fix: preserve newlines in config_updater insert/update — no extra newlines on key replacement, add leading nl when inserting at end without trailing nl

src/config_updater.c

@@ -151,19 +151,23 @@ static int insert_or_replace_option(char **buf, size_t *buf_len, size_t *buf_cap
     char *opt_pos = find_option(*buf, *buf_len, option);
 
     if (opt_pos) {
-        // Find option line end
-        char *line_end = strchr(opt_pos, '\n');
-
-        if (!line_end) line_end = *buf + *buf_len;
+        // Determine if old line has trailing \n
+        char *nl = strchr(opt_pos, '\n');
+        int has_trailing_nl = (nl != NULL);
+        char *line_end = nl ? nl : *buf + *buf_len;
 
         // Prepare new line
         char new_line[MAX_LINE_LEN];
-        int new_len = snprintf(new_line, sizeof(new_line), "%s=%s\n", option, value);
-
+        int new_len;
+        if (has_trailing_nl) {
+            new_len = snprintf(new_line, sizeof(new_line), "%s=%s\n", option, value);
+        } else {
+            new_len = snprintf(new_line, sizeof(new_line), "%s=%s", option, value);
+        }
         if (new_len <= 0 || new_len >= (int)sizeof(new_line)) return -1;
 
         // Calculate length difference
-        size_t old_line_len = line_end - opt_pos + 1;
+        size_t old_line_len = has_trailing_nl ? (line_end - opt_pos + 1) : (line_end - opt_pos);
         long len_diff = new_len - old_line_len;
 
         // Ensure buffer capacity
@@ -177,9 +181,9 @@ static int insert_or_replace_option(char **buf, size_t *buf_len, size_t *buf_cap
             opt_pos = find_option(*buf, *buf_len, option);
 
             if (!opt_pos) return -1;
-            line_end = strchr(opt_pos, '\n');
-
-            if (!line_end) line_end = *buf + *buf_len;
+            nl = strchr(opt_pos, '\n');
+            has_trailing_nl = (nl != NULL);
+            line_end = nl ? nl : *buf + *buf_len;
         }
 
         // Move content and insert new line
@@ -197,9 +201,13 @@ static int insert_or_replace_option(char **buf, size_t *buf_len, size_t *buf_cap
 
         // Find end of global section (next [ or end of buffer)
         char *insert_pos = strchr(section_end, '[');
+        int need_leading_nl = 0;
 
-        if (!insert_pos) insert_pos = *buf + *buf_len;
-        else {
+        if (!insert_pos) {
+            insert_pos = *buf + *buf_len;
+            if (*buf_len > 0 && (*buf)[*buf_len - 1] != '\n')
+                need_leading_nl = 1;
+        } else {
             // Move to beginning of next line
             char *prev_nl = insert_pos;
             while (prev_nl > *buf && *prev_nl != '\n') prev_nl--;
@@ -208,8 +216,11 @@ static int insert_or_replace_option(char **buf, size_t *buf_len, size_t *buf_cap
 
         // Prepare new line
         char new_line[MAX_LINE_LEN];
-        int new_len = snprintf(new_line, sizeof(new_line), "%s=%s\n", option, value);
-
+        int new_len;
+        if (need_leading_nl)
+            new_len = snprintf(new_line, sizeof(new_line), "\n%s=%s\n", option, value);
+        else
+            new_len = snprintf(new_line, sizeof(new_line), "%s=%s\n", option, value);
         if (new_len <= 0 || new_len >= (int)sizeof(new_line)) return -1;
 
         // Ensure buffer capacity
@@ -224,9 +235,13 @@ static int insert_or_replace_option(char **buf, size_t *buf_len, size_t *buf_cap
 
             if (!section_end) return -1;
             insert_pos = strchr(section_end, '[');
+            need_leading_nl = 0;
 
-            if (!insert_pos) insert_pos = *buf + *buf_len;
-            else {
+            if (!insert_pos) {
+                insert_pos = *buf + *buf_len;
+                if (*buf_len > 0 && (*buf)[*buf_len - 1] != '\n')
+                    need_leading_nl = 1;
+            } else {
                 char *prev_nl = insert_pos;
                 while (prev_nl > *buf && *prev_nl != '\n') prev_nl--;
                 if (*prev_nl == '\n') insert_pos = prev_nl + 1;