We normally check errors for all operations. But for some cleanup calls, eg
"defer file.Close()", we didn't. Now we also check and log most of those.
Partially because those errors can point to some mishandling or unexpected code
paths (eg file unexpected already closed). And in part to make it easier to use
"errcheck" to find the real missing error checks, there is too much noise now.
The log.Check function can now be used unconditionally for checking and logging
about errors. It adjusts the log level if the error is caused by a network
connection being closed, or a context is canceled or its deadline reached, or a
socket deadline is reached.
we don't want external software to include internal details like mlog.
slog.Logger is/will be the standard.
we still have mlog for its helper functions, and its handler that logs in
concise logfmt used by mox.
packages that are not meant for reuse still pass around mlog.Log for
convenience.
we use golang.org/x/exp/slog because we also support the previous Go toolchain
version. with the next Go release, we'll switch to the builtin slog.
message.Part now has a ReaderUTF8OrBinary() along with the existing Reader().
the new function returns a reader of decoded content. we now use it in a few
places, including search. we only support the charsets in
golang.org/x/text/encoding/ianaindex.
search has also been changed to not read the entire message in memory. instead,
we make one 8k buffer for reading and search in that, and we keep the buffer
around for all messages. saves quite some allocations when searching large
mailboxes.
