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Cygwin: FIFO: support opening multiple readers 

Although we can have multiple readers open because of dup/fork/exec,
the current code does not support multiple readers opening a FIFO by
explicitly calling 'open'.

The main complication in supporting this is that when a blocking
reader tries to open and there's already one open, it has to check
whether there any writers open.  It can't rely on the write_ready
event, whose state hasn't changed since the first writer opened.

To fix this, add two new named events, check_write_ready_evt and
write_ready_ok_evt, and a new method, check_write_ready().

The first event signals the owner's reader thread to call
check_write_ready(), which polls the fc_handler list to check for
connected writers.  If it finds none, it checks to see if there's a
writer in the process and then sets/resets write_ready appropriately.

When check_write_ready() finishes it sets write_ready_ok_evt to signal
the reader that write_ready has been updated.

The polling is done via fifo_client_handler::pipe_state().  As long as
it's calling that function anyway, check_write_ready() updates the
state of each handler.

Also add a new lock to prevent a race if two readers are trying to
open simultaneously.
This commit is contained in:
Ken Brown 2020-05-06 11:31:39 -04:00
parent bf66a56cca
commit 4811889e0c
2 changed files with 127 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
winsup/cygwin/fhandler.h
View File

@ -1324,7 +1324,7 @@ class fifo_shmem_t
{
LONG _nreaders;
fifo_reader_id_t _owner, _prev_owner, _pending_owner;
af_unix_spinlock_t _owner_lock, _reading_lock;
af_unix_spinlock_t _owner_lock, _reading_lock, _reader_opening_lock;
/* Info about shared memory block used for temporary storage of the
owner's fc_handler list. */
@ -1346,6 +1346,8 @@ public:
void owner_unlock () { _owner_lock.unlock (); }
void reading_lock () { _reading_lock.lock (); }
void reading_unlock () { _reading_lock.unlock (); }
void reader_opening_lock () { _reader_opening_lock.lock (); }
void reader_opening_unlock () { _reader_opening_lock.unlock (); }
int get_shared_nhandlers () const { return (int) _sh_nhandlers; }
void set_shared_nhandlers (int n) { InterlockedExchange (&_sh_nhandlers, n); }
@ -1371,6 +1373,8 @@ class fhandler_fifo: public fhandler_base
HANDLE owner_needed_evt; /* The owner is closing. */
HANDLE owner_found_evt; /* A new owner has taken over. */
HANDLE update_needed_evt; /* shared_fc_handler needs updating. */
HANDLE check_write_ready_evt; /* write_ready needs to be checked. */
HANDLE write_ready_ok_evt; /* check_write_ready is done. */
/* Handles to non-shared events needed for fifo_reader_threads. */
HANDLE cancel_evt; /* Signal thread to terminate. */
@ -1448,6 +1452,9 @@ class fhandler_fifo: public fhandler_base
{ return shmem->shared_fc_handler_updated (); }
void shared_fc_handler_updated (bool val)
{ shmem->shared_fc_handler_updated (val); }
void check_write_ready ();
void reader_opening_lock () { shmem->reader_opening_lock (); }
void reader_opening_unlock () { shmem->reader_opening_unlock (); }
public:
fhandler_fifo ();

