FileMode.OpenOrCreate: don't change existing files

src/libraries/Common/src/Interop/Unix/System.Native/Interop.PosixFAllocate.cs

@@ -1,7 +1,6 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-using System.IO;
 using System.Runtime.InteropServices;
 using Microsoft.Win32.SafeHandles;
 
@@ -13,6 +12,6 @@ internal static partial class Sys
         /// Returns -1 on ENOSPC, -2 on EFBIG. On success or ignorable error, 0 is returned.
         /// </summary>
         [DllImport(Libraries.SystemNative, EntryPoint = "SystemNative_PosixFAllocate", SetLastError = false)]
-        internal static extern int PosixFAllocate(SafeFileHandle fd, long offset, long length, FileMode mode);
+        internal static extern int PosixFAllocate(SafeFileHandle fd, long offset, long length);
     }
 }

src/libraries/Native/Unix/System.Native/pal_io.c

@@ -1008,29 +1008,11 @@ int32_t SystemNative_PosixFAdvise(intptr_t fd, int64_t offset, int64_t length, i
 #endif
 }
 
-int32_t SystemNative_PosixFAllocate(intptr_t fd, int64_t offset, int64_t length, int32_t mode)
+int32_t SystemNative_PosixFAllocate(intptr_t fd, int64_t offset, int64_t length)
 {
     assert_msg(offset == 0, "Invalid offset value", (int)offset);
 
     int fileDescriptor = ToFileDescriptor(fd);
-#if !HAVE_POSIX_FALLOCATE64 && !HAVE_POSIX_FALLOCATE
-    // posix_fallocate does not allow for shrinking file
-    // when it's not available, we need to ensure that the file won't be shrinked
-    if (mode == 4) // OpenOrCreate
-    {
-        struct stat_ fileStat;
-        int fstatResult;
-        while ((fstatResult = fstat_(fileDescriptor, &fileStat)) < 0 && errno == EINTR);
-
-        if (fstatResult != 0 || (fileStat.st_mode & S_IFMT) != S_IFREG || (int64_t)fileStat.st_size >= length)
-        {
-            return 0;
-        }
-    }
-#else
-    (void)mode;
-#endif
-
     int32_t result;
 #if HAVE_POSIX_FALLOCATE64 // 64-bit Linux
     while ((result = posix_fallocate64(fileDescriptor, (off64_t)offset, (off64_t)length)) == EINTR);

src/libraries/Native/Unix/System.Native/pal_io.h

@@ -624,7 +624,7 @@ PALEXPORT int32_t SystemNative_PosixFAdvise(intptr_t fd, int64_t offset, int64_t
  *
  * Returns -1 on ENOSPC, -2 on EFBIG. On success or ignorable error, 0 is returned.
  */
-PALEXPORT int32_t SystemNative_PosixFAllocate(intptr_t fd, int64_t offset, int64_t length, int32_t mode);
+PALEXPORT int32_t SystemNative_PosixFAllocate(intptr_t fd, int64_t offset, int64_t length);
 
 /**
  * Reads the number of bytes specified into the provided buffer from the specified, opened file descriptor.

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

@@ -114,9 +114,9 @@ public void PreallocationSizeIsIgnoredForFileModeOpenAndAppend(FileMode mode, lo
         }
 
         [Theory]
-        [InlineData(FileMode.OpenOrCreate, 20L)] // this can extend the file
-        [InlineData(FileMode.OpenOrCreate, 5L)] // this should NOT shink the file
-        public void WhenFileIsBeingOpenedWithOpenOrCreateModeTheLengthCanBeOnlyExtended(FileMode mode, long preallocationSize)
+        [InlineData(FileMode.OpenOrCreate, 20L)] // preallocationSize > initialSize
+        [InlineData(FileMode.OpenOrCreate, 5L)] // preallocationSize < initialSize
+        public void WhenExistingFileIsBeingOpenedWithOpenOrCreateModeTheLengthRemainsUnchanged(FileMode mode, long preallocationSize)
         {
             const int initialSize = 10;
             string filePath = GetPathToNonExistingFile();
@@ -125,14 +125,12 @@ public void WhenFileIsBeingOpenedWithOpenOrCreateModeTheLengthCanBeOnlyExtended(
 
             using (var fs = new FileStream(filePath, GetOptions(mode, FileAccess.ReadWrite, FileShare.None, FileOptions.None, preallocationSize)))
             {
-                Assert.Equal(Math.Max(initialSize, preallocationSize), fs.Length); // it was not shrinked
+                Assert.Equal(initialSize, fs.Length); // it was not changed
                 Assert.Equal(0, fs.Position);
 
                 byte[] actualContent = new byte[initialData.Length];
                 Assert.Equal(actualContent.Length, fs.Read(actualContent));
                 AssertExtensions.SequenceEqual(initialData, actualContent); // the initial content was not changed
-
-                AssertFileContentHasBeenZeroed(initialSize, (int)fs.Length, fs);
             }
         }
 

