Re: should frontend tools use syncfs() ?

> +    if (!user_opts.sync_method)
> +        user_opts.sync_method = pg_strdup("fsync");

why pstrdup?

> +parse_sync_method(const char *optarg, SyncMethod *sync_method)
> +{
> +    if (strcmp(optarg, "fsync") == 0)
> +        *sync_method = SYNC_METHOD_FSYNC;
> +#ifdef HAVE_SYNCFS
> +    else if (strcmp(optarg, "syncfs") == 0)
> +        *sync_method = SYNC_METHOD_SYNCFS;
> +#endif
> +    else
> +    {
> +        pg_log_error("unrecognized sync method: %s", optarg);
> +        return false;
> +    }

This should probably give a distinct error when syncfs is not supported
than when it's truely recognized.

The patch should handle pg_dumpall, too.

Note that /bin/sync doesn't try to de-duplicate, it does just what you
tell it.

$ strace -e openat,syncfs,fsync sync / / / -f
...
openat(AT_FDCWD, "/", O_RDONLY|O_NONBLOCK) = 3
syncfs(3)                               = 0
openat(AT_FDCWD, "/", O_RDONLY|O_NONBLOCK) = 3
syncfs(3)                               = 0
openat(AT_FDCWD, "/", O_RDONLY|O_NONBLOCK) = 3
syncfs(3)                               = 0
+++ exited with 0 +++