InfluxdbWriter: actually verify TLS server certificates

And add a new option ssl_insecure_noverify to explicitly disable it if desired.

doc/09-object-types.md

@@ -1640,6 +1640,7 @@ Configuration Attributes:
   username                  | String                | **Optional.** InfluxDB user name. Defaults to `none`.
   password                  | String                | **Optional.** InfluxDB user password.  Defaults to `none`.
   ssl\_enable               | Boolean               | **Optional.** Whether to use a TLS stream. Defaults to `false`.
+  ssl\_insecure\_noverify   | Boolean               | **Optional.** Disable TLS peer verification.
   ssl\_ca\_cert             | String                | **Optional.** Path to CA certificate to validate the remote host.
   ssl\_cert                 | String                | **Optional.** Path to host certificate to present to the remote host for mutual verification.
   ssl\_key                  | String                | **Optional.** Path to host key to accompany the ssl\_cert.

lib/perfdata/influxdbwriter.cpp

@@ -211,6 +211,18 @@ OptionalTlsStream InfluxdbWriter::Connect()
 				<< "TLS handshake with host '" << GetHost() << "' failed.";
 			throw;
 		}
+
+		if (!GetSslInsecureNoverify()) {
+			if (!tlsStream.GetPeerCertificate()) {
+				BOOST_THROW_EXCEPTION(std::runtime_error("InfluxDB didn't present any TLS certificate."));
+			}
+
+			if (!tlsStream.IsVerifyOK()) {
+				BOOST_THROW_EXCEPTION(std::runtime_error(
+					"TLS certificate validation failed: " + std::string(tlsStream.GetVerifyError())
+				));
+			}
+		}
 	}
 
 	return std::move(stream);

lib/perfdata/influxdbwriter.ti

@@ -29,6 +29,9 @@ class InfluxdbWriter : ConfigObject
 	[config] bool ssl_enable {
 		default {{{ return false; }}}
 	};
+	[config] bool ssl_insecure_noverify {
+		default {{{ return false; }}}
+	};
 	[config] String ssl_ca_cert {
 		default {{{ return ""; }}}
 	};