Hi,

While trying to understand the reason for slow completion with lsp-mode
in some situations, I noticed that the language server response is
split into 64k chunks despite setting `read-process-output-max' to 1M
(as recommended by lsp-mode maintainers).

The problem is not lsp-mode specific and can be reproduced without
additionnal packages by using the two scripts attached to this email:
- "write-server.py" is the server: it waits for a number
(requested_size) on its input and write a string of "requested_size"
bytes on its output. The file must be executable and located in the
PATH.
- "real-pipe-size.el" launch "write-server.py" and communicate with the
process using different values for `read-process-output-max'. It prints
the maximum chunk size used for each value.

On my system, I got the following output executing 'emacs -Q --script
~/Projets/emacs-pipe/real-pipe-size.el':
Real pipe size (with read-process-output-max = 4096): 4096
Real pipe size (with read-process-output-max = 32768): 32768
Real pipe size (with read-process-output-max = 65536): 65536
Real pipe size (with read-process-output-max = 65537): 65536
Real pipe size (with read-process-output-max = 204800): 65536
Real pipe size (with read-process-output-max = 1048576): 65536
Real pipe size (with read-process-output-max = 2097152): 65536

This 64k limit is GNU/Linux specific and is explained in "Pipe
capacity" section of pipe(7) manpage and can be changed calling
`fcntl'. So I added a call to `fcntl' for `F_SETPIPE_SZ' in
`create_process' function just after other uses of `fcntl' (I'm not
really sure if it's the right place). The patch is attached to this
email.

With the patch, the limit is now the value of `/proc/sys/fs/pipe-max-
size' (which default to 1M and can be change by users with the
CAP_SYS_RESOURCE capability). Output of 'emacs -Q --script
~/Projets/emacs-pipe/real-pipe-size.el' is now:
Real pipe size (with read-process-output-max = 4096): 4096
Real pipe size (with read-process-output-max = 32768): 32768
Real pipe size (with read-process-output-max = 65536): 65536
Real pipe size (with read-process-output-max = 65537): 65537
Real pipe size (with read-process-output-max = 204800): 204800
Real pipe size (with read-process-output-max = 1048576): 1048576
Real pipe size (with read-process-output-max = 2097152): 65536

And my initial problem of slow completion with lsp-mode is fixed.

P.S. I signed the FSF copyright assignment agreement for Emacs last
year.

In GNU Emacs 28.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.33,
cairo version 1.16.0)
 of 2022-05-30 built on guiriden
Repository revision: 5a223c7f2ef4c31abbd46367b6ea83cd19d30aa7
Repository branch: heads/emacs-28.1
Windowing system distributor 'The X.Org Foundation', version
11.0.12201000
System Description: Debian GNU/Linux bookworm/sid

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