fix(backend): correctly load configuration using CONFIG_FILE env variable

Fixes #1466

cmd/karma/tests/testscript/config_file_flag_over_env.txt

@@ -0,0 +1,28 @@
+# Loads config file from flags when CONFIG_FILE env is also set
+env CONFIG_FILE=env.yaml
+karma.bin-should-work --check-config --config.file=flag.yaml
+! stdout .
+stderr 'msg="Reading configuration file flag.yaml"'
+stderr 'msg="Configuration is valid"'
+stderr 'msg="\[flag\] Configured Alertmanager source at http://localhost:8080 \(proxied: false\, readonly: false\)"'
+! stderr 'level=error'
+! stderr 'msg="Reading configuration file karma.yaml"'
+! stderr 'msg="Reading configuration file env.yaml"'
+
+-- flag.yaml --
+alertmanager:
+  servers:
+    - name: flag
+      uri: "http://localhost:8080"
+
+-- env.yaml --
+alertmanager:
+  servers:
+    - name: env
+      uri: "http://localhost:8080"
+
+-- karma.yaml --
+alertmanager:
+  servers:
+    - name: karma
+      uri: "http://localhost:8080"

cmd/karma/tests/testscript/config_file_from_env.txt

@@ -0,0 +1,15 @@
+# Loads config file from CONFIG_FILE env
+env CONFIG_FILE=foo.yaml
+karma.bin-should-work --check-config
+! stdout .
+stderr 'msg="Reading configuration file foo.yaml"'
+stderr 'msg="Configuration is valid"'
+stderr 'msg="\[cwd\] Configured Alertmanager source at http://localhost:8080 \(proxied: true\, readonly: false\)"'
+! stderr 'level=error'
+
+-- foo.yaml --
+alertmanager:
+  servers:
+    - name: cwd
+      uri: "http://localhost:8080"
+      proxy: true

cmd/karma/tests/testscript/config_file_from_flag.txt

@@ -0,0 +1,15 @@
+# Loads config file from --config.file flag
+env CONFIG_FILE=foo.yaml
+karma.bin-should-work --check-config --config.file foo.yaml
+! stdout .
+stderr 'msg="Reading configuration file foo.yaml"'
+stderr 'msg="Configuration is valid"'
+stderr 'msg="\[cwd\] Configured Alertmanager source at http://localhost:8080 \(proxied: true\, readonly: false\)"'
+! stderr 'level=error'
+
+-- foo.yaml --
+alertmanager:
+  servers:
+    - name: cwd
+      uri: "http://localhost:8080"
+      proxy: true

cmd/karma/tests/testscript/invalid_flag.txt

@@ -0,0 +1,6 @@
+# Fails on invalid flag usage
+env CONFIG_FILE=foo.yaml
+karma.bin-should-fail --check-config --invalid.flag
+! stdout .
+! stderr 'msg="Configuration is valid"'
+stderr 'level=error msg="unknown flag: --invalid.flag"'

internal/config/config.go

@@ -121,9 +121,17 @@ func SetupFlags(f *pflag.FlagSet) {
 }
 
 func readConfigFile(k *koanf.Koanf, flags *pflag.FlagSet) string {
-	configFile, _ := flags.GetString("config.file")
-	// if config.file is not passed via flags then see if there's karma.yaml in
-	// current working directory
+	var configFile string
+
+	// 1. Load file from flags is set
+	configFile, _ = flags.GetString("config.file")
+	// 2. Fallback to CONFIG_FILE env if there's no flag value
+	if configFile == "" {
+		if v, found := os.LookupEnv("CONFIG_FILE"); found {
+			configFile = v
+		}
+	}
+	// 3 see if there's karma.yaml in current working directory
 	if configFile == "" {
 		if _, err := os.Stat("karma.yaml"); !os.IsNotExist(err) {
 			configFile = "karma.yaml"