overhaul cancellation to fix resource leaks and dangerous behavior with signals

this commit addresses two issues:

1. a race condition, whereby a cancellation request occurring after a
syscall returned from kernelspace but before the subsequent
CANCELPT_END would cause cancellable resource-allocating syscalls
(like open) to leak resources.

2. signal handlers invoked while the thread was blocked at a
cancellation point behaved as if asynchronous cancellation mode wer in
effect, resulting in potentially dangerous state corruption if a
cancellation request occurs.

the glibc/nptl implementation of threads shares both of these issues.

with this commit, both are fixed. however, cancellation points
encountered in a signal handler will not be acted upon if the signal
was received while the thread was already at a cancellation point.
they will of course be acted upon after the signal handler returns, so
in real-world usage where signal handlers quickly return, it should
not be a problem. it's possible to solve this problem too by having
sigaction() wrap all signal handlers with a function that uses a
pthread_cleanup handler to catch cancellation, patch up the saved
context, and return into the cancellable function that will catch and
act upon the cancellation. however that would be a lot of complexity
for minimal if any benefit...

2 files changed, 2 insertions, 0 deletions

diff --git a/src/select/pselect.c b/src/select/pselect.c
index 63ea0695..155a6eb0 100644
--- a/src/select/pselect.c
+++ b/src/select/pselect.c
@@ -10,6 +10,7 @@ int pselect(int n, fd_set *rfds, fd_set *wfds, fd_set *efds, const struct timesp
 	if (ts) ts_tmp = *ts;
 	CANCELPT_BEGIN;
 	r = syscall(SYS_pselect6, n, rfds, wfds, efds, ts ? &ts_tmp : 0, data);
+	CANCELPT_TRY;
 	CANCELPT_END;
 	return r;
 }
diff --git a/src/select/select.c b/src/select/select.c
index 12322718..b38e7fd2 100644
--- a/src/select/select.c
+++ b/src/select/select.c
@@ -7,6 +7,7 @@ int select(int n, fd_set *rfds, fd_set *wfds, fd_set *efds, struct timeval *tv)
 	int r;
 	CANCELPT_BEGIN;
 	r = syscall(SYS_select, n, rfds, wfds, efds, tv);
+	CANCELPT_TRY;
 	CANCELPT_END;
 	return r;
 }