os: allow case only renames on case-insensitive filesystems

Fixes #35222

src/os/file_unix.go

@@ -27,13 +27,17 @@ func rename(oldname, newname string) error {
 		// At this point we've determined the newname is bad.
 		// But just in case oldname is also bad, prioritize returning
 		// the oldname error because that's what we did historically.
-		if _, err := Lstat(oldname); err != nil {
+		// However, if the old name and new name are not the same, yet
+		// they refer to the same file, it implies a case-only
+		// rename on a case-insensitive filesystem, which is ok.
+		if ofi, err := Lstat(oldname); err != nil {
 			if pe, ok := err.(*PathError); ok {
 				err = pe.Err
 			}
 			return &LinkError{"rename", oldname, newname, err}
+		} else if newname == oldname || !SameFile(fi, ofi) {
+			return &LinkError{"rename", oldname, newname, syscall.EEXIST}
 		}
-		return &LinkError{"rename", oldname, newname, syscall.EEXIST}
 	}
 	err = syscall.Rename(oldname, newname)
 	if err != nil {

src/os/os_test.go

@@ -973,6 +973,67 @@ func TestRenameToDirFailed(t *testing.T) {
 	}
 }
 
+func TestRenameCaseDifference(pt *testing.T) {
+	from, to := "renameFROM", "RENAMEfrom"
+	tests := []struct {
+		name   string
+		create func() error
+	}{
+		{"dir", func() error {
+			return Mkdir(from, 0777)
+		}},
+		{"file", func() error {
+			fd, err := Create(from)
+			if err != nil {
+				return err
+			}
+			return fd.Close()
+		}},
+	}
+
+	for _, test := range tests {
+		pt.Run(test.name, func(t *testing.T) {
+			defer chtmpdir(t)()
+
+			if err := test.create(); err != nil {
+				t.Fatalf("failed to create test file: %s", err)
+			}
+
+			if _, err := Stat(to); err != nil {
+				// Sanity check that the underlying filesystem is not case sensitive.
+				if IsNotExist(err) {
+					t.Skipf("case sensitive filesystem")
+				}
+				t.Fatalf("stat %q, got: %q", to, err)
+			}
+
+			if err := Rename(from, to); err != nil {
+				t.Fatalf("unexpected error when renaming from %q to %q: %s", from, to, err)
+			}
+
+			fd, err := Open(".")
+			if err != nil {
+				t.Fatalf("Open .: %s", err)
+			}
+
+			// Stat does not return the real case of the file (it returns what the called asked for)
+			// So we have to use readdir to get the real name of the file.
+			dirNames, err := fd.Readdirnames(-1)
+			if err != nil {
+				t.Fatalf("readdirnames: %s", err)
+			}
+
+			if dirNamesLen := len(dirNames); dirNamesLen != 1 {
+				t.Fatalf("unexpected dirNames len, got %q, want %q", dirNamesLen, 1)
+			}
+
+			if dirNames[0] != to {
+				t.Errorf("unexpected name, got %q, want %q", dirNames[0], to)
+			}
+		})
+	}
+}
+
 func exec(t *testing.T, dir, cmd string, args []string, expect string) {
 	r, w, err := Pipe()
 	if err != nil {