Allow setting libexecdir (/usr/libexec) from build flags

This allows building packages with different values of libexecdir with the
daemon binary automatically adapting the channel plugin dir to it, i.e. so
there is no need to specify it in the config file.

This is relevant for Suse where %{_libexecdir} expands to /usr/lib instead.

Makefile

@@ -10,9 +10,13 @@ libexecdir = $(prefix)/libexec
 datadir    = $(prefix)/share
 sysconfdir = /etc
 
+all: pkg = github.com/icinga/icinga-notifications/internal
 all:
 	mkdir -p build
-	go build -o build/ ./cmd/icinga-notifications
+	go build \
+		-o build/ \
+		-ldflags "-X '$(pkg).LibExecDir=$(libexecdir)'" \
+		./cmd/icinga-notifications
 	go build -o build/channel/ ./cmd/channel/...
 
 test:

config.example.yml

@@ -7,7 +7,7 @@
 #debug-password: "put-something-secret-here"
 
 icingaweb2-url: http://localhost/icingaweb2/
-channel-plugin-dir: /usr/libexec/icinga-notifications/channel
+#channel-plugin-dir: /usr/libexec/icinga-notifications/channel
 api-timeout: 1m
 
 database:

internal/daemon/config.go

@@ -6,20 +6,31 @@ import (
 	"github.com/goccy/go-yaml"
 	"github.com/icinga/icinga-go-library/database"
 	"github.com/icinga/icinga-go-library/logging"
+	"github.com/icinga/icinga-notifications/internal"
 	"os"
 	"time"
 )
 
 type ConfigFile struct {
 	Listen           string          `yaml:"listen" default:"localhost:5680"`
 	DebugPassword    string          `yaml:"debug-password"`
-	ChannelPluginDir string          `yaml:"channel-plugin-dir" default:"/usr/libexec/icinga-notifications/channel"`
+	ChannelPluginDir string          `yaml:"channel-plugin-dir"`
 	ApiTimeout       time.Duration   `yaml:"api-timeout" default:"1m"`
 	Icingaweb2URL    string          `yaml:"icingaweb2-url"`
 	Database         database.Config `yaml:"database"`
 	Logging          logging.Config  `yaml:"logging"`
 }
 
+// SetDefaults implements the defaults.Setter interface.
+func (c *ConfigFile) SetDefaults() {
+	if defaults.CanUpdate(c.ChannelPluginDir) {
+		c.ChannelPluginDir = internal.LibExecDir + "/icinga-notifications/channel"
+	}
+}
+
+// Assert interface compliance.
+var _ defaults.Setter = (*ConfigFile)(nil)
+
 // config holds the configuration state as a singleton. It is used from LoadConfig and Config
 var config *ConfigFile
 

internal/paths.go

@@ -0,0 +1,6 @@
+package internal
+
+// This variable exists to allow overwriting the path using `go build -ldflags "-X ...", see Makefile.
+var (
+	LibExecDir = "/usr/libexec"
+)