!956 Optimize fgets API in C library

Merge pull request !956 from Wang Yaofeng/fgets_opt

musl_src.gni

@@ -2183,6 +2183,7 @@ musl_src_porting_file = [
   "src/stdio/__toread.c",
   "src/stdio/__towrite.c",
   "src/stdio/stderr.c",
+  "src/stdio/fgets.c",
   "src/internal/stdio_impl.h",
   "src/internal/vdso.c",
   "src/time/clock_gettime.c",

porting/linux/user/src/stdio/fgets.c

+#include "stdio_impl.h"
+#include <string.h>
+
+#define MIN(a,b) ((a)<(b) ? (a) : (b))
+
+char *fgets(char *restrict s, int n, FILE *restrict f)
+{
+	char *p = s;
+	unsigned char *z;
+	size_t k;
+	int c;
+
+	FLOCK(f);
+
+	if (n--<=1) {
+		f->mode |= f->mode-1;
+		FUNLOCK(f);
+		if (n) {
+			return NULL;
+		}
+		*s = '\0';
+		return s;
+	}
+
+	while (n) {
+		if (f->rpos == f->rend && __fill_buffer(f)) {
+			if (p == s || !feof(f)) {
+				s = 0;
+			}
+			break;
+		}
+
+		/* search minimal length */
+		k = MIN(f->rend - f->rpos, n);
+		z = memchr(f->rpos, '\n', k);
+		if (z != NULL) {
+			k = z - f->rpos + 1;
+		}
+		memcpy(p, f->rpos, k);
+		f->rpos += k;
+		p += k;
+		n -= k;
+		if (z || !n) {
+			break;
+		}
+	}
+	if (s) {
+		*p = 0;
+	}
+
+	FUNLOCK(f);
+
+	return s;
+}
+
+weak_alias(fgets, fgets_unlocked);