import environment variables that are present

previously docker did import environment variables if they were present
but created them if they were not when it was asked via a --env-file
cli option to import but not create them.

fix is to only import the variable into the environment if it is present.

additionally do not import variable names of zero-length (which are lines
w/ a potential variable definition w/o a variable name).

refs:

- https://github.com/docker/for-linux/issues/284

Signed-off-by: Tom Klingenberg <tklingenberg@lastflood.net>

opts/envfile.go

@@ -18,5 +18,5 @@ import (
 // environment variables, that's why we just strip leading whitespace and
 // nothing more.
 func ParseEnvFile(filename string) ([]string, error) {
-	return parseKeyValueFile(filename, os.Getenv)
+	return parseKeyValueFile(filename, os.LookupEnv)
 }

opts/file.go

@@ -21,7 +21,7 @@ func (e ErrBadKey) Error() string {
 	return fmt.Sprintf("poorly formatted environment: %s", e.msg)
 }
 
-func parseKeyValueFile(filename string, emptyFn func(string) string) ([]string, error) {
+func parseKeyValueFile(filename string, emptyFn func(string) (string, bool)) ([]string, error) {
 	fh, err := os.Open(filename)
 	if err != nil {
 		return []string{}, err
@@ -53,19 +53,25 @@ func parseKeyValueFile(filename string, emptyFn func(string) string) ([]string,
 			if strings.ContainsAny(variable, whiteSpaces) {
 				return []string{}, ErrBadKey{fmt.Sprintf("variable '%s' has white spaces", variable)}
 			}
+			if len(variable) == 0 {
+				return []string{}, ErrBadKey{fmt.Sprintf("no variable name on line '%s'", line)}
+			}
 
 			if len(data) > 1 {
 				// pass the value through, no trimming
 				lines = append(lines, fmt.Sprintf("%s=%s", variable, data[1]))
 			} else {
 				var value string
+				var present bool
 				if emptyFn != nil {
-					value = emptyFn(line)
+					value, present = emptyFn(line)
 				}
+				if present {
 					// if only a pass-through variable is given, clean it up.
 					lines = append(lines, fmt.Sprintf("%s=%s", strings.TrimSpace(line), value))
 				}
 			}
 		}
+	}
 	return lines, scanner.Err()
 }

opts/parse.go

@@ -19,10 +19,10 @@ func ReadKVStrings(files []string, override []string) ([]string, error) {
 // present in the file with additional pairs specified in the override parameter.
 // If a key has no value, it will get the value from the environment.
 func ReadKVEnvStrings(files []string, override []string) ([]string, error) {
-	return readKVStrings(files, override, os.Getenv)
+	return readKVStrings(files, override, os.LookupEnv)
 }
 
-func readKVStrings(files []string, override []string, emptyFn func(string) string) ([]string, error) {
+func readKVStrings(files []string, override []string, emptyFn func(string) (string, bool)) ([]string, error) {
 	variables := []string{}
 	for _, ef := range files {
 		parsedVars, err := parseKeyValueFile(ef, emptyFn)