c - Writing programs to cope with I/O errors causing lost writes on Linux
TL;DR: If the Linux kernel loses a buffered I/O write, is there any way for the application to find out?

fsync() returns -EIO if the kernel lost a write
(Note: early part references older kernels; updated below to reflect modern kernels)
It looks like async buffer write-out in end_buffer_async_write(...) failures set an -EIO flag on the failed dirty buffer page for the file:
set_bit(AS_EIO, &page->mapping->flags);
set_buffer_write_io_error(bh);
clear_buffer_uptodate(bh);
SetPageError(page);

which is then detected by wait_on_page_writeback_range(...) as called by do_sync_mapping_range(...) as called by sys_sync_file_range(...) as called by sys_sync_file_range2(...) to implement the C library call fsync().
But only once!
This comment on  sys_sync_file_range
168  * SYNC_FILE_RANGE_WAIT_BEFORE and SYNC_FILE_RANGE_WAIT_AFTER will detect any
169  * I/O errors or ENOSPC conditions and will return those to the caller, after
170  * clearing the EIO and ENOSPC flags in the address_space.

suggests that when fsync() returns -EIO or (undocumented in the manpage) -ENOSPC, it will clear the error state so a subsequent fsync() will report success even though the pages never got written.
Sure enough wait_on_page_writeback_range(...) clears the error bits when it tests them:
301         /* Check for outstanding write errors */
302         if (test_and_clear_bit(AS_ENOSPC, &mapping->flags))
303                 ret = -ENOSPC;
304         if (test_and_clear_bit(AS_EIO, &mapping->flags))
305                 ret = -EIO;

So if the application expects it can re-try fsync() until it succeeds and trust that the data is on-disk, it is terribly wrong.
I'm pretty sure this is the source of the data corruption I found in the DBMS. It retries fsync() and thinks all will be well when it succeeds.
Is this allowed?
The POSIX/SuS docs on fsync() don't really specify this either way:

If the fsync() function fails, outstanding I/O operations are not guaranteed to have been completed.

Linux's man-page for fsync() just doesn't say anything about what happens on failure.
So it seems that the meaning of fsync() errors is "I don't know what happened to your writes, might've worked or not, better try again to be sure".
Newer kernels
On 4.9 end_buffer_async_write sets -EIO on the page, just via mapping_set_error.
    buffer_io_error(bh, ", lost async page write");
    mapping_set_error(page->mapping, -EIO);
    set_buffer_write_io_error(bh);
    clear_buffer_uptodate(bh);
    SetPageError(page);

On the sync side I think it's similar, though the structure is now pretty complex to follow. filemap_check_errors in mm/filemap.c now does:
    if (test_bit(AS_EIO, &mapping->flags) &&
        test_and_clear_bit(AS_EIO, &mapping->flags))
            ret = -EIO;

which has much the same effect. Error checks seem to all go through filemap_check_errors which does a test-and-clear:
    if (test_bit(AS_EIO, &mapping->flags) &&
        test_and_clear_bit(AS_EIO, &mapping->flags))
            ret = -EIO;
    return ret;

I'm using btrfs on my laptop, but when I create an ext4 loopback for testing on /mnt/tmp and set up a perf probe on it:
sudo dd if=/dev/zero of=/tmp/ext bs=1M count=100
sudo mke2fs -j -T ext4 /tmp/ext
sudo mount -o loop /tmp/ext /mnt/tmp

sudo perf probe filemap_check_errors

sudo perf record -g -e probe:end_buffer_async_write -e probe:filemap_check_errors dd if=/dev/zero of=/mnt/tmp/test bs=4k count=1 conv=fsync

I find the following call stack in perf report -T:
        ---__GI___libc_fsync
           entry_SYSCALL_64_fastpath
           sys_fsync
           do_fsync
           vfs_fsync_range
           ext4_sync_file
           filemap_write_and_wait_range
           filemap_check_errors

A read-through suggests that yeah, modern kernels behave the same.
This seems to mean that if fsync() (or presumably write() or close()) returns -EIO, the file is in some undefined state between when you last successfully fsync()d or close()d it and its most recently write()ten state.
Test
I've implemented a test case to demonstrate this behaviour.
Implications
A DBMS can cope with this by entering crash recovery. How on earth is a normal user application supposed to cope with this? The fsync() man page gives no warning that it means "fsync-if-you-feel-like-it" and I expect a lot of apps won't cope well with this behaviour.
Bug reports

https://bugzilla.kernel.org/show_bug.cgi?id=194755
https://bugzilla.kernel.org/show_bug.cgi?id=194757

Further reading
lwn.net touched on this in the article "Improved block-layer error handling".
postgresql.org mailing list thread.
