remove cancellation points in stdio

commit 58165923 added these optional
cancellation points on the basis that cancellable stdio could be
useful, to unblock threads stuck on stdio operations that will never
complete. however, the only way to ensure that cancellation can
achieve this is to violate the rules for side effects when
cancellation is acted upon, discarding knowledge of any partial data
transfer already completed. our implementation exhibited this behavior
and was thus non-conforming.

in addition to improving correctness, removing these cancellation
points moderately reduces code size, and should significantly improve
performance on i386, where sysenter/syscall instructions can be used
instead of "int $128" for non-cancellable syscalls.

src/stdio/__stdio_read.c

 #include "stdio_impl.h"
 #include <sys/uio.h>
-#include <pthread.h>
-
-static void cleanup(void *p)
-{
-	FILE *f = p;
-	if (!f->lockcount) __unlockfile(f);
-}
 
 size_t __stdio_read(FILE *f, unsigned char *buf, size_t len)
 {
@@ -16,9 +9,7 @@ size_t __stdio_read(FILE *f, unsigned char *buf, size_t len)
 	};
 	ssize_t cnt;
 
-	pthread_cleanup_push(cleanup, f);
-	cnt = syscall_cp(SYS_readv, f->fd, iov, 2);
-	pthread_cleanup_pop(0);
+	cnt = syscall(SYS_readv, f->fd, iov, 2);
 	if (cnt <= 0) {
 		f->flags |= F_EOF ^ ((F_ERR^F_EOF) & cnt);
 		return cnt;

src/stdio/__stdio_write.c

 #include "stdio_impl.h"
 #include <sys/uio.h>
-#include <pthread.h>
-
-static void cleanup(void *p)
-{
-	FILE *f = p;
-	if (!f->lockcount) __unlockfile(f);
-}
 
 size_t __stdio_write(FILE *f, const unsigned char *buf, size_t len)
 {
@@ -19,9 +12,7 @@ size_t __stdio_write(FILE *f, const unsigned char *buf, size_t len)
 	int iovcnt = 2;
 	ssize_t cnt;
 	for (;;) {
-		pthread_cleanup_push(cleanup, f);
-		cnt = syscall_cp(SYS_writev, f->fd, iov, iovcnt);
-		pthread_cleanup_pop(0);
+		cnt = syscall(SYS_writev, f->fd, iov, iovcnt);
 		if (cnt == rem) {
 			f->wend = f->buf + f->buf_size;
 			f->wpos = f->wbase = f->buf;
@@ -34,11 +25,8 @@ size_t __stdio_write(FILE *f, const unsigned char *buf, size_t len)
 		}
 		rem -= cnt;
 		if (cnt > iov[0].iov_len) {
-			f->wpos = f->wbase = f->buf;
 			cnt -= iov[0].iov_len;
 			iov++; iovcnt--;
-		} else if (iovcnt == 2) {
-			f->wbase += cnt;
 		}
 		iov[0].iov_base = (char *)iov[0].iov_base + cnt;
 		iov[0].iov_len -= cnt;

src/stdio/fopen.c

@@ -18,7 +18,7 @@ FILE *fopen(const char *restrict filename, const char *restrict mode)
 	/* Compute the flags to pass to open() */
 	flags = __fmodeflags(mode);
 
-	fd = sys_open_cp(filename, flags, 0666);
+	fd = sys_open(filename, flags, 0666);
 	if (fd < 0) return 0;
 	if (flags & O_CLOEXEC)
 		__syscall(SYS_fcntl, fd, F_SETFD, FD_CLOEXEC);