Errors serving rpc client

[jchavarri]

Using latest dune from main branch (commit eed25bf), I am seeing these errors every time I run it on watch mode:

Error: encountered error serving rpc client (id 0)
File "src/csexp_rpc/io_buffer.ml", line 45, characters 2-8: Assertion
failed
backtrace:
Raised at Csexp_rpc__io_buffer.write_char_exn in file
"src/csexp_rpc/io_buffer.ml", line 45, characters 2-41
Called from Stdlib__List.iter in file "list.ml", line 110, characters
12-15
Called from Csexp_rpc__io_buffer.write_csexps.loop in file
"src/csexp_rpc/io_buffer.ml", line 76, characters 6-29
Called from Stdlib__List.iter in file "list.ml", line 110, characters
12-15
Called from Csexp_rpc__io_buffer.write_csexps.loop in file
"src/csexp_rpc/io_buffer.ml", line 76, characters 6-29
Called from Stdlib__List.iter in file "list.ml", line 110, characters
12-15
Called from Csexp_rpc__io_buffer.write_csexps.loop in file
"src/csexp_rpc/io_buffer.ml", line 76, characters 6-29
Called from Stdlib__List.iter in file "list.ml", line 110, characters
12-15
Called from Csexp_rpc__io_buffer.write_csexps.loop in file
"src/csexp_rpc/io_buffer.ml", line 76, characters 6-29
Called from Stdlib__List.iter in file "list.ml", line 110, characters
12-15
Called from Csexp_rpc__io_buffer.write_csexps.loop in file
"src/csexp_rpc/io_buffer.ml", line 76, characters 6-29
Called from Stdlib__List.iter in file "list.ml", line 110, characters
12-15
Called from Csexp_rpc.Session.write in file "src/csexp_rpc/csexp_rpc.ml",
line 240, characters 10-46
Called from Fiber__scheduler.exec in file "otherlibs/fiber/src/scheduler.ml",
line 73, characters 8-11

Also, this one appears after the first compiler error message:

Error: encountered error serving rpc client (id 0)
Sys_error("Mutex.lock: Resource deadlock avoided")
backtrace:
Raised by primitive operation at Csexp_rpc.Session.write in file
"src/csexp_rpc/csexp_rpc.ml", line 239, characters 10-32
Called from Fiber__scheduler.exec in file "otherlibs/fiber/src/scheduler.ml",
line 73, characters 8-11

[nojb]

Maybe related to #7319? cc @rgrinberg

[rgrinberg]

Indeed it's related.