RFR: 8303942: FileMapInfo::write_bytes aborts on a short os::write

[Afshin Zafari]

On Sun, 2 Apr 2023 22:57:19 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> `os::write` is called in a loop until all the requested size is written. The number of bytes parameter(`size_t nbytes`) is casted to `ssize_t size` to be able to check `> 0` condition. To enable pointer arithmetic, the `const void *` is casted to `const char *` for addition and then recasted back.
>> 
>> ### Test
>> local: hotspot:tier1
>> mach5 tier1-5
>
> src/hotspot/share/cds/filemap.cpp line 1767:
> 
>> 1765: void FileMapInfo::write_bytes(const void* buffer, size_t nbytes) {
>> 1766:   assert(_file_open, "must be");
>> 1767:   ssize_t size = (ssize_t)nbytes;
> 
> I'm very confused as to how the underlying OS `write` can take in an unsigned count of bytes to write, yet the actual count of what was written is signed and so cannot reflect the true count for large values! Can we at least assert here that size is > 0 ?

Can this be the rationale for it?
The return value can be `-1` so the type is `ssize_t`. And it is also large enough to hold the maximum bytes transfer of `write()`:
>From the [write() man page](https://man7.org/linux/man-pages/man2/write.2.html#:~:text=of%20bytes%20written.-,On%20Linux%2C,-write()%20(and%20similar)) 

       On Linux, write() (and similar system calls) will transfer at
       most 0x7ffff000 (2,147,479,552) bytes, returning the number of
       bytes actually transferred.  (This is true on both 32-bit and
       64-bit systems.)

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/13188#discussion_r1155616420