string-util: add helper for extracting n'th line of a string

src/basic/string-util.c

@@ -1131,3 +1131,64 @@ int string_truncate_lines(const char *s, size_t n_lines, char **ret) {
         *ret = copy;
         return truncation_applied;
 }
+
+int string_extract_line(const char *s, size_t i, char **ret) {
+        const char *p = s;
+        size_t c = 0;
+
+        /* Extract the i'nth line from the specified string. Returns > 0 if there are more lines after that,
+         * and == 0 if we are looking at the last line or already beyond the last line. As special
+         * optimization, if the first line is requested and the string only consists of one line we return
+         * NULL, indicating the input string should be used as is, and avoid a memory allocation for a very
+         * common case. */
+
+        for (;;) {
+                const char *q;
+
+                q = strchr(p, '\n');
+                if (i == c) {
+                        /* The line we are looking for! */
+
+                        if (q) {
+                                char *m;
+
+                                m = strndup(p, q - p);
+                                if (!m)
+                                        return -ENOMEM;
+
+                                *ret = m;
+                                return !isempty(q + 1); /* more coming? */
+                        } else {
+                                if (p == s)
+                                        *ret = NULL; /* Just use the input string */
+                                else {
+                                        char *m;
+
+                                        m = strdup(p);
+                                        if (!m)
+                                                return -ENOMEM;
+
+                                        *ret = m;
+                                }
+
+                                return 0; /* The end */
+                        }
+                }
+
+                if (!q) {
+                        char *m;
+
+                        /* No more lines, return empty line */
+
+                        m = strdup("");
+                        if (!m)
+                                return -ENOMEM;
+
+                        *ret = m;
+                        return 0; /* The end */
+                }
+
+                p = q + 1;
+                c++;
+        }
+}