+#include "strbuf.h"
+
+struct update_info_ctx {
+ FILE *cur_fp;
+ FILE *old_fp; /* becomes NULL if it differs from cur_fp */
+ struct strbuf cur_sb;
+ struct strbuf old_sb;
+};
+
+static void uic_mark_stale(struct update_info_ctx *uic)
+{
+ fclose(uic->old_fp);
+ uic->old_fp = NULL;
+}
+
+static int uic_is_stale(const struct update_info_ctx *uic)
+{
+ return uic->old_fp == NULL;
+}
+
+static int uic_printf(struct update_info_ctx *uic, const char *fmt, ...)
+{
+ va_list ap;
+ int ret = -1;
+
+ va_start(ap, fmt);
+
+ if (uic_is_stale(uic)) {
+ ret = vfprintf(uic->cur_fp, fmt, ap);
+ } else {
+ ssize_t r;
+ struct strbuf *cur = &uic->cur_sb;
+ struct strbuf *old = &uic->old_sb;
+
+ strbuf_reset(cur);
+ strbuf_vinsertf(cur, 0, fmt, ap);
+
+ strbuf_reset(old);
+ strbuf_grow(old, cur->len);
+ r = fread(old->buf, 1, cur->len, uic->old_fp);
+ if (r != cur->len || memcmp(old->buf, cur->buf, r))
+ uic_mark_stale(uic);
+
+ if (fwrite(cur->buf, 1, cur->len, uic->cur_fp) == cur->len)
+ ret = 0;
+ }
+
+ va_end(ap);
+
+ return ret;
+}