer.Length, stream.Position);
             Assert.Equal(stream.Position, createdFromHandle.Position);
         }
 
@@ -187,17 +187,22 @@ public async Task WriteByteFlushesTheBufferWhenItBecomesFull()
             byte[] allBytes = File.ReadAllBytes(filePath);
             Assert.Equal(writtenBytes.ToArray(), allBytes);
         }
-        
+
         [Fact]
         public void WhenFileStreamFailsToPreallocateDiskSpaceTheErrorMessageContainsAllTheDetails()
         {
             const long tooMuch = 1024L * 1024L * 1024L * 1024L; // 1 TB
 
             string filePath = GetTestFilePath();
-            IOException ex = Assert.Throws<IOException>(() => new FileStream(filePath, FileMode.OpenOrCreate, FileAccess.Write, FileShare.None, BufferSize, Options, tooMuch));
+
+            Assert.False(File.Exists(filePath));
+
+            IOException ex = Assert.Throws<IOException>(() => new FileStream(filePath, FileMode.Create, FileAccess.Write, FileShare.None, BufferSize, Options, tooMuch));
             Assert.Contains("disk was full", ex.Message);
             Assert.Contains(filePath, ex.Message);
             Assert.Contains(AllocationSize.ToString(), ex.Message);
+
+            Assert.False(File.Exists(filePath));
         }
     }
 

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/FileStreamHelpers.Unix.cs

@@ -37,7 +37,7 @@ internal static SafeFileHandle OpenHandle(string path, FileMode mode, FileAccess
 
             // Open the file and store the safe handle.
             SafeFileHandle handle = SafeFileHandle.Open(path!, openFlags, (int)OpenPermissions);
-            if (allocationSize > 0 && (mode == FileMode.Create || mode == FileMode.CreateNew || mode == FileMode.Truncate))
+            if (allocationSize > 0 && (mode == FileMode.Create || mode == FileMode.CreateNew || mode == FileMode.Truncate || mode == FileMode.OpenOrCreate))
             {
                 int allocationResult = Interop.Sys.FAllocate(handle, 0, allocationSize);