fix multiple stdio functions' behavior on zero-length operations

previously, fgets, fputs, fread, and fwrite completely omitted locking
and access to the FILE object when their arguments yielded a zero
length read or write operation independent of the FILE state. this
optimization was invalid; it wrongly skipped marking the stream as
byte-oriented (a C conformance bug) and exposed observably missing
synchronization (a POSIX conformance bug) where one of these functions
could wrongly complete despite another thread provably holding the
lock.

(cherry picked from commit 6e2bb7acf42589fb7130b039d0623e2ca42503dd)

src/stdio/fgets.c

@@ -10,14 +10,16 @@ char *fgets(char *restrict s, int n, FILE *restrict f)
 	size_t k;
 	int c;
 
+	FLOCK(f);
+
 	if (n--<=1) {
+		f->mode |= f->mode-1;
+		FUNLOCK(f);
 		if (n) return 0;
 		*s = 0;
 		return s;
 	}
 
-	FLOCK(f);
-
 	while (n) {
 		z = memchr(f->rpos, '\n', f->rend - f->rpos);
 		k = z ? z - f->rpos + 1 : f->rend - f->rpos;

src/stdio/fputs.c

@@ -3,9 +3,7 @@
 
 int fputs(const char *restrict s, FILE *restrict f)
 {
-	size_t l = strlen(s);
-	if (!l) return 0;
-	return (int)fwrite(s, l, 1, f) - 1;
+	return (int)fwrite(s, strlen(s), 1, f) - 1;
 }
 
 weak_alias(fputs, fputs_unlocked);

src/stdio/fread.c

@@ -8,11 +8,10 @@ size_t fread(void *restrict destv, size_t size, size_t nmemb, FILE *restrict f)
 	unsigned char *dest = destv;
 	size_t len = size*nmemb, l = len, k;
 
-	/* Never touch the file if length is zero.. */
-	if (!l) return 0;
-
 	FLOCK(f);
 
+	f->mode |= f->mode-1;
+
 	if (f->rend - f->rpos > 0) {
 		/* First exhaust the buffer. */
 		k = MIN(f->rend - f->rpos, l);

src/stdio/fwrite.c

@@ -28,7 +28,6 @@ size_t __fwritex(const unsigned char *restrict s, size_t l, FILE *restrict f)
 size_t fwrite(const void *restrict src, size_t size, size_t nmemb, FILE *restrict f)
 {
 	size_t k, l = size*nmemb;
-	if (!l) return l;
 	FLOCK(f);
 	k = __fwritex(src, l, f);
 	FUNLOCK(f);