net: prometheus: Add way to format output by a metric

Instead of requiring one big buffer for formatting the output,
have a walk function that can be used to generate output by
one metric at a time.

Signed-off-by: Jukka Rissanen <[email protected]>

Author: jukkar

include/zephyr/net/prometheus/collector.h

@@ -116,6 +116,60 @@ int prometheus_collector_register_metric(struct prometheus_collector *collector,
 const void *prometheus_collector_get_metric(struct prometheus_collector *collector,
 					    const char *name);
 
+/** @cond INTERNAL_HIDDEN */
+
+enum prometheus_walk_state {
+	PROMETHEUS_WALK_START,
+	PROMETHEUS_WALK_CONTINUE,
+	PROMETHEUS_WALK_STOP,
+};
+
+struct prometheus_collector_walk_context {
+	struct prometheus_collector *collector;
+	struct prometheus_metric *metric;
+	struct prometheus_metric *tmp;
+	enum prometheus_walk_state state;
+};
+
+/** @endcond */
+
+/**
+ * @brief Walk through all metrics in a Prometheus collector and format them
+ *        into a buffer.
+ *
+ * @param ctx Pointer to the walker context.
+ * @param buffer Pointer to the buffer to store the formatted metrics.
+ * @param buffer_size Size of the buffer.
+ * @return 0 if successful and we went through all metrics, -EAGAIN if we
+ *	 need to call this function again, any other negative error code
+ *	 means an error occurred.
+ */
+int prometheus_collector_walk_metrics(struct prometheus_collector_walk_context *ctx,
+				      uint8_t *buffer, size_t buffer_size);
+
+/**
+ * @brief Initialize the walker context to walk through all metrics.
+ *
+ * @param ctx Pointer to the walker context.
+ * @param collector Pointer to the collector to walk through.
+ *
+ * @return 0 if successful, otherwise a negative error code.
+ */
+static inline int prometheus_collector_walk_init(struct prometheus_collector_walk_context *ctx,
+						 struct prometheus_collector *collector)
+{
+	if (collector == NULL) {
+		return -EINVAL;
+	}
+
+	ctx->collector = collector;
+	ctx->state = PROMETHEUS_WALK_START;
+	ctx->metric = NULL;
+	ctx->tmp = NULL;
+
+	return 0;
+}
+
 /**
  * @}
  */

include/zephyr/net/prometheus/formatter.h

@@ -22,7 +22,7 @@
  * @brief Format exposition data for Prometheus
  *
  * Formats the exposition data collected by the specified collector into the provided buffer.
- * Function to format metric data according to Prometheus text-based format
+ * Function will format metric data according to Prometheus text-based format
  *
  * @param collector Pointer to the collector containing the data to format.
  * @param buffer Pointer to the buffer where the formatted exposition data will be stored.
@@ -33,6 +33,22 @@
 int prometheus_format_exposition(struct prometheus_collector *collector, char *buffer,
 				 size_t buffer_size);
 
+/**
+ * @brief Format exposition data for one metric for Prometheus
+ *
+ * Formats the exposition data of one specific metric into the provided buffer.
+ * Function will format metric data according to Prometheus text-based format.
+ *
+ * @param metric Pointer to the metric containing the data to format.
+ * @param buffer Pointer to the buffer where the formatted exposition data will be stored.
+ * @param buffer_size Size of the buffer.
+ * @param written How many bytes have been written to the buffer.
+ *
+ * @return 0 on success, negative errno on error.
+ */
+int prometheus_format_one_metric(struct prometheus_metric *metric, char *buffer,
+				 size_t buffer_size, int *written);
+
 /**
  * @}
  */

subsys/net/lib/prometheus/collector.c

@@ -12,6 +12,7 @@
 #include <zephyr/net/prometheus/summary.h>
 #include <zephyr/net/prometheus/counter.h>
 #include <zephyr/net/prometheus/gauge.h>
+#include <zephyr/net/prometheus/formatter.h>
 
 #include <stdlib.h>
 #include <string.h>
@@ -157,3 +158,77 @@ const void *prometheus_collector_get_metric(struct prometheus_collector *collect
 out:
 	return NULL;
 }
+
+int prometheus_collector_walk_metrics(struct prometheus_collector_walk_context *ctx,
+				      uint8_t *buffer, size_t buffer_size)
+{
+	int ret = 0;
+
+	if (ctx->collector == NULL) {
+		LOG_ERR("Invalid arguments");
+		return -EINVAL;
+	}
+
+	if (ctx->state == PROMETHEUS_WALK_START) {
+		k_mutex_lock(&ctx->collector->lock, K_FOREVER);
+		ctx->state = PROMETHEUS_WALK_CONTINUE;
+
+		/* Start of the loop is taken from
+		 * SYS_SLIST_FOR_EACH_CONTAINER_SAFE macro to simulate
+		 * a loop.
+		 */
+
+		ctx->metric = Z_GENLIST_PEEK_HEAD_CONTAINER(slist,
+							    &ctx->collector->metrics,
+							    ctx->metric,
+							    node);
+		ctx->tmp = Z_GENLIST_PEEK_NEXT_CONTAINER(slist,
+							 ctx->metric,
+							 node);
+	}
+
+	if (ctx->state == PROMETHEUS_WALK_CONTINUE) {
+		int len = 0;
+
+		ctx->metric = ctx->tmp;
+		ctx->tmp = Z_GENLIST_PEEK_NEXT_CONTAINER(slist,
+							 ctx->metric,
+							 node);
+
+		if (ctx->metric == NULL) {
+			ctx->state = PROMETHEUS_WALK_STOP;
+			goto out;
+		}
+
+		/* If there is a user callback, use it to update the metric data. */
+		if (ctx->collector->user_cb) {
+			ret = ctx->collector->user_cb(ctx->collector, ctx->metric,
+						      ctx->collector->user_data);
+			if (ret < 0) {
+				if (ret != -EAGAIN) {
+					ctx->state = PROMETHEUS_WALK_STOP;
+					goto out;
+				}
+
+				/* Skip this metric for now */
+				goto out;
+			}
+		}
+
+		ret = prometheus_format_one_metric(ctx->metric, buffer, buffer_size, &len);
+		if (ret < 0) {
+			ctx->state = PROMETHEUS_WALK_STOP;
+			goto out;
+		}
+
+		ret = -EAGAIN;
+	}
+
+out:
+	if (ctx->state == PROMETHEUS_WALK_STOP) {
+		k_mutex_unlock(&ctx->collector->lock);
+		ret = 0;
+	}
+
+	return ret;
+}