[newlib-cygwin/main] Cygwin: fix SSH hangs

[Corinna Vinschen]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=11a84cc757ef766505ed037c88c21d87b457862d

commit 11a84cc757ef766505ed037c88c21d87b457862d
Author:     Johannes Schindelin <johannes.schindelin@gmx.de>
AuthorDate: Mon Feb 24 13:22:36 2025 -0800
Commit:     Corinna Vinschen <corinna@vinschen.de>
CommitDate: Mon Feb 24 22:33:44 2025 +0100

    Cygwin: fix SSH hangs
    
    It was reported in https://github.com/git-for-windows/git/issues/5199
    that as of v3.5.4, cloning or fetching via SSH is hanging indefinitely.
    
    Bisecting the problem points to 555afcb2f3 (Cygwin: select: set pipe
    writable only if PIPE_BUF bytes left, 2024-08-18). That commit's
    intention seems to look at the write buffer, and only report the pipe as
    writable if there are more than one page (4kB) available.
    
    However, the number that is looked up is the number of bytes that are
    already in the buffer, ready to be read, and further analysis
    shows that in the scenario described in the report, the number of
    available bytes is substantially below `PIPE_BUF`, but as long as they
    are not handled, there is apparently a dead-lock.
    
    Since the old logic worked, and the new logic causes a dead-lock, let's
    essentially revert 555afcb2f3a6 ("Cygwin: select: set pipe writable only if
    PIPE_BUF bytes left").
    
    Note: This is not a straight revert, as the code in question has been
    modified subsequently, and trying to revert the original commit would
    cause merge conflicts. Therefore, the diff looks very different from the
    reverse diff of the commit whose logic is reverted.
    
    Fixes: 555afcb2f3a6 ("Cygwin: select: set pipe writable only if PIPE_BUF bytes left")
    Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Diff:
---
 winsup/cygwin/select.cc | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/winsup/cygwin/select.cc b/winsup/cygwin/select.cc
index 353f074c735a..012578886011 100644
--- a/winsup/cygwin/select.cc
+++ b/winsup/cygwin/select.cc
@@ -778,7 +778,7 @@ out:
 	}
       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);
+      gotone += s->write_ready = (n > 0);
       if (n < 0 && s->except_selected)
 	gotone += s->except_ready = true;
     }
@@ -992,7 +992,7 @@ out:
       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);
+      gotone += s->write_ready = (n > 0);
       if (n < 0 && s->except_selected)
 	gotone += s->except_ready = true;
     }
@@ -1418,7 +1418,7 @@ out:
     {
       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);
+      gotone += s->write_ready = (n > 0);
       if (n < 0 && s->except_selected)
 	gotone += s->except_ready = true;
     }