_       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.10.4 (2024-06-04)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |


julia> using FileWatching

julia> import Base: OS_HANDLE, preserve_handle, unpreserve_handle

julia> using Downloads

julia> import Downloads.Curl: curl_socket_t, connect_semaphore_acquire, init!,
           Multi,
           Easy,
           @check,
           CURL_POLL_IN, CURL_POLL_OUT, CURL_POLL_INOUT, CURL_POLL_REMOVE,
           CURL_CSELECT_IN, CURL_CSELECT_OUT, CURL_CSELECT_ERR,
           check_multi_info, curl_multi_assign, curl_multi_socket_action, curl_multi_add_handle

julia> _close = Ref{Bool}(true)
Base.RefValue{Bool}(true)

julia> function simulate_closed_socket(sock)
           if _close[]
               ccall(:close, Cint, (Cint,), sock)
           end
       end
simulate_closed_socket (generic function with 1 method)

julia> function Downloads.Curl.socket_callback(
           easy_h    :: Ptr{Cvoid},
           sock      :: curl_socket_t,
           action    :: Cint,
           multi_p   :: Ptr{Cvoid},
           watcher_p :: Ptr{Cvoid},
       )::Cint
           try
               if action ∉ (CURL_POLL_IN, CURL_POLL_OUT, CURL_POLL_INOUT, CURL_POLL_REMOVE)
                   @async @error("socket_callback: unexpected action", action, maxlog=1_000)
                   return -1
               end
               multi = unsafe_pointer_to_objref(multi_p)::Multi
               if watcher_p != C_NULL
                   old_watcher = unsafe_pointer_to_objref(watcher_p)::FDWatcher
                   @check curl_multi_assign(multi.handle, sock, C_NULL)
                   unpreserve_handle(old_watcher)
               end
               if action in (CURL_POLL_IN, CURL_POLL_OUT, CURL_POLL_INOUT)
                   readable = action in (CURL_POLL_IN,  CURL_POLL_INOUT)
                   writable = action in (CURL_POLL_OUT, CURL_POLL_INOUT)
                   os_handle = OS_HANDLE(sock)
                   simulate_closed_socket(sock)
                   watcher = FDWatcher(os_handle, readable, writable)
                   preserve_handle(watcher)
                   watcher_p = pointer_from_objref(watcher)
                   @check curl_multi_assign(multi.handle, sock, watcher_p)
                   task = @async while watcher.readable || watcher.writable # isopen(watcher)
                       events = try
                           wait(watcher)
                       catch err
                           err isa EOFError && return
                           err isa Base.IOError || rethrow()
                           FileWatching.FDEvent()
                       end
                       flags = CURL_CSELECT_IN  * isreadable(events) +
                               CURL_CSELECT_OUT * iswritable(events) +
                               CURL_CSELECT_ERR * (events.disconnect || events.timedout)
                       lock(multi.lock) do
                           watcher.readable || watcher.writable || return # !isopen
                           @check curl_multi_socket_action(multi.handle, sock, flags)
                           check_multi_info(multi)
                       end
                   end
                   @isdefined(errormonitor) && errormonitor(task)
               end
               @isdefined(old_watcher) && close(old_watcher)
               return 0
           catch err
               @async @error("socket_callback: unexpected error", err=err, maxlog=1_000)
               return -1
           end
       end

julia> _close[] = false                     # normal condition
false

julia> t0 = @async Downloads.request("google.com")     # this should go thru normally
Task (runnable) @0x00007f71631cc4c0

julia> wait(t0)

julia> @info("t0 done: $(istaskdone(t0))")
[ Info: t0 done: true

julia> _close[] = true                     # simulate a closed connection
true

julia> t1 = @async Downloads.request("google.com")     # this will cause the task to hang
┌ Error: socket_callback: unexpected error
│   err = IOError: FDWatcher: bad file descriptor (EBADF)
└ @ Main REPL[7]:50
┌ Error: curl_multi_socket_action: 11
└ @ Downloads.Curl /data/Work/julia/binaries/julia-1.10.4/share/julia/stdlib/v1.10/Downloads/src/Curl/utils.jl:57
Task (runnable) @0x00007f70f73de270

julia> sleep(10)

julia> @info("t1 done: $(istaskdone(t1))")
[ Info: t1 done: false

julia> @info("while t1 is hung, any new requests will also get hung")
[ Info: while t1 is hung, any new requests will also get hung

julia> t2 = @async Downloads.request("google.com")     # these will also hang
┌ Error: curl_multi_add_handle: 11
└ @ Downloads.Curl /data/Work/julia/binaries/julia-1.10.4/share/julia/stdlib/v1.10/Downloads/src/Curl/utils.jl:57
Task (runnable) @0x00007f70f6f9f080

julia> t3 = @async Downloads.request("google.com")     # these will also hang
┌ Error: curl_multi_add_handle: 11
└ @ Downloads.Curl /data/Work/julia/binaries/julia-1.10.4/share/julia/stdlib/v1.10/Downloads/src/Curl/utils.jl:57
Task (runnable) @0x00007f70f6f9c330

julia> sleep(10)

julia> @info("t2 done: $(istaskdone(t2))")
[ Info: t2 done: false

julia> @info("t3 done: $(istaskdone(t3))")
[ Info: t3 done: false