fix failure of ungetc and ungetwc to work on files in eof status

these functions were written to handle clearing eof status, but failed
to account for the __toread function's handling of eof. with this
patch applied, __toread still returns EOF when the file is in eof
status, so that read operations will fail, but it also sets up valid
buffer pointers for read mode, which are set to the end of the buffer
rather than the beginning in order to make the whole buffer available
to ungetc/ungetwc.

minor changes to __uflow were needed since it's now possible to have
non-zero buffer pointers while in eof status. as made, these changes
remove a 'fast path' bypassing the function call to __toread, which
could be reintroduced with slightly different logic, but since
ordinary files have a syscall in f->read, optimizing the code path
does not seem worthwhile.

the __stdio_read function is also updated not to zero the read buffer
pointers on eof/error. while not necessary for correctness, this
change avoids the overhead of calling __toread in ungetc after
reaching eof, and it also reduces code size and increases consistency
with the fmemopen read operation which does not zero the pointers.

src/stdio/__stdio_read.c

@@ -21,7 +21,6 @@ size_t __stdio_read(FILE *f, unsigned char *buf, size_t len)
 	pthread_cleanup_pop(0);
 	if (cnt <= 0) {
 		f->flags |= F_EOF ^ ((F_ERR^F_EOF) & cnt);
-		f->rpos = f->rend = 0;
 		return cnt;
 	}
 	if (cnt <= iov[0].iov_len) return cnt;

src/stdio/__toread.c

@@ -5,12 +5,12 @@ int __toread(FILE *f)
 	f->mode |= f->mode-1;
 	if (f->wpos > f->buf) f->write(f, 0, 0);
 	f->wpos = f->wbase = f->wend = 0;
-	if (f->flags & (F_EOF|F_NORD)) {
-		if (f->flags & F_NORD) f->flags |= F_ERR;
+	if (f->flags & F_NORD) {
+		f->flags |= F_ERR;
 		return EOF;
 	}
-	f->rpos = f->rend = f->buf;
-	return 0;
+	f->rpos = f->rend = f->buf + f->buf_size;
+	return (f->flags & F_EOF) ? EOF : 0;
 }
 
 void __stdio_exit_needed(void);

src/stdio/__uflow.c

 #include "stdio_impl.h"
 
-/* This function will never be called if there is already data
- * buffered for reading. Thus we can get by with very few branches. */
+/* This function assumes it will never be called if there is already
+ * data buffered for reading. */
 
 int __uflow(FILE *f)
 {
 	unsigned char c;
-	if ((f->rend || !__toread(f)) && f->read(f, &c, 1)==1) return c;
+	if (!__toread(f) && f->read(f, &c, 1)==1) return c;
 	return EOF;
 }

src/stdio/ungetc.c

@@ -6,7 +6,8 @@ int ungetc(int c, FILE *f)
 
 	FLOCK(f);
 
-	if ((!f->rend && __toread(f)) || f->rpos <= f->buf - UNGET) {
+	if (!f->rpos) __toread(f);
+	if (!f->rpos || f->rpos <= f->buf - UNGET) {
 		FUNLOCK(f);
 		return EOF;
 	}

src/stdio/ungetwc.c

@@ -19,7 +19,8 @@ wint_t ungetwc(wint_t c, FILE *f)
 
 	f->mode |= f->mode+1;
 
-	if ((!f->rend && __toread(f)) || f->rpos < f->buf - UNGET + l) {
+	if (!f->rpos) __toread(f);
+	if (!f->rpos || f->rpos < f->buf - UNGET + l) {
 		FUNLOCK(f);
 		return EOF;
 	}