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Cygwin: pipe: improve writing when pipe buffer is almost full 

So far fhandler_pipe_fifo::raw_write always returns -1/EINTR
if a signal arrived.  Linux does not do that if there's still
space left in the pipe buffer.

The Linux buffer handling can't be emulated by Cygwin, but we
can do something similar which makes it much more likely to still
write successfully even if the buffer is almost full.

Utilize pipe_data_available to return valid pipe buffer usage
to raw_write, allowing a more sophisticated way to fill the
buffer while maintaining comaptibility with non-Cygwin pipes.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
This commit is contained in:
Corinna Vinschen 2024-08-18 22:07:16 +02:00
parent 1c5f4dcdc5
commit 170e6badb6
3 changed files with 75 additions and 18 deletions
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42
winsup/cygwin/fhandler/pipe.cc
View File

@ -13,6 +13,7 @@ details. */
#include "security.h"
#include "path.h"
#include "fhandler.h"
#include "select.h"
#include "dtable.h"
#include "cygheap.h"
#include "pinfo.h"
@ -433,6 +434,7 @@ fhandler_pipe_fifo::raw_write (const void *ptr, size_t len)
NTSTATUS status = STATUS_SUCCESS;
IO_STATUS_BLOCK io;
HANDLE evt;
bool short_write_once = false;
if (!len)
return 0;
@ -471,6 +473,7 @@ fhandler_pipe_fifo::raw_write (const void *ptr, size_t len)
len1 = chunk;
else
len1 = (ULONG) left;
/* NtWriteFile returns success with # of bytes written == 0 if writing
on a non-blocking pipe fails because the pipe buffer doesn't have
sufficient space.
@ -517,6 +520,13 @@ fhandler_pipe_fifo::raw_write (const void *ptr, size_t len)
}
else
cygwait (select_sem, 10, cw_cancel);
/* If we got a timeout in the blocking case, and we already
did a short write, we got a signal in the previous loop. */
if (waitret == WAIT_TIMEOUT && short_write_once)
{
waitret = WAIT_SIGNALED;
break;
}
}
/* Loop in case of blocking write or SA_RESTART */
while (waitret == WAIT_TIMEOUT || waitret == WAIT_SIGNALED);
@ -533,13 +543,39 @@ fhandler_pipe_fifo::raw_write (const void *ptr, size_t len)
status = STATUS_THREAD_CANCELED;
else if (waitret == WAIT_SIGNALED)
status = STATUS_THREAD_SIGNALED;
else if (waitret == WAIT_TIMEOUT && io.Status == STATUS_CANCELLED)
status = STATUS_SUCCESS;
else
status = io.Status;
}
if (!is_nonblocking () || !NT_SUCCESS (status) || io.Information > 0
|| len <= PIPE_BUF)
if (status != STATUS_THREAD_SIGNALED && !NT_SUCCESS (status))
break;
if (io.Information > 0 || len <= PIPE_BUF || short_write_once)
break;
/* Independent of being blocking or non-blocking, if we're here,
the pipe has less space than requested. If the pipe is a
non-Cygwin pipe, just try the old strategy of trying a half
write. If the pipe has at
least PIPE_BUF bytes available, try to write all matching
PIPE_BUF sized blocks. If it's less than PIPE_BUF, try
the next less power of 2 bytes. This is not really the Linux
strategy because Linux is filling the pages of a pipe buffer
in a very implementation-defined way we can't emulate, but it
resembles it closely enough to get useful results. */
ssize_t avail = pipe_data_available (-1, this, get_handle (),
PDA_WRITE);
if (avail < 1) /* error or pipe closed */
break;
if (avail >= len1) /* somebody read from the pipe */
avail = len1;
if (avail == 1) /* 1 byte left or non-Cygwin pipe */
len1 >>= 1;
else if (avail >= PIPE_BUF)
len1 = avail & ~(PIPE_BUF - 1);
else
len1 = 1 << (31 - __builtin_clzl (avail));
if (!is_nonblocking ())
short_write_once = true;
}
if (isclosed ()) /* A signal handler might have closed the fd. */
{
@ -569,7 +605,7 @@ fhandler_pipe_fifo::raw_write (const void *ptr, size_t len)
else
__seterrno_from_nt_status (status);
if (nbytes_now == 0)
if (nbytes_now == 0 || short_write_once)
break;
}
out:

5
winsup/cygwin/local_includes/select.h
View File

@ -139,4 +139,9 @@ public:
extern "C" int cygwin_select (int , fd_set *, fd_set *, fd_set *,
struct timeval *to);
ssize_t pipe_data_available (int, fhandler_base *, HANDLE, int);
#define PDA_WRITE 0x01
#define PDA_SELECT 0x02
#endif /* _SELECT_H_ */

