MINOR: stats: support rate in stats-file

Implement support for FN_RATE stat column into stat-file.

For the output part, only minimal change is required. Reuse the function
read_freq_ctr() to print the same value in both stats output and
stats-file dump.

For counter preloading, define a new utility function
preload_freq_ctr(). This can be used to initialize a freq-ctr type by
preloading previous period value. Reuse this function in load_ctr()
during stats-file parsing.

At the moment, no rate column is defined as generic. Thus, this commit
does not have functional change. This will be changed as soon as FN_RATE
are converted to generic columns.

include/haproxy/freq_ctr.h

@@ -32,6 +32,14 @@ ullong freq_ctr_total(const struct freq_ctr *ctr, uint period, int pend);
 int freq_ctr_overshoot_period(const struct freq_ctr *ctr, uint period, uint freq);
 uint update_freq_ctr_period_slow(struct freq_ctr *ctr, uint period, uint inc);
 
+/* Only usable during single threaded startup phase. */
+static inline void preload_freq_ctr(struct freq_ctr *ctr, uint value)
+{
+	ctr->curr_ctr = 0;
+	ctr->prev_ctr = value;
+	ctr->curr_tick = now_ms & ~1;
+}
+
 /* Update a frequency counter by <inc> incremental units. It is automatically
  * rotated if the period is over. It is important that it correctly initializes
  * a null area.

src/stats-file.c

@@ -211,6 +211,10 @@ static int load_ctr(const struct stat_col *col, const struct ist token,
 		value.u.u64 = read_uint64(&ptr, istend(token));
 		break;
 
+	case FF_U32:
+		value.u.u32 = read_uint(&ptr, istend(token));
+		break;
+
 	default:
 		/* Unsupported field nature. */
 		return 1;
@@ -223,6 +227,9 @@ static int load_ctr(const struct stat_col *col, const struct ist token,
 	if (fn == FN_COUNTER && ff == FF_U64) {
 		*(uint64_t *)counter = value.u.u64;
 	}
+	else if (fn == FN_RATE && ff == FF_U32) {
+		preload_freq_ctr(counter, value.u.u32);
+	}
 	else {
 		/* Unsupported field format/nature combination. */
 		return 1;

src/stats.c

@@ -801,6 +801,11 @@ static struct field me_generate_field(const struct stat_col *col,
 			ABORT_NOW();
 		}
 	}
+	else if (fn == FN_RATE) {
+		/* freq-ctr always uses FF_U32 */
+		BUG_ON(stcol_format(col) != FF_U32);
+		value = mkf_u32(FN_RATE, read_freq_ctr(counter));
+	}
 	else {
 		/* No generic column available for other field nature. */
 		ABORT_NOW();