Values without a backing env var should not be expanded

cmd/backup/config_provider.go

@@ -4,6 +4,7 @@
 package main
 
 import (
+	"bufio"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -99,11 +100,7 @@ func loadConfigsFromEnvFiles(directory string) ([]*Config, error) {
 			continue
 		}
 		p := filepath.Join(directory, item.Name())
-		f, err := os.ReadFile(p)
-		if err != nil {
-			return nil, errwrap.Wrap(err, fmt.Sprintf("error reading %s", item.Name()))
-		}
-		envFile, err := godotenv.Unmarshal(os.ExpandEnv(string(f)))
+		envFile, err := source(p)
 		if err != nil {
 			return nil, errwrap.Wrap(err, fmt.Sprintf("error reading config file %s", p))
 		}
@@ -125,3 +122,34 @@ func loadConfigsFromEnvFiles(directory string) ([]*Config, error) {
 
 	return configs, nil
 }
+
+func source(path string) (map[string]string, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, errwrap.Wrap(err, fmt.Sprintf("error opening %s", path))
+	}
+
+	result := map[string]string{}
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := scanner.Text()
+		line = expand(line, os.LookupEnv)
+		m, err := godotenv.Unmarshal(line)
+		if err != nil {
+			return nil, errwrap.Wrap(err, fmt.Sprintf("error sourcing %s", path))
+		}
+		for key, value := range m {
+			currentValue, currentOk := os.LookupEnv(key)
+			defer func() {
+				if currentOk {
+					os.Setenv(key, currentValue)
+					return
+				}
+				os.Unsetenv(key)
+			}()
+			result[key] = value
+			os.Setenv(key, value)
+		}
+	}
+	return result, nil
+}

cmd/backup/expand.go

@@ -0,0 +1,100 @@
+// This code comes from golang standard library:
+// https://github.com/golang/go/blob/0e85fd7561de869add933801c531bf25dee9561c/src/os/env.go#L16-L96
+// This file is explicitly excluded from the top repository license.
+
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// General environment variables.
+
+package main
+
+import "fmt"
+
+// Expand replaces ${var} or $var in the string based on the mapping function.
+func expand(s string, mapping func(string) (string, bool)) string {
+	var buf []byte
+	// ${} is all ASCII, so bytes are fine for this operation.
+	i := 0
+	for j := 0; j < len(s); j++ {
+		if s[j] == '$' && j+1 < len(s) {
+			if buf == nil {
+				buf = make([]byte, 0, 2*len(s))
+			}
+			buf = append(buf, s[i:j]...)
+			shellNameInput := s[j+1:]
+			name, w := getShellName(shellNameInput)
+			if name == "" && w > 0 {
+				// Encountered invalid syntax; eat the
+				// characters.
+			} else if name == "" {
+				// Valid syntax, but $ was not followed by a
+				// name. Leave the dollar character untouched.
+				buf = append(buf, s[j])
+			} else {
+				replacement, ok := mapping(name)
+				if ok {
+					buf = append(buf, replacement...)
+				} else {
+					// preserve enclosing {}
+					if shellNameInput[0] == '{' {
+						buf = append(buf, fmt.Sprintf("${%s}", name)...)
+					} else {
+						buf = append(buf, fmt.Sprintf("$%s", name)...)
+					}
+				}
+			}
+			j += w
+			i = j + 1
+		}
+	}
+	if buf == nil {
+		return s
+	}
+	return string(buf) + s[i:]
+}
+
+// isShellSpecialVar reports whether the character identifies a special
+// shell variable such as $*.
+func isShellSpecialVar(c uint8) bool {
+	switch c {
+	case '*', '#', '$', '@', '!', '?', '-', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
+		return true
+	}
+	return false
+}
+
+// isAlphaNum reports whether the byte is an ASCII letter, number, or underscore
+func isAlphaNum(c uint8) bool {
+	return c == '_' || '0' <= c && c <= '9' || 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z'
+}
+
+// getShellName returns the name that begins the string and the number of bytes
+// consumed to extract it. If the name is enclosed in {}, it's part of a ${}
+// expansion and two more bytes are needed than the length of the name.
+func getShellName(s string) (string, int) {
+	switch {
+	case s[0] == '{':
+		if len(s) > 2 && isShellSpecialVar(s[1]) && s[2] == '}' {
+			return s[1:2], 3
+		}
+		// Scan to closing brace
+		for i := 1; i < len(s); i++ {
+			if s[i] == '}' {
+				if i == 1 {
+					return "", 2 // Bad syntax; eat "${}"
+				}
+				return s[1:i], i + 1
+			}
+		}
+		return "", 1 // Bad syntax; eat "${"
+	case isShellSpecialVar(s[0]):
+		return s[0:1], 1
+	}
+	// Scan alphanumerics.
+	var i int
+	for i = 0; i < len(s) && isAlphaNum(s[i]); i++ {
+	}
+	return s[:i], i
+}

test/confd/02backup.env

@@ -1,2 +1,3 @@
 NAME="other"
 BACKUP_CRON_EXPRESSION="*/1 * * * *"
+BACKUP_FILENAME="override-$NAME.tar.gz"

test/confd/run.sh

@@ -20,7 +20,7 @@ if [ ! -f "$LOCAL_DIR/conf.tar.gz" ]; then
 fi
 pass "Config from file was used."
 
-if [ ! -f "$LOCAL_DIR/other.tar.gz" ]; then
+if [ ! -f "$LOCAL_DIR/override-other.tar.gz" ]; then
   fail "Run on same schedule did not succeed."
 fi
 pass "Run on same schedule succeeded."