Improve reading config file (#38)

* Fix config file reading for GCF

* Allow specifying config path via env variable

* Automatically fall back to other file paths if config file can't be found

core/configuration/dt_config_file_reader.go

@@ -17,7 +17,9 @@ package configuration
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"io/ioutil"
+	"os"
 )
 
 type fileConfig struct {
@@ -55,10 +57,46 @@ type configFileReader interface {
 type jsonConfigFileReader struct {
 }
 
-// ReadConfigFromFile looks for a config file "dtconfig.json" in the current directory and attempts to parse it.
+// readConfigFromFile looks for a config file "dtconfig.json" and attempts to parse it.
 // Returns an error if the file can't be read or the parsing fails.
-func (j *jsonConfigFileReader) readConfigFromFile() (fileConfig, error) {
-	return j.readConfigFromFileByPath("./dtconfig.json")
+func (j *jsonConfigFileReader) readConfigFromFile() (cfg fileConfig, err error) {
+	filePaths := j.configFilePaths()
+
+	if len(filePaths) == 0 {
+		return fileConfig{}, errors.New("could not determine any file paths to read config file from")
+	}
+
+	for _, filePath := range filePaths {
+		cfg, err = j.readConfigFromFileByPath(filePath)
+		if err == nil {
+			return cfg, nil
+		}
+	}
+
+	return cfg, err
+}
+
+// configFilePaths returns all possible file paths to look for the config file in.
+func (j *jsonConfigFileReader) configFilePaths() []string {
+	var filePaths []string
+
+	if configFilePathFromEnv := os.Getenv("DT_CONFIG_FILE_PATH"); configFilePathFromEnv != "" {
+		filePaths = append(filePaths, configFilePathFromEnv)
+	}
+
+	// When running in a Google Cloud Functions Go runtime, we need to find the config file at a different path.
+	// For reference on the K_SERVICE environment variable, see:
+	// https://cloud.google.com/functions/docs/configuring/env-var#runtime_environment_variables_set_automatically
+	_, inGcf := os.LookupEnv("K_SERVICE")
+	if inGcf {
+		// In the GCF Go runtime, the root directory of your function source code is
+		// beneath the current working directory at ./serverless_function_source_code
+		// See https://cloud.google.com/functions/docs/concepts/execution-environment#memory-file-system
+		filePaths = append(filePaths, "./serverless_function_source_code/dtconfig.json")
+	}
+
+	filePaths = append(filePaths, "./dtconfig.json")
+	return filePaths
 }
 
 func (j *jsonConfigFileReader) readConfigFromFileByPath(filePath string) (fileConfig, error) {

core/configuration/dt_config_file_reader_test.go

@@ -15,6 +15,7 @@
 package configuration
 
 import (
+	"os"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -26,6 +27,16 @@ func TestJsonConfigFileReader_NoErrorForValidFile(t *testing.T) {
 	assert.NoError(t, err)
 }
 
+func TestJsonConfigFileReader_CustomPath(t *testing.T) {
+	os.Setenv("DT_CONFIG_FILE_PATH", "./testdata/subfolder/dtconfig_test_valid.json")
+	defer os.Unsetenv("DT_CONFIG_FILE_PATH")
+
+	reader := jsonConfigFileReader{}
+	cfg, err := reader.readConfigFromFile()
+	assert.NoError(t, err)
+	assert.Equal(t, "subfolder_config", cfg.Tenant)
+}
+
 func TestJsonConfigFileReader_ErrorForInvalidFile(t *testing.T) {
 	reader := jsonConfigFileReader{}
 	_, err := reader.readConfigFromFileByPath("./testdata/dtconfig_test_invalid.json")

core/configuration/testdata/subfolder/dtconfig_test_valid.json

@@ -0,0 +1,29 @@
+{
+    "AgentActive": true,
+    "ClusterID": 12345,
+    "Tenant": "subfolder_config",
+    "Connection": {
+        "AuthToken": "dt0a01.schnitzel.xsdffdedr",
+        "BaseUrl": "https://ag.xyz.com"
+    },
+    "RUM": {
+        "ClientIpHeaders": [
+            "x-forwarded-for"
+        ]
+    },
+    "Testability": {
+        "SpanProcessingIntervalMs": 3000,
+        "KeepAliveIntervalMs": 30000,
+        "MetricCollectionIntervalMs": 10000,
+        "MetricCollectionsPerExport": 6
+    },
+    "Logging": {
+        "Destination": "stderr",
+        "Go": {
+            "Flags": "Exporter=true,Propagator=false"
+        }
+    },
+    "Debug": {
+        "AddStackOnStart": true
+    }
+}