write_prometheus plugin: Fall back to IPv4 socket if IPv6 fails.

Unfortunately, for libmicrohttpd "dual stack" means "enforce IPv6 (but
allow IPv4, too)". That means that on systems without IPv6,
libmicrohttpd'd initialization fails (when the dual stack flag is set).
To work around this limitation we open the socket ourselves and pass it
to libmicrohttpd to use.

Fixes: #2149

diff --git a/src/write_prometheus.c b/src/write_prometheus.c
index de1c389..fe6f1cc 100644 (file)
--- a/src/write_prometheus.c
+++ b/src/write_prometheus.c
@@ -36,6 +36,10 @@
 
 #include <microhttpd.h>
 
+#include <netdb.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+
 #ifndef PROMETHEUS_DEFAULT_STALENESS_DELTA
 #define PROMETHEUS_DEFAULT_STALENESS_DELTA TIME_T_TO_CDTIME_T_STATIC(300)
 #endif
@@ -727,6 +731,64 @@ metric_family_get(data_set_t const *ds, value_list_t const *vl, size_t ds_index,
 }
 /* }}} */
 
+static int prom_open_socket(int domain, struct sockaddr const *addr,
+                            socklen_t addrlen) {
+  int fd = socket(domain, SOCK_STREAM | SOCK_CLOEXEC, 0);
+  if (fd == -1)
+    return errno;
+
+  if (bind(fd, addr, addrlen) != 0) {
+    close(fd);
+    return -1;
+  }
+
+  if (listen(fd, /* backlog = */ 16) != 0) {
+    close(fd);
+    return -1;
+  }
+
+  return fd;
+}
+
+static struct MHD_Daemon *prom_start_daemon() {
+  struct sockaddr_in6 sa6 = {
+      .sin6_family = AF_INET6,
+      .sin6_port = htons(httpd_port),
+      .sin6_addr = IN6ADDR_ANY_INIT,
+  };
+  int fd = prom_open_socket(PF_INET6, (void *)&sa6, sizeof(sa6));
+
+  if (fd == -1) {
+    struct sockaddr_in sa4 = {
+        .sin_family = AF_INET,
+        .sin_port = htons(httpd_port),
+        .sin_addr =
+            {
+                .s_addr = INADDR_ANY,
+            },
+    };
+    fd = prom_open_socket(PF_INET, (void *)&sa4, sizeof(sa4));
+  }
+
+  if (fd == -1) {
+    ERROR("write_prometheus plugin: Opening a listening socket failed.");
+    return NULL;
+  }
+
+  struct MHD_Daemon *d =
+      MHD_start_daemon(MHD_USE_THREAD_PER_CONNECTION, 0,
+                       /* MHD_AcceptPolicyCallback = */ NULL,
+                       /* MHD_AcceptPolicyCallback arg = */ NULL, http_handler,
+                       NULL, MHD_OPTION_LISTEN_SOCKET, fd, MHD_OPTION_END);
+  if (d == NULL) {
+    ERROR("write_prometheus plugin: MHD_start_daemon() failed.");
+    close(fd);
+    return NULL;
+  }
+
+  return d;
+}
+
 /*
  * collectd callbacks
  */
@@ -760,15 +822,7 @@ static int prom_init() {
   }
 
   if (httpd == NULL) {
-    unsigned int flags = MHD_USE_THREAD_PER_CONNECTION;
-#if MHD_VERSION >= 0x00093300
-    flags |= MHD_USE_DUAL_STACK;
-#endif
-
-    httpd = MHD_start_daemon(flags, httpd_port,
-                             /* MHD_AcceptPolicyCallback = */ NULL,
-                             /* MHD_AcceptPolicyCallback arg = */ NULL,
-                             http_handler, NULL, MHD_OPTION_END);
+    httpd = prom_start_daemon();
     if (httpd == NULL) {
       ERROR("write_prometheus plugin: MHD_start_daemon() failed.");
       return -1;