io-util: add new helper fputs_with_newline()

src/basic/io-util.c

@@ -306,3 +306,19 @@ ssize_t sparse_write(int fd, const void *p, size_t sz, size_t run_length) {
 
         return q - (const uint8_t*) p;
 }
+
+int fputs_with_newline(const char *s, FILE *f) {
+        assert(s);
+        assert(f);
+
+        /* This is like fputs() but outputs a trailing newline char, but only if the string doesn't end in a
+         * newline anyway. Just like fputs() returns EOF on error. Otherwise returns 0 in case we didn't
+         * append a newline, > 0 otherwise. */
+
+        if (fputs(s, f) == EOF)
+                return EOF;
+        if (endswith(s, "\n"))
+                return 0;
+
+        return fputc('\n', f) == EOF ? EOF : 1;
+}

src/basic/io-util.h

@@ -5,6 +5,7 @@
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
+#include <stdio.h>
 #include <sys/types.h>
 
 #include "macro.h"
@@ -44,3 +45,5 @@ static inline bool FILE_SIZE_VALID_OR_INFINITY(uint64_t l) {
         return FILE_SIZE_VALID(l);
 
 }
+
+int fputs_with_newline(const char *s, FILE *f);