fix authentication failure when the password contains the $ symbol

config/config.go

@@ -123,7 +123,22 @@ func (ch *MySqlConfigHandler) ReloadConfig(filename string, mysqldAddress string
 		}
 	}
 
-	cfg.ValueMapper = os.ExpandEnv
+	cfg.ValueMapper = func(val string) string {
+		i := 0
+		for i < len(val) && val[i] == '$' {
+			i++
+		}
+
+		if i == 0 {
+			return val
+		}
+
+		if i%2 == 0 {
+			return strings.Repeat("$", i/2) + val[i:]
+		}
+
+		return os.ExpandEnv(val[i-1:])
+	}
 	config := &Config{}
 	m := make(map[string]MySqlConfig)
 	for _, sec := range cfg.Sections() {

config/config_test.go

@@ -153,6 +153,57 @@ func TestValidateConfig(t *testing.T) {
 		convey.So(section.Password, convey.ShouldEqual, "foo")
 		convey.So(section.EnableCleartextPlugin, convey.ShouldBeTrue)
 	})
+
+	convey.Convey("Expand variables", t, func() {
+		c := MySqlConfigHandler{
+			Config: &Config{},
+		}
+		os.Setenv("MYSQLD_EXPORTER_PASSWORD", "supersecretpassword")
+		if err := c.ReloadConfig("testdata/expand_variables.cnf", "localhost:3306", "", true, promslog.NewNopLogger()); err != nil {
+			t.Error(err)
+		}
+
+		cfg := c.GetConfig()
+		section := cfg.Sections["client.server1"]
+		convey.So(section.User, convey.ShouldEqual, "test")
+		convey.So(section.Password, convey.ShouldEqual, "foo")
+
+		section = cfg.Sections["client.env"]
+		convey.So(section.User, convey.ShouldEqual, "test2")
+		convey.So(section.Password, convey.ShouldEqual, "supersecretpassword")
+
+		section = cfg.Sections["client.notExpandEnv"]
+		convey.So(section.User, convey.ShouldEqual, "mysql_exporter")
+		convey.So(section.Password, convey.ShouldEqual, "SECRET_PA$SWORD")
+
+		section = cfg.Sections["client.envNotExists"]
+		convey.So(section.User, convey.ShouldEqual, "test")
+		convey.So(section.Password, convey.ShouldEqual, "")
+
+		section = cfg.Sections["client.envEscaped"]
+		convey.So(section.User, convey.ShouldEqual, "test")
+		convey.So(section.Password, convey.ShouldEqual, "$MYSQLD_EXPORTER_PASSWORD")
+
+		section = cfg.Sections["client.password$$Escaped"]
+		convey.So(section.User, convey.ShouldEqual, "test")
+		convey.So(section.Password, convey.ShouldEqual, "$$MYSQLD_EXPORTER_PASSWORD")
+
+		section = cfg.Sections["client.password$$$Env"]
+		convey.So(section.User, convey.ShouldEqual, "test")
+		convey.So(section.Password, convey.ShouldEqual, "supersecretpassword")
+
+		section = cfg.Sections["client.password$$$Escaped"]
+		convey.So(section.User, convey.ShouldEqual, "test")
+		convey.So(section.Password, convey.ShouldEqual, "$$$MYSQLD_EXPORTER_PASSWORD")
+
+		section = cfg.Sections["client.password$$$$Escaped"]
+		convey.So(section.User, convey.ShouldEqual, "test")
+		convey.So(section.Password, convey.ShouldEqual, "$$$$MYSQLD_EXPORTER_PASSWORD")
+
+		section = cfg.Sections["client.password$$$$$Env"]
+		convey.So(section.User, convey.ShouldEqual, "test")
+		convey.So(section.Password, convey.ShouldEqual, "supersecretpassword")
+	})
 }
 
 func TestFormDSN(t *testing.T) {

config/testdata/expand_variables.cnf

@@ -0,0 +1,30 @@
+[client.server1]
+user = test
+password = foo
+[client.env]
+user = test2
+password = $MYSQLD_EXPORTER_PASSWORD
+[client.notExpandEnv]
+user = mysql_exporter
+password = SECRET_PA$SWORD
+[client.envNotExists]
+user = test
+password = $ENV_NOT_EXISTS
+[client.envEscaped]
+user = test
+password = $$MYSQLD_EXPORTER_PASSWORD
+[client.password$$Escaped]
+user = test
+password = $$$$MYSQLD_EXPORTER_PASSWORD
+[client.password$$$Env]
+user = test
+password = $$$MYSQLD_EXPORTER_PASSWORD
+[client.password$$$Escaped]
+user = test
+password = $$$$$$MYSQLD_EXPORTER_PASSWORD
+[client.password$$$$Escaped]
+user = test
+password = $$$$$$$$MYSQLD_EXPORTER_PASSWORD
+[client.password$$$$$Env]
+user = test
+password = $$$$$MYSQLD_EXPORTER_PASSWORD