Prometheus: Allow plaintext Content-Type

In general, the openmetrics Content-Type is preferred. The code makes
an exception out of browsers, however.

The full logic is

- If the request has no Accept header, Content-Type will be openmetrics
  1.0.0.

- If the Accept header expects neither openmetrics nor plaintext,
  Content-Type will be plaintext 0.0.4.

This is because the client is usually a browser (which seem to typically
lack Openmetrics handlers) or a generic HTTP client like curl (which
don't really care about Content-Type).

- Otherwise Fort will decide between openmetrics 1.0.0 or plaintext
  0.0.4, depending on q-values. (Requested version will be ignored,
  because only one is supported for each.) If their q is the same,
  openmetrics will be preferred.

For #50.

diff --git a/src/prometheus.c b/src/prometheus.c
index 561a443eb9f37e853fbcd54419b547a588f00a91..093cc7edbf54c377681949740d322356958446b5 100644 (file)
--- a/src/prometheus.c
+++ b/src/prometheus.c
@@ -1,5 +1,6 @@
 #include "prometheus.h"
 
+#include <stdio.h>
 #include <string.h>
 #include <microhttpd.h>
 
@@ -13,7 +14,10 @@
 #define MHD_RESULT int
 #endif
 
-#define CONTENT_TYPE "application/openmetrics-text; version=1.0.0; charset=utf-8"
+#define OPENMETRICS_CT \
+       "application/openmetrics-text; version=1.0.0; charset=utf-8"
+#define PLAINTEXT_CT \
+       "text/plain; version=0.0.4; charset=utf-8"
 
 static struct MHD_Daemon *prometheus_daemon;
 
@@ -31,6 +35,58 @@ respond(struct MHD_Connection *conn, char *msg, unsigned int status)
        return result;
 }
 
+static float
+find_q(char const *accept, char const *ct)
+{
+       char const *value;
+       char const *limit;
+       char const *qstr;
+       float q;
+
+       value = strstr(accept, ct);
+       if (!value)
+               return 0;
+
+       limit = strchr(value, ',');
+       if (!limit)
+               limit = value + strlen(value);
+
+       qstr = strstr(value, ";q=");
+       if (!qstr || qstr > limit)
+               return 1;
+       return (sscanf(qstr, ";q=%f", &q) == EOF) ? 0.5 /* Shrug */ : q;
+}
+
+static void
+set_content_type(struct MHD_Connection *conn, struct MHD_Response *res)
+{
+       char const *accept;
+       float om_q, txt_q;
+       char const *ct;
+       MHD_RESULT ret;
+
+       accept = MHD_lookup_connection_value (conn, MHD_HEADER_KIND,
+           MHD_HTTP_HEADER_ACCEPT);
+       if (accept != NULL) {
+               om_q = find_q(accept, "application/openmetrics-text");
+               txt_q = find_q(accept, "text/plain");
+
+               if (om_q < 0.001f && txt_q < 0.001f)
+                       /* Likely a browser; these tend to prefer plaintext. */
+                       ct = PLAINTEXT_CT;
+               else
+                       ct = (om_q >= txt_q) ? OPENMETRICS_CT : PLAINTEXT_CT;
+       } else {
+               ct = OPENMETRICS_CT;
+       }
+
+       ret = MHD_add_response_header(res, "Content-Type", ct);
+       if (ret != MHD_YES) {
+               pr_op_debug("Could not set Content-Type HTTP header.");
+               /* Keep going; maybe the client won't care. */
+       }
+}
+
 static MHD_RESULT
 send_metrics(struct MHD_Connection *conn)
 {
@@ -41,6 +97,7 @@ send_metrics(struct MHD_Connection *conn)
        pr_op_debug("Handling Prometheus request...");
 
        stats = stats_export();
+
 #if MHD_VERSION > 0x00096000
        res = MHD_create_response_from_buffer_with_free_callback(strlen(stats),
            stats, free);
@@ -49,17 +106,13 @@ send_metrics(struct MHD_Connection *conn)
            MHD_RESPMEM_MUST_FREE);
 #endif
 
-       ret = MHD_add_response_header(res, "Content-Type", CONTENT_TYPE);
-       if (ret != MHD_YES) {
-               pr_op_debug("Could not set Content-Type HTTP header.");
-               /* Keep going; maybe the client won't care. */
-       }
+       set_content_type(conn, res);
 
        ret = MHD_queue_response(conn, MHD_HTTP_OK, res);
        MHD_destroy_response(res);
 
        pr_op_debug("Prometheus request handled.");
-       return MHD_YES;
+       return ret;
 }
 
 static MHD_RESULT