129
winsup/cygwin/fhandler_fifo.cc
View File

@ -75,6 +75,7 @@ fhandler_fifo::fhandler_fifo ():
fhandler_base (),
read_ready (NULL), write_ready (NULL), writer_opening (NULL),
owner_needed_evt (NULL), owner_found_evt (NULL), update_needed_evt (NULL),
check_write_ready_evt (NULL), write_ready_ok_evt (NULL),
cancel_evt (NULL), thr_sync_evt (NULL), _maybe_eof (false),
fc_handler (NULL), shandlers (0), nhandlers (0),
reader (false), writer (false), duplexer (false),
@ -441,6 +442,45 @@ fhandler_fifo::update_shared_handlers ()
return 0;
}
/* The write_ready event gets set when a writer opens, to indicate
that a blocking reader can open. If a second reader wants to open,
we need to see if there are still any writers open. */
void
fhandler_fifo::check_write_ready ()
{
bool set = false;
fifo_client_lock ();
for (int i = 0; i < nhandlers && !set; i++)
switch (fc_handler[i].pipe_state ())
{
case FILE_PIPE_CONNECTED_STATE:
fc_handler[i].state = fc_connected;
set = true;
break;
case FILE_PIPE_INPUT_AVAILABLE_STATE:
fc_handler[i].state = fc_input_avail;
set = true;
break;
case FILE_PIPE_DISCONNECTED_STATE:
fc_handler[i].state = fc_disconnected;
break;
case FILE_PIPE_LISTENING_STATE:
fc_handler[i].state = fc_listening;
case FILE_PIPE_CLOSING_STATE:
fc_handler[i].state = fc_closing;
default:
fc_handler[i].state = fc_error;
break;
}
fifo_client_unlock ();
if (set || IsEventSignalled (writer_opening))
SetEvent (write_ready);
else
ResetEvent (write_ready);
SetEvent (write_ready_ok_evt);
}
static DWORD WINAPI
fifo_reader_thread (LPVOID param)
{
@ -526,13 +566,15 @@ fhandler_fifo::fifo_reader_thread_func ()
IO_STATUS_BLOCK io;
bool cancel = false;
bool update = false;
bool check = false;
status = NtFsControlFile (fc.h, conn_evt, NULL, NULL, &io,
FSCTL_PIPE_LISTEN, NULL, 0, NULL, 0);
if (status == STATUS_PENDING)
{
HANDLE w[3] = { conn_evt, update_needed_evt, cancel_evt };
switch (WaitForMultipleObjects (3, w, false, INFINITE))
HANDLE w[4] = { conn_evt, update_needed_evt,
check_write_ready_evt, cancel_evt };
switch (WaitForMultipleObjects (4, w, false, INFINITE))
{
case WAIT_OBJECT_0:
status = io.Status;
@ -544,6 +586,10 @@ fhandler_fifo::fifo_reader_thread_func ()
update = true;
break;
case WAIT_OBJECT_0 + 2:
status = STATUS_WAIT_2;
check = true;
break;
case WAIT_OBJECT_0 + 3:
status = STATUS_THREAD_IS_TERMINATING;
cancel = true;
update = true;
@ -570,6 +616,7 @@ fhandler_fifo::fifo_reader_thread_func ()
break;
case STATUS_THREAD_IS_TERMINATING:
case STATUS_WAIT_1:
case STATUS_WAIT_2:
/* Try to connect a bogus client. Otherwise fc is still
listening, and the next connection might not get recorded. */
status1 = open_pipe (ph);
@ -602,6 +649,8 @@ fhandler_fifo::fifo_reader_thread_func ()
NtClose (ph);
if (update && update_shared_handlers () < 0)
api_fatal ("Can't update shared handlers, %E");
if (check)
check_write_ready ();
if (cancel)
goto canceled;
}
@ -833,14 +882,19 @@ fhandler_fifo::open (int flags, mode_t)
and start the fifo_reader_thread. */
if (reader)
{
bool first = true;
SetEvent (read_ready);
if (create_shmem () < 0)
goto err_close_writer_opening;
if (create_shared_fc_handler () < 0)
goto err_close_shmem;
inc_nreaders ();
/* Reinitialize _sh_fc_handler_updated, which starts as 0. */
shared_fc_handler_updated (true);
reader_opening_lock ();
if (inc_nreaders () == 1)
/* Reinitialize _sh_fc_handler_updated, which starts as 0. */
shared_fc_handler_updated (true);
else
first = false;
npbuf[0] = 'n';
if (!(owner_needed_evt = CreateEvent (sa_buf, true, false, npbuf)))
{
@ -862,9 +916,23 @@ fhandler_fifo::open (int flags, mode_t)
__seterrno ();
goto err_close_owner_found_evt;
}
npbuf[0] = 'c';
if (!(check_write_ready_evt = CreateEvent (sa_buf, false, false, npbuf)))
{
debug_printf ("CreateEvent for %s failed, %E", npbuf);
__seterrno ();
goto err_close_update_needed_evt;
}
npbuf[0] = 'k';
