merge revision(s) 50637: [Backport #11172]

	* win32/win32.c (setup_overlapped): seek to the file end only when
	  writing (mode:a), not reading (mode:a+, read).


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_2@50836 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

1 files changed, 8 insertions, 4 deletions

diff --git a/win32/win32.c b/win32/win32.c
index b7d7e32935..fc2c99edf1 100644
--- a/win32/win32.c
+++ b/win32/win32.c
@@ -6318,12 +6318,16 @@ rb_w32_close(int fd)
 }
 
 static int
-setup_overlapped(OVERLAPPED *ol, int fd)
+setup_overlapped(OVERLAPPED *ol, int fd, int iswrite)
 {
     memset(ol, 0, sizeof(*ol));
     if (!(_osfile(fd) & (FDEV | FPIPE))) {
 	LONG high = 0;
-	DWORD method = _osfile(fd) & FAPPEND ? FILE_END : FILE_CURRENT;
+	/* On mode:a, it can write only FILE_END.
+	 * On mode:a+, though it can write only FILE_END,
+	 * it can read from everywhere.
+	 */
+	DWORD method = ((_osfile(fd) & FAPPEND) && iswrite) ? FILE_END : FILE_CURRENT;
 	DWORD low = SetFilePointer((HANDLE)_osfhnd(fd), 0, &high, method);
 #ifndef INVALID_SET_FILE_POINTER
 #define INVALID_SET_FILE_POINTER ((DWORD)-1)
@@ -6420,7 +6424,7 @@ rb_w32_read(int fd, void *buf, size_t size)
 
     /* if have cancel_io, use Overlapped I/O */
     if (cancel_io) {
-	if (setup_overlapped(&ol, fd)) {
+	if (setup_overlapped(&ol, fd, FALSE)) {
 	    MTHREAD_ONLY(LeaveCriticalSection(&_pioinfo(fd)->lock));
 	    return -1;
 	}
@@ -6550,7 +6554,7 @@ rb_w32_write(int fd, const void *buf, size_t size)
 
     /* if have cancel_io, use Overlapped I/O */
     if (cancel_io) {
-	if (setup_overlapped(&ol, fd)) {
+	if (setup_overlapped(&ol, fd, TRUE)) {
 	    MTHREAD_ONLY(LeaveCriticalSection(&_pioinfo(fd)->lock));
 	    return -1;
 	}