Failure of rename() may cause delete files.

[fjpanag]

There is a case were rename() may fail, but also delete files in the process that it shouldn't.

This can happen when oldpath refers to a non-existent file, while newpath refers to a file that is actually there.
The code will first delete newpath, and then try to rename the first file. If this fails, newpath is lost.

The following snippet demonstrates this:

#define TEST_FILE1 "/mnt/sdcard0/file1"
#define TEST_FILE2 "/mnt/sdcard0/file2"

int fd1 = open(TEST_FILE1, O_WRONLY | O_CREAT, 0666);
write(fd1, "file1", sizeof("file1") - 1);
close(fd1);

int fd2 = open(TEST_FILE2, O_WRONLY | O_CREAT, 0666);
write(fd2, "file2", sizeof("file2") - 1);
close(fd2);

/* Rename file 1 to file 2. File 2 should be deleted
* and file 1 should take its place.
*/

rename(TEST_FILE1, TEST_FILE2);

/* Now only test file 2 should exist. */

/* Since there is no file 1, renaming bellow should fail. */

assert(rename(TEST_FILE1, TEST_FILE2) < 0);

/* ERROR! File 2 has been deleted! */

struct stat f_stat;
assert(stat(TEST_FILE2, &f_stat) == 0);