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* [newlib-cygwin] Cygwin: select: don't report read ready on a FIFO never opened for writing
@ 2022-10-19 12:30 Ken Brown
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From: Ken Brown @ 2022-10-19 12:30 UTC (permalink / raw)
To: cygwin-cvs
https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=5781871775fcba783938bc4ac645eba866711fc5
commit 5781871775fcba783938bc4ac645eba866711fc5
Author: Ken Brown <kbrown@cornell.edu>
Date: Fri Sep 23 10:24:04 2022 -0400
Cygwin: select: don't report read ready on a FIFO never opened for writing
According to POSIX and the Linux man page, select(2) is supposed to
report read ready if a file is at EOF. In the case of a FIFO, this
means that the pipe is empty and there are no writers. But there
seems to be an undocumented exception, observed on Linux and other
platforms: If no writer has ever been opened, then select(2) does not
report read ready. This can happen if a reader is opened with
O_NONBLOCK before any writers have opened.
This commit makes Cygwin consistent with those other platforms by
introducing a special EOF test, fhandler_fifo::select_hit_eof, which
returns false if there's never been a writer opened.
To implement this we use a new variable '_writer_opened' in the FIFO's
shared memory, which is set to 1 the first time a writer opens. New
methods writer_opened() and set_writer_opened() are used to test and
set this variable.
Addresses: https://cygwin.com/pipermail/cygwin/2022-September/252223.html
Diff:
---
winsup/cygwin/fhandler/fifo.cc | 1 +
winsup/cygwin/local_includes/fhandler.h | 9 +++++++++
winsup/cygwin/release/3.3.4 | 3 +++
winsup/cygwin/select.cc | 12 +++++++++++-
4 files changed, 24 insertions(+), 1 deletion(-)
diff --git a/winsup/cygwin/fhandler/fifo.cc b/winsup/cygwin/fhandler/fifo.cc
index 1d3e42908..efea508ae 100644
--- a/winsup/cygwin/fhandler/fifo.cc
+++ b/winsup/cygwin/fhandler/fifo.cc
@@ -1043,6 +1043,7 @@ writer_shmem:
set_pipe_non_blocking (get_handle (), flags & O_NONBLOCK);
nwriters_lock ();
inc_nwriters ();
+ set_writer_opened ();
SetEvent (write_ready);
ResetEvent (writer_opening);
nwriters_unlock ();
diff --git a/winsup/cygwin/local_includes/fhandler.h b/winsup/cygwin/local_includes/fhandler.h
index aad7f4c37..b012c6e8f 100644
--- a/winsup/cygwin/local_includes/fhandler.h
+++ b/winsup/cygwin/local_includes/fhandler.h
@@ -1327,6 +1327,8 @@ struct fifo_reader_id_t
class fifo_shmem_t
{
LONG _nreaders, _nwriters;
+ /* Set to 1 the first time a writer opens. */
+ LONG _writer_opened;
fifo_reader_id_t _owner, _prev_owner, _pending_owner;
af_unix_spinlock_t _owner_lock, _reading_lock, _nreaders_lock, _nwriters_lock;
@@ -1343,6 +1345,9 @@ public:
int inc_nwriters () { return (int) InterlockedIncrement (&_nwriters); }
int dec_nwriters () { return (int) InterlockedDecrement (&_nwriters); }
+ bool writer_opened () const { return (bool) _writer_opened; }
+ void set_writer_opened () { InterlockedExchange (&_writer_opened, 1); }
+
fifo_reader_id_t get_owner () const { return _owner; }
void set_owner (fifo_reader_id_t fr_id) { _owner = fr_id; }
fifo_reader_id_t get_prev_owner () const { return _prev_owner; }
@@ -1425,6 +1430,8 @@ class fhandler_fifo: public fhandler_pipe_fifo
int nwriters () const { return shmem->nwriters (); }
int inc_nwriters () { return shmem->inc_nwriters (); }
int dec_nwriters () { return shmem->dec_nwriters (); }
+ bool writer_opened () const { return shmem->writer_opened (); }
+ void set_writer_opened () { shmem->set_writer_opened (); }
void nreaders_lock () { shmem->nreaders_lock (); }
void nreaders_unlock () { shmem->nreaders_unlock (); }
void nwriters_lock () { shmem->nwriters_lock (); }
@@ -1480,6 +1487,8 @@ public:
/* Called if we appear to be at EOF after polling fc_handlers. */
bool hit_eof () const
{ return !nwriters () && !IsEventSignalled (writer_opening); }
+ /* Special EOF test needed by select.cc:peek_fifo(). */
+ bool select_hit_eof () const { return hit_eof () && writer_opened (); }
int get_nhandlers () const { return nhandlers; }
fifo_client_handler &get_fc_handler (int i) { return fc_handler[i]; }
PUNICODE_STRING get_pipe_name ();
diff --git a/winsup/cygwin/release/3.3.4 b/winsup/cygwin/release/3.3.4
index bc74e6914..616f73ff0 100644
--- a/winsup/cygwin/release/3.3.4
+++ b/winsup/cygwin/release/3.3.4
@@ -37,3 +37,6 @@ Bug Fixes
- Fix a permission problem when writing DOS attributes on Samba.
Addresses: https://cygwin.com/pipermail/cygwin/2022-January/250629.html
+- For Linux compatibility, select(2) no longer reports read ready on a
+ FIFO that has never been opened for writing.
+ Addresses: https://cygwin.com/pipermail/cygwin/2022-September/252223.html
diff --git a/winsup/cygwin/select.cc b/winsup/cygwin/select.cc
index 76ab91bbb..2fd7b72b6 100644
--- a/winsup/cygwin/select.cc
+++ b/winsup/cygwin/select.cc
@@ -950,7 +950,17 @@ peek_fifo (select_record *s, bool from_select)
}
}
fh->fifo_client_unlock ();
- if (!nconnected && fh->hit_eof ())
+ /* According to POSIX and the Linux man page, we're supposed to
+ report read ready if the FIFO is at EOF, i.e., if the pipe is
+ empty and there are no writers. But there seems to be an
+ undocumented exception, observed on Linux and other platforms
+ (https://cygwin.com/pipermail/cygwin/2022-September/252223.html):
+ If no writer has ever been opened, then we do not report read
+ ready. This can happen if a reader is opened with O_NONBLOCK
+ before any writers have opened. To be consistent with other
+ platforms, we use a special EOF test that returns false if
+ there's never been a writer opened. */
+ if (!nconnected && fh->select_hit_eof ())
{
select_printf ("read: %s, saw EOF", fh->get_name ());
gotone += s->read_ready = true;
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2022-10-19 12:30 [newlib-cygwin] Cygwin: select: don't report read ready on a FIFO never opened for writing Ken Brown
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