bug#6149: bug#24531: process-send-string seems to truncate lines over 4096 characters

[Spencer Baugh <sbaugh@janestreet.com>]

Eli Zaretskii <eliz@gnu.org> writes:
>> Date: Thu, 27 Jul 2023 04:48:18 +0300
>> Cc: 24531@debbugs.gnu.org, 6149@debbugs.gnu.org,
>>  Stefan Monnier <monnier@iro.umontreal.ca>, jidanni@jidanni.org
>> From: Dmitry Gutov <dmitry@gutov.dev>
>> 
>> If some syscall or etc limits the length of a string to 4096, can't we 
>> detect this case, split the string and emit said call multiple times?
>> 
>> This function's docstring already mentions the case of
>> 
>>    If STRING is larger than the input buffer of the process, ...
>>    it is sent in several bunches

Alas it's far more cursed than that.  The length of a *line* is limited
to 4096 characters.  So regardless of how big or small your buffers for
writing are, if you write more than 4095 characters before writing a
newline, the remaining characters will be discarded.  There is no way to
prevent this with ptys.

So even if we wrote one character at a time, characters would start
getting dropped after writing 4095 non-newline characters.

>
> AFAIU, that is based on the errno value returned by a 'write' call
> which attempts to write too many bytes (see the would_block function).
> I guess writes to PTYs don't do that?

Writes to PTYs do tell us when the data has been truncated.  There's
just nothing we can do with that information.