This project began as a an attempt to allow a FIFO to be opened multiple times for reading. The initial motivation was that Midnight Commander running under tcsh does this (unsuccessfully on Cygwin). See
https://sourceware.org/pipermail/cygwin/2019-December/243317.html It quickly became apparent, however, that the current code doesn't even properly handle the case where the FIFO is *explicitly* opened only once for reading, but additional readers are created via dup/fork/exec. This explained some of the bugs reported by Kristian Ivarsson. See, for example, the thread starting here: https://sourceware.org/pipermail/cygwin/2020-March/000206.html as well as later similar threads. [The discussion continued in private email, with many bug reports and test programs by Kristian. I'm very grateful to him for his reports and testing.] The first 10 patches in this series make some improvements and bug fixes that came up along the way and don't specifically relate to multiple readers. The next 10 patches, with the exception of "allow fc_handler list to grow dynamically", add the support for multiple readers. The last one updates the commentary at the beginning of fhandler_fifo.cc that tries to explain how it all works. The key ideas in these patches are: 1. Use shared memory, so that all readers have the necessary information about the writers that are open. 2. Designate one reader as the "owner". This reader runs a thread that listens for connections and keeps track of the writers. 3. Use a second shared memory block to be used for transfer of ownership. Ownership must be transferred when the owner closes or execs. And a reader that wants to read or run select must take ownership in order to be able to poll the writers for input. The patches in this series have been applied to the topic/fifo branch in case it's easier to review/test them there. Ken Brown (21): Cygwin: FIFO: minor change - use NtClose Cygwin: FIFO: simplify the fifo_client_handler structure Cygwin: FIFO: change the fifo_client_connect_state enum Cygwin: FIFO: simplify the listen_client_thread code Cygwin: FIFO: remove the arm method Cygwin: FIFO: honor the flags argument in dup Cygwin: FIFO: dup/fork/exec: make sure child starts unlocked Cygwin: FIFO: fix hit_eof Cygwin: FIFO: make opening a writer more robust Cygwin: FIFO: use a cygthread instead of a homemade thread Cygwin: FIFO: add shared memory Cygwin: FIFO: keep track of the number of readers Cygwin: FIFO: introduce a new type, fifo_reader_id_t Cygwin: FIFO: designate one reader as owner Cygwin: FIFO: allow fc_handler list to grow dynamically Cygwin: FIFO: add a shared fifo_client_handler list Cygwin: FIFO: take ownership on exec Cygwin: FIFO: find a new owner when closing Cygwin: FIFO: allow any reader to take ownership Cygwin: FIFO: support opening multiple readers Cygwin: FIFO: update commentary winsup/cygwin/fhandler.h | 208 ++++- winsup/cygwin/fhandler_fifo.cc | 1564 ++++++++++++++++++++++---------- winsup/cygwin/select.cc | 48 +- 3 files changed, 1311 insertions(+), 509 deletions(-) -- 2.21.0
