Add ability to load config from the environment (#20)

In certain platforms, setting application secrets is easier with
environment variables. Add a new function that can use environment
variables to override configuration values loaded from other places,
like a file.

Author: bluekeyes

githubapp/config.go

@@ -14,6 +14,11 @@
 
 package githubapp
 
+import (
+	"os"
+	"strconv"
+)
+
 type Config struct {
 	WebURL   string `yaml:"web_url" json:"webUrl"`
 	V3APIURL string `yaml:"v3_api_url" json:"v3ApiUrl"`
@@ -30,3 +35,33 @@ type Config struct {
 		ClientSecret string `yaml:"client_secret" json:"clientSecret"`
 	} `yaml:"oauth" json:"oauth"`
 }
+
+// SetValuesFromEnv sets values in the configuration from coresponding
+// environment variables, if they exist. The optional prefix is added to the
+// start of the environment variable names.
+func (c *Config) SetValuesFromEnv(prefix string) {
+	setStringFromEnv("GITHUB_WEB_URL", prefix, &c.WebURL)
+	setStringFromEnv("GITHUB_V3_API_URL", prefix, &c.V3APIURL)
+	setStringFromEnv("GITHUB_V4_API_URL", prefix, &c.V4APIURL)
+
+	setIntFromEnv("GITHUB_APP_INTEGRATION_ID", prefix, &c.App.IntegrationID)
+	setStringFromEnv("GITHUB_APP_WEBHOOK_SECRET", prefix, &c.App.WebhookSecret)
+	setStringFromEnv("GITHUB_APP_PRIVATE_KEY", prefix, &c.App.PrivateKey)
+
+	setStringFromEnv("GITHUB_OAUTH_CLIENT_ID", prefix, &c.OAuth.ClientID)
+	setStringFromEnv("GITHUB_OAUTH_CLIENT_SECRET", prefix, &c.OAuth.ClientSecret)
+}
+
+func setStringFromEnv(key, prefix string, value *string) {
+	if v, ok := os.LookupEnv(prefix + key); ok {
+		*value = v
+	}
+}
+
+func setIntFromEnv(key, prefix string, value *int) {
+	if v, ok := os.LookupEnv(prefix + key); ok {
+		if i, err := strconv.Atoi(v); err == nil {
+			*value = i
+		}
+	}
+}

githubapp/config_test.go

@@ -0,0 +1,131 @@
+// Copyright 2019 Palantir Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package githubapp
+
+import (
+	"os"
+	"reflect"
+	"testing"
+)
+
+func TestSetValuesFromEnv(t *testing.T) {
+	tests := map[string]struct {
+		Input     func(*Config)
+		Prefix    string
+		Variables map[string]string
+		Output    func(*Config)
+	}{
+		"noVariables": {
+			Input: func(c *Config) {
+				c.WebURL = "https://github.com"
+				c.App.WebhookSecret = "secrethookvalue"
+			},
+			Output: func(c *Config) {
+				c.WebURL = "https://github.com"
+				c.App.WebhookSecret = "secrethookvalue"
+			},
+		},
+		"overwriteExisting": {
+			Input: func(c *Config) {
+				c.WebURL = "https://github.com"
+			},
+			Variables: map[string]string{
+				"GITHUB_WEB_URL": "https://github.company.domain",
+			},
+			Output: func(c *Config) {
+				c.WebURL = "https://github.company.domain"
+			},
+		},
+		"allVariables": {
+			Variables: map[string]string{
+				"GITHUB_WEB_URL":             "https://github.company.domain",
+				"GITHUB_V3_API_URL":          "https://github.company.domain/api/v3",
+				"GITHUB_V4_API_URL":          "https://github.company.domain/api/graphql",
+				"GITHUB_APP_INTEGRATION_ID":  "4",
+				"GITHUB_APP_WEBHOOK_SECRET":  "secrethookvalue",
+				"GITHUB_APP_PRIVATE_KEY":     "-----BEGIN RSA PRIVATE KEY-----\nxxx\nxxx\nxxx\n-----END RSA PRIVATE KEY-----",
+				"GITHUB_OAUTH_CLIENT_ID":     "92faf4b9146f3278",
+				"GITHUB_OAUTH_CLIENT_SECRET": "b00f7ea6d59dd5c9578c48f9391e71db",
+			},
+			Output: func(c *Config) {
+				c.WebURL = "https://github.company.domain"
+				c.V3APIURL = "https://github.company.domain/api/v3"
+				c.V4APIURL = "https://github.company.domain/api/graphql"
+				c.App.IntegrationID = 4
+				c.App.WebhookSecret = "secrethookvalue"
+				c.App.PrivateKey = "-----BEGIN RSA PRIVATE KEY-----\nxxx\nxxx\nxxx\n-----END RSA PRIVATE KEY-----"
+				c.OAuth.ClientID = "92faf4b9146f3278"
+				c.OAuth.ClientSecret = "b00f7ea6d59dd5c9578c48f9391e71db"
+			},
+		},
+		"withPrefix": {
+			Input: func(c *Config) {
+				c.WebURL = "https://github.com"
+			},
+			Prefix: "TEST_",
+			Variables: map[string]string{
+				"TEST_GITHUB_WEB_URL": "https://github.company.domain",
+			},
+			Output: func(c *Config) {
+				c.WebURL = "https://github.company.domain"
+			},
+		},
+		"emptyValues": {
+			Input: func(c *Config) {
+				c.WebURL = "https://github.com"
+			},
+			Variables: map[string]string{
+				"GITHUB_WEB_URL": "",
+			},
+			Output: func(c *Config) {
+				c.WebURL = ""
+			},
+		},
+	}
+
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			for k, v := range test.Variables {
+				if err := os.Setenv(k, v); err != nil {
+					t.Fatalf("failed to set environment variable: %s: %v", k, err)
+				}
+			}
+
+			defer func() {
+				for k := range test.Variables {
+					if err := os.Unsetenv(k); err != nil {
+						t.Fatalf("failed to clear environment variable: %s: %v", k, err)
+					}
+				}
+			}()
+
+			var in Config
+			if test.Input != nil {
+				test.Input(&in)
+			}
+
+			var out Config
+			if test.Output != nil {
+				test.Output(&out)
+			}
+
+			in.SetValuesFromEnv(test.Prefix)
+
+			if !reflect.DeepEqual(out, in) {
+				t.Errorf("incorrect configuration\nexpected: %+v\n  actual: %+v", out, in)
+			}
+		})
+	}
+}