Re: "how did it ever work"... on Solaris, because of the st_mtime failure, it doesn't even get a chance to fail on the subsequent st_atime.  I suspect the only platform that's exercised the utimensat() to date is Linux, and either a) os.stat() doesn't affect atime on Linux, b) everyone has atime disabled, c) the two stat calls happen quick enough that no measurable difference is observed against st_atime.

As for POSIX:

http://pubs.opengroup.org/onlinepubs/9699919799/toc.htm

"The fstat() function shall update any time-related fields (as described in XBD File Times Update ), before writing into the stat structure."

The referenced section:

4.8 File Times Update

Each file has three distinct associated timestamps: the time of last data access, the time of last data modification, and the time the file status last changed. These values are returned in the file characteristics structure struct stat, as described in <sys/stat.h> .

Each function or utility in POSIX.1-2008 that reads or writes data (even if the data does not change) or performs an operation to change file status (even if the file status does not change) indicates which of the appropriate timestamps shall be marked for update. If an implementation of such a function or utility marks for update one of these timestamps in a place or time not specified by POSIX.1-2008, this shall be documented, except that any changes caused by pathname resolution need not be documented. For the other functions or utilities in POSIX.1-2008 (those that are not explicitly required to read or write file data or change file status, but that in some implementations happen to do so), the effect is unspecified.

An implementation may update timestamps that are marked for update immediately, or it may update such timestamps periodically. At the point in time when an update occurs, any marked timestamps shall be set to the current time and the update marks shall be cleared. All timestamps that are marked for update shall be updated when the file ceases to be open by any process or before a fstat(), fstatat(), fsync(), futimens(), lstat(), stat(), utime(), utimensat(), or utimes() is successfully performed on the file. Other times at which updates are done are unspecified. Marks for update, and updates themselves, shall not be done for files on read-only file systems; see Read-Only File System .

The resolution of timestamps of files in a file system is implementation-defined, but shall be no coarser than one-second resolution. The three timestamps shall always have values that are supported by the file system. Whenever any of a file's timestamps are to be set to a value V according to the rules of the preceding paragraphs of this section, the implementation shall immediately set the timestamp to the greatest value supported by the file system that is not greater than V.