if (!(write_ready_ok_evt = CreateEvent (sa_buf, false, false, npbuf)))
{
debug_printf ("CreateEvent for %s failed, %E", npbuf);
__seterrno ();
goto err_close_check_write_ready_evt;
}
/* Make cancel and sync inheritable for exec. */
if (!(cancel_evt = create_event (true)))
goto err_close_update_needed_evt;
goto err_close_write_ready_ok_evt;
if (!(thr_sync_evt = create_event (true)))
goto err_close_cancel_evt;
me.winpid = GetCurrentProcessId ();
@ -908,9 +976,19 @@ fhandler_fifo::open (int flags, mode_t)
}
}
}
/* Not a duplexer; wait for a writer to connect. */
else if (!wait (write_ready))
goto err_close_reader;
/* Not a duplexer; wait for a writer to connect if we're blocking. */
else if (!(flags & O_NONBLOCK))
{
if (!first)
{
/* Ask the owner to update write_ready. */
SetEvent (check_write_ready_evt);
WaitForSingleObject (write_ready_ok_evt, INFINITE);
}
if (!wait (write_ready))
goto err_close_reader;
}
reader_opening_unlock ();
goto success;
}
@ -984,6 +1062,10 @@ err_close_reader:
return 0;
err_close_cancel_evt:
NtClose (cancel_evt);
err_close_write_ready_ok_evt:
NtClose (write_ready_ok_evt);
err_close_check_write_ready_evt:
NtClose (check_write_ready_evt);
err_close_update_needed_evt:
NtClose (update_needed_evt);
err_close_owner_found_evt:
@ -993,6 +1075,7 @@ err_close_owner_needed_evt:
err_dec_nreaders:
if (dec_nreaders () == 0)
ResetEvent (read_ready);
reader_opening_unlock ();
/* err_close_shared_fc_handler: */
NtUnmapViewOfSection (NtCurrentProcess (), shared_fc_handler);
NtClose (shared_fc_hdl);
@ -1396,6 +1479,10 @@ fhandler_fifo::close ()
NtClose (owner_found_evt);
if (update_needed_evt)
NtClose (update_needed_evt);
if (check_write_ready_evt)
NtClose (check_write_ready_evt);
if (write_ready_ok_evt)
NtClose (write_ready_ok_evt);
if (cancel_evt)
NtClose (cancel_evt);
if (thr_sync_evt)
@ -1519,8 +1606,22 @@ fhandler_fifo::dup (fhandler_base *child, int flags)
__seterrno ();
goto err_close_owner_found_evt;
}
if (!DuplicateHandle (GetCurrentProcess (), check_write_ready_evt,
GetCurrentProcess (), &fhf->check_write_ready_evt,
0, !(flags & O_CLOEXEC), DUPLICATE_SAME_ACCESS))
{
__seterrno ();
goto err_close_update_needed_evt;
}
if (!DuplicateHandle (GetCurrentProcess (), write_ready_ok_evt,
GetCurrentProcess (), &fhf->write_ready_ok_evt,
0, !(flags & O_CLOEXEC), DUPLICATE_SAME_ACCESS))
{
__seterrno ();
goto err_close_check_write_ready_evt;
}
if (!(fhf->cancel_evt = create_event (true)))
goto err_close_update_needed_evt;
goto err_close_write_ready_ok_evt;
if (!(fhf->thr_sync_evt = create_event (true)))
goto err_close_cancel_evt;
inc_nreaders ();
@ -1530,6 +1631,10 @@ fhandler_fifo::dup (fhandler_base *child, int flags)
return 0;
err_close_cancel_evt:
NtClose (fhf->cancel_evt);
err_close_write_ready_ok_evt:
NtClose (fhf->write_ready_ok_evt);
err_close_check_write_ready_evt:
NtClose (fhf->check_write_ready_evt);
err_close_update_needed_evt:
NtClose (fhf->update_needed_evt);
err_close_owner_found_evt:
@ -1575,6 +1680,8 @@ fhandler_fifo::fixup_after_fork (HANDLE parent)
fork_fixup (parent, owner_needed_evt, "owner_needed_evt");
fork_fixup (parent, owner_found_evt, "owner_found_evt");
fork_fixup (parent, update_needed_evt, "update_needed_evt");
fork_fixup (parent, check_write_ready_evt, "check_write_ready_evt");
fork_fixup (parent, write_ready_ok_evt, "write_ready_ok_evt");
if (close_on_exec ())
/* Prevent a later attempt to close the non-inherited
pipe-instance handles copied from the parent. */
@ -1658,6 +1765,8 @@ fhandler_fifo::set_close_on_exec (bool val)
set_no_inheritance (owner_needed_evt, val);
set_no_inheritance (owner_found_evt, val);
set_no_inheritance (update_needed_evt, val);
set_no_inheritance (check_write_ready_evt, val);
set_no_inheritance (write_ready_ok_evt, val);
set_no_inheritance (cancel_evt, val);
set_no_inheritance (thr_sync_evt, val);
fifo_client_lock ();