Disallow invalid identifiers as environment keys

Since the keys under `environment` will end up as the identifiers in
`export` lines, we should ensure they are valid identifiers.

    export not valid=value
    export worse; echo dangerous; _='uh oh'

env.go

@@ -3,11 +3,14 @@ package ejson2env
 import (
 	"errors"
 	"fmt"
+	"regexp"
 )
 
 var errNoEnv = errors.New("environment is not set in ejson")
 var errEnvNotMap = errors.New("environment is not a map[string]interface{}")
 
+var validIdentifierPattern = regexp.MustCompile(`\A[a-zA-Z_][a-zA-Z0-9_]*\z`)
+
 // ExtractEnv extracts the environment values from the map[string]interface{}
 // containing all secrets, and returns a map[string]string containing the
 // key value pairs. If there's an issue (the environment key doesn't exist, for
@@ -26,6 +29,12 @@ func ExtractEnv(secrets map[string]interface{}) (map[string]string, error) {
 	envSecrets := make(map[string]string, len(envMap))
 
 	for key, rawValue := range envMap {
+		// Reject keys that would be invalid environment variable identifiers
+		if !validIdentifierPattern.MatchString(key) {
+			err := fmt.Errorf("invalid identifier as key in environment: %q", key)
+
+			return nil, err
+		}
 
 		// Only export values that convert to strings properly.
 		if value, ok := rawValue.(string); ok {

env_test.go

@@ -94,6 +94,12 @@ func TestInvalidEnvironments(t *testing.T) {
 		"environment": "bad",
 	}
 
+	testBadInvalidKey := map[string]interface{}{
+		"environment": map[string]interface{}{
+			"invalid key": "test_value",
+		},
+	}
+
 	var testNoEnv map[string]interface{}
 
 	_, err := ExtractEnv(testBadNonMap)
@@ -103,6 +109,13 @@ func TestInvalidEnvironments(t *testing.T) {
 		t.Errorf("wrong error when passed a non-map environment: %s", err)
 	}
 
+	_, err = ExtractEnv(testBadInvalidKey)
+	if nil == err {
+		t.Errorf("no error when passed an environment with invalid key")
+	} else if `invalid identifier as key in environment: "invalid key"` != err.Error() {
+		t.Errorf("wrong error when passed an environment with invalid key: %s", err)
+	}
+
 	_, err = ExtractEnv(testNoEnv)
 	if nil == err {
 		t.Errorf("no error when passed a non-existent environment")
@@ -133,3 +146,45 @@ func TestEscaping(t *testing.T) {
 	}
 
 }
+
+func TestIdentifierPattern(t *testing.T) {
+	key := "ALL_CAPS123"
+	if !validIdentifierPattern.MatchString(key) {
+		t.Errorf("key should match pattern %q: %q", validIdentifierPattern, key)
+	}
+
+	key = "lowercase"
+	if !validIdentifierPattern.MatchString(key) {
+		t.Errorf("key should match pattern %q: %q", validIdentifierPattern, key)
+	}
+
+	key = "a"
+	if !validIdentifierPattern.MatchString(key) {
+		t.Errorf("key should match pattern %q: %q", validIdentifierPattern, key)
+	}
+
+	key = "_leading_underscore"
+	if !validIdentifierPattern.MatchString(key) {
+		t.Errorf("key should match pattern %q: %q", validIdentifierPattern, key)
+	}
+
+	key = "1_leading_digit"
+	if validIdentifierPattern.MatchString(key) {
+		t.Errorf("key should not match pattern %q: %q", validIdentifierPattern, key)
+	}
+
+	key = "contains whitespace"
+	if validIdentifierPattern.MatchString(key) {
+		t.Errorf("key should not match pattern %q: %q", validIdentifierPattern, key)
+	}
+
+	key = "contains-dash"
+	if validIdentifierPattern.MatchString(key) {
+		t.Errorf("key should not match pattern %q: %q", validIdentifierPattern, key)
+	}
+
+	key = "contains_special_character;"
+	if validIdentifierPattern.MatchString(key) {
+		t.Errorf("key should not match pattern %q: %q", validIdentifierPattern, key)
+	}
+}