fix ftello result for append streams with unflushed output

when there is unflushed output, ftello (and ftell) compute the logical
stream position as the underlying file descriptor's offset plus an
adjustment for the amount of buffered data. however, this can give the
wrong result for append-mode streams where the unflushed writes should
adjust the logical position to be at the end of the file, as if a seek
to end-of-file takes place before the write.

the solution turns out to be a simple trick: when ftello (indirectly)
calls lseek to determine the current file offset, use SEEK_END instead
of SEEK_CUR if the stream is append-mode and there's unwritten
buffered data.

the ISO C rules regarding switching between reading and writing for a
stream opened in an update mode, along with the POSIX rules regarding
switching "active handles", conveniently leave undefined the
hypothetical usage cases where this fix might lead to observably
incorrect offsets.

the bug being fixed was discovered via the test case for glibc issue

src/internal/stdio_impl.h

@@ -17,6 +17,7 @@
 #define F_EOF 16
 #define F_ERR 32
 #define F_SVB 64
+#define F_APP 128
 
 struct _IO_FILE {
 	unsigned flags;

src/stdio/__fdopen.c

@@ -33,6 +33,7 @@ FILE *__fdopen(int fd, const char *mode)
 	if (*mode == 'a') {
 		int flags = __syscall(SYS_fcntl, fd, F_GETFL);
 		__syscall(SYS_fcntl, fd, F_SETFL, flags | O_APPEND);
+		f->flags |= F_APP;
 	}
 
 	f->fd = fd;

src/stdio/ftell.c

@@ -4,7 +4,9 @@
 
 off_t __ftello_unlocked(FILE *f)
 {
-	off_t pos = f->seek(f, 0, SEEK_CUR);
+	off_t pos = f->seek(f, 0,
+		(f->flags & F_APP) && f->wpos > f->wbase
+		? SEEK_END : SEEK_CUR);
 	if (pos < 0) return pos;
 
 	/* Adjust for data in buffer. */