44
winsup/cygwin/select.cc
View File

@ -584,13 +584,14 @@ no_verify (select_record *, fd_set *, fd_set *, fd_set *)
return 0;
}
static int
pipe_data_available (int fd, fhandler_base *fh, HANDLE h, bool writing)
ssize_t
pipe_data_available (int fd, fhandler_base *fh, HANDLE h, int flags)
{
if (fh->get_device () == FH_PIPER)
{
DWORD nbytes_in_pipe;
if (!writing && PeekNamedPipe (h, NULL, 0, NULL, &nbytes_in_pipe, NULL))
if (!(flags & PDA_WRITE)
&& PeekNamedPipe (h, NULL, 0, NULL, &nbytes_in_pipe, NULL))
return nbytes_in_pipe;
return -1;
}
@ -609,9 +610,17 @@ pipe_data_available (int fd, fhandler_base *fh, HANDLE h, bool writing)
access on the write end. */
select_printf ("fd %d, %s, NtQueryInformationFile failed, status %y",
fd, fh->get_name (), status);
return writing ? PIPE_BUF : -1;
switch (flags)
{
case PDA_WRITE:
return 1;
case PDA_SELECT | PDA_WRITE:
return PIPE_BUF;
default:
return -1;
}
if (writing)
}
if (flags & PDA_WRITE)
{
/* If there is anything available in the pipe buffer then signal
that. This means that a pipe could still block since you could
@ -638,20 +647,26 @@ pipe_data_available (int fd, fhandler_base *fh, HANDLE h, bool writing)
/* Note: Do not use NtQueryInformationFile() for query_hdl because
NtQueryInformationFile() seems to interfere with reading pipes
in non-cygwin apps. Instead, use PeekNamedPipe() here. */
/* Note 2: we return the number of available bytes. Select for writing
returns writable *only* if at least PIPE_BUF bytes are left in the
buffer. If we can't fetch the real number of available bytes, the
number of bytes returned depends on the caller. For select we return
PIPE_BUF to fake writability, for writing we return 1 to allow
handling this fact. */
if (fh->get_device () == FH_PIPEW && fpli.WriteQuotaAvailable == 0)
{
HANDLE query_hdl = ((fhandler_pipe *) fh)->get_query_handle ();
if (!query_hdl)
query_hdl = ((fhandler_pipe *) fh)->temporary_query_hdl ();
if (!query_hdl)
return PIPE_BUF; /* We cannot know actual write pipe space. */
if (!query_hdl) /* We cannot know actual write pipe space. */
return (flags & PDA_SELECT) ? PIPE_BUF : 1;
DWORD nbytes_in_pipe;
BOOL res =
PeekNamedPipe (query_hdl, NULL, 0, NULL, &nbytes_in_pipe, NULL);
if (!((fhandler_pipe *) fh)->get_query_handle ())
CloseHandle (query_hdl); /* Close temporary query_hdl */
if (!res)
return PIPE_BUF; /* We cannot know actual write pipe space. */
if (!res) /* We cannot know actual write pipe space. */
return (flags & PDA_SELECT) ? PIPE_BUF : 1;
fpli.WriteQuotaAvailable = fpli.InboundQuota - nbytes_in_pipe;
}
if (fpli.WriteQuotaAvailable > 0)
@ -715,10 +730,10 @@ peek_pipe (select_record *s, bool from_select)
gotone = s->read_ready = true;
goto out;
}
int n = pipe_data_available (s->fd, fh, h, false);
ssize_t n = pipe_data_available (s->fd, fh, h, PDA_SELECT);
/* On PTY masters, check if input from the echo pipe is available. */
if (n == 0 && fh->get_echo_handle ())
n = pipe_data_available (s->fd, fh, fh->get_echo_handle (), false);
n = pipe_data_available (s->fd, fh, fh->get_echo_handle (), PDA_SELECT);
if (n < 0)
{
@ -759,7 +774,7 @@ out:
gotone += s->except_ready = true;
return gotone;
}
int n = pipe_data_available (s->fd, fh, h, true);
ssize_t n = pipe_data_available (s->fd, fh, h, PDA_SELECT | PDA_WRITE);
select_printf ("write: %s, n %d", fh->get_name (), n);
gotone += s->write_ready = (n >= PIPE_BUF);
if (n < 0 && s->except_selected)
@ -972,7 +987,8 @@ peek_fifo (select_record *s, bool from_select)
out:
if (s->write_selected)
{
int n = pipe_data_available (s->fd, fh, fh->get_handle (), true);
ssize_t n = pipe_data_available (s->fd, fh, fh->get_handle (),
PDA_SELECT | PDA_WRITE);
select_printf ("write: %s, n %d", fh->get_name (), n);
gotone += s->write_ready = (n >= PIPE_BUF);
if (n < 0 && s->except_selected)
@ -1398,7 +1414,7 @@ out:
HANDLE h = ptys->get_output_handle ();
if (s->write_selected)
{
int n = pipe_data_available (s->fd, fh, h, true);
ssize_t n = pipe_data_available (s->fd, fh, h, PDA_SELECT | PDA_WRITE);
select_printf ("write: %s, n %d", fh->get_name (), n);
gotone += s->write_ready = (n >= PIPE_BUF);
if (n < 0 && s->except_selected)