vfs: default to Unix semantics in MemFS

Default to Unix semantics in MemFS. The Windows semantics are left as
configurable for unit tests that seek to ensure we don't remove open
files, when possible.

There are existing cases where we cannot satisfy Windows semantics, and
Go does not support opening files with the appropriate
`FILE_SHARE_DELETE` permission bit (see golang/go#34681,
golang/go#32088). The MemFS's implementation of Windows semantics is
useful for ensuring we don't regress in the cases where we can satisfy
Windows semantics.

Close cockroachdb#2064.
Informs cockroachdb#1236.

ingest_test.go

@@ -1630,6 +1630,7 @@ func TestIngestCleanup(t *testing.T) {
 	for _, tc := range testCases {
 		t.Run("", func(t *testing.T) {
 			mem := vfs.NewMem()
+			mem.UseWindowsSemantics(true)
 
 			// Create the files in the VFS.
 			metaMap := make(map[base.FileNum]vfs.File)

vfs/mem_fs.go

@@ -79,10 +79,23 @@ type MemFS struct {
 
 	strict      bool
 	ignoreSyncs bool
+	// Windows has peculiar semantics with respect to hard links and deleting
+	// open files. In tests meant to exercise this behavior, this flag can be
+	// set to error if removing an open file.
+	windowsSemantics bool
 }
 
 var _ FS = &MemFS{}
 
+// UseWindowsSemantics configures whether the MemFS implements Windows-style
+// semantics, in particular with respect to whether any of an open file's links
+// may be removed. Windows semantics default to off.
+func (y *MemFS) UseWindowsSemantics(windowsSemantics bool) {
+	y.mu.Lock()
+	defer y.mu.Unlock()
+	y.windowsSemantics = windowsSemantics
+}
+
 // String dumps the contents of the MemFS.
 func (y *MemFS) String() string {
 	y.mu.Lock()
@@ -305,11 +318,14 @@ func (y *MemFS) Remove(fullname string) error {
 			if !ok {
 				return oserror.ErrNotExist
 			}
-			// Disallow removal of open files/directories which implements Windows
-			// semantics. This ensures that we don't regress in the ordering of
-			// operations and try to remove a file while it is still open.
-			if n := atomic.LoadInt32(&child.refs); n > 0 {
-				return oserror.ErrInvalid
+			if y.windowsSemantics {
+				// Disallow removal of open files/directories which implements
+				// Windows semantics. This ensures that we don't regress in the
+				// ordering of operations and try to remove a file while it is
+				// still open.
+				if n := atomic.LoadInt32(&child.refs); n > 0 {
+					return oserror.ErrInvalid
+				}
 			}
 			if len(child.children) > 0 {
 				return errNotEmpty