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

@@ -324,9 +324,9 @@ private void Init(string path, FileMode mode, FileAccess access, FileShare share
             }
 
             // If preallocationSize has been provided for a creatable and writeable file
-            if (FileStreamHelpers.ShouldPreallocate(preallocationSize, access, mode))
+            if (FileStreamHelpers.ShouldPreallocate(preallocationSize, access, mode, this))
             {
-                int fallocateResult = Interop.Sys.PosixFAllocate(this, 0, preallocationSize, mode);
+                int fallocateResult = Interop.Sys.PosixFAllocate(this, 0, preallocationSize);
                 if (fallocateResult != 0)
                 {
                     Dispose();

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

@@ -34,7 +34,7 @@ internal static unsafe SafeFileHandle Open(string fullPath, FileMode mode, FileA
                 // of converting DOS to NT file paths (RtlDosPathNameToRelativeNtPathName_U_WithStatus is not documented)
                 SafeFileHandle fileHandle = CreateFile(fullPath, mode, access, share, options);
 
-                if (FileStreamHelpers.ShouldPreallocate(preallocationSize, access, mode))
+                if (FileStreamHelpers.ShouldPreallocate(preallocationSize, access, mode, fileHandle))
                 {
                     Preallocate(fullPath, preallocationSize, fileHandle, mode);
                 }
@@ -106,20 +106,8 @@ private static unsafe SafeFileHandle CreateFile(string fullPath, FileMode mode,
             return fileHandle;
         }
 
-        // preallocationSize must be ignored for non-seekable files, unsupported file systems
-        // and other failures other than ERROR_DISK_FULL and ERROR_FILE_TOO_LARGE
         private static unsafe void Preallocate(string fullPath, long preallocationSize, SafeFileHandle fileHandle, FileMode mode)
         {
-            if (mode == FileMode.OpenOrCreate)
-            {
-                Interop.Kernel32.FILE_STANDARD_INFO info;
-                if (!Interop.Kernel32.GetFileInformationByHandleEx(fileHandle, Interop.Kernel32.FileStandardInfo, &info, (uint)sizeof(Interop.Kernel32.FILE_STANDARD_INFO))
-                    || info.EndOfFile >= preallocationSize) // existing files must NOT be shrinked
-                {
-                    return;
-                }
-            }
-
             var eofInfo = new Interop.Kernel32.FILE_END_OF_FILE_INFO
             {
                 EndOfFile = preallocationSize
@@ -131,6 +119,8 @@ private static unsafe void Preallocate(string fullPath, long preallocationSize,
                 &eofInfo,
                 (uint)sizeof(Interop.Kernel32.FILE_END_OF_FILE_INFO)))
             {
+                // preallocationSize must be ignored for non-seekable files, unsupported file systems
+                // and other failures other than ERROR_DISK_FULL and ERROR_FILE_TOO_LARGE
                 int errorCode = Marshal.GetLastPInvokeError();
                 if (errorCode != Interop.Errors.ERROR_DISK_FULL && errorCode != Interop.Errors.ERROR_FILE_TOO_LARGE)
                 {

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

@@ -58,10 +58,11 @@ e is UnauthorizedAccessException ||
             e is NotSupportedException ||
             (e is ArgumentException && !(e is ArgumentNullException));
 
-        internal static bool ShouldPreallocate(long preallocationSize, FileAccess access, FileMode mode)
+        internal static bool ShouldPreallocate(long preallocationSize, FileAccess access, FileMode mode, SafeFileHandle fileHandle)
             => preallocationSize > 0
                && (access & FileAccess.Write) != 0
-               && mode != FileMode.Open && mode != FileMode.Append;
+               && mode != FileMode.Open && mode != FileMode.Append
+               && (mode != FileMode.OpenOrCreate || fileHandle.CanSeek && RandomAccess.GetFileLength(fileHandle) == 0); // allow to extend only new files
 
         internal static void ValidateArguments(string path, FileMode mode, FileAccess access, FileShare share, int bufferSize, FileOptions options, long preallocationSize)
         {