repart: add --append-fstab parameter (#37238)

If --generate-fstab=PATH is used, there is the possibility that the
fstab file already exists, making systemd-repart fail.

This commit will add a new --append-fstab= parameter, that will read
the file and merge it with the new generated content.  Using the
comments, the command can separate the automatic-generated section from
the user-provided section, allowing for the next append the replacement
only of the automatic-generated section, keeping the user one.

src/basic/string-util.c

@@ -1383,6 +1383,46 @@ char* find_line_startswith(const char *haystack, const char *needle) {
         return p + strlen(needle);
 }
 
+char* find_line(const char *haystack, const char *needle) {
+        char *p;
+
+        assert(haystack);
+        assert(needle);
+
+        /* Finds the first line in 'haystack' that match the specified string. Returns a pointer to the
+         * beginning of the line */
+
+        p = find_line_startswith(haystack, needle);
+        if (!p)
+                return NULL;
+
+        if (*p == 0 || strchr(NEWLINE, *p))
+                return p - strlen(needle);
+
+        return NULL;
+}
+
+char* find_line_after(const char *haystack, const char *needle) {
+        char *p;
+
+        assert(haystack);
+        assert(needle);
+
+        /* Finds the first line in 'haystack' that match the specified string. Returns a pointer to the
+         * next line after it */
+
+        p = find_line_startswith(haystack, needle);
+        if (!p)
+                return NULL;
+
+        if (*p == 0)
+                return p;
+        if (strchr(NEWLINE, *p))
+                return p + 1;
+
+        return NULL;
+}
+
 bool version_is_valid(const char *s) {
         if (isempty(s))
                 return false;

src/basic/string-util.h

@@ -279,6 +279,8 @@ char* strdupcspn(const char *a, const char *reject);
         })
 
 char* find_line_startswith(const char *haystack, const char *needle);
+char* find_line(const char *haystack, const char *needle);
+char* find_line_after(const char *haystack, const char *needle);
 
 bool version_is_valid(const char *s);