Add option in Config to not remove prefix when expanding default values which resolve to other environment variables

envconfig.go

@@ -15,7 +15,7 @@
 // Package envconfig populates struct fields based on environment variable
 // values (or anything that responds to "Lookup"). Structs declare their
 // environment dependencies using the "env" tag with the key being the name of
-// the environment variable, case sensitive.
+// the environment variable, case-sensitive.
 //
 //	type MyStruct struct {
 //	  A string `env:"A"` // resolves A to $A
@@ -264,8 +264,13 @@ type Config struct {
 	// default value is false.
 	DefaultRequired bool
 
-	// Mutators is an optiona list of mutators to apply to lookups.
+	// Mutators is an optional list of mutators to apply to lookups.
 	Mutators []Mutator
+
+	// KeepPrefixExpansion specifies if prefixes are removed when expanding default
+	// values that reference other environment variables. Set to true to keep any
+	// prefix.
+	KeepPrefixExpansion bool
 }
 
 // Process decodes the struct using values from environment variables. See
@@ -336,6 +341,8 @@ func processWith(ctx context.Context, c *Config) error {
 	structDecodeUnset := c.DefaultDecodeUnset
 	structRequired := c.DefaultRequired
 
+	unwrapOnExpand := !c.KeepPrefixExpansion
+
 	mutators := c.Mutators
 
 	for i := 0; i < t.NumField(); i++ {
@@ -428,7 +435,7 @@ func processWith(ctx context.Context, c *Config) error {
 			// Lookup the value, ignoring an error if the key isn't defined. This is
 			// required for nested structs that don't declare their own `env` keys,
 			// but have internal fields with an `env` defined.
-			val, found, usedDefault, err := lookup(key, required, opts.Default, l)
+			val, found, usedDefault, err := lookup(key, required, opts.Default, unwrapOnExpand, l)
 			if err != nil && !errors.Is(err, ErrMissingKey) {
 				return fmt.Errorf("%s: %w", tf.Name, err)
 			}
@@ -483,7 +490,7 @@ func processWith(ctx context.Context, c *Config) error {
 			continue
 		}
 
-		val, found, usedDefault, err := lookup(key, required, opts.Default, l)
+		val, found, usedDefault, err := lookup(key, required, opts.Default, unwrapOnExpand, l)
 		if err != nil {
 			return fmt.Errorf("%s: %w", tf.Name, err)
 		}
@@ -591,7 +598,7 @@ LOOP:
 // first boolean parameter indicates whether the value was found in the
 // lookuper. The second boolean parameter indicates whether the default value
 // was used.
-func lookup(key string, required bool, defaultValue string, l Lookuper) (string, bool, bool, error) {
+func lookup(key string, required bool, defaultValue string, unwrapOnExpand bool, l Lookuper) (string, bool, bool, error) {
 	if key == "" {
 		// The struct has something like `env:",required"`, which is likely a
 		// mistake. We could try to infer the envvar from the field name, but that
@@ -620,7 +627,7 @@ func lookup(key string, required bool, defaultValue string, l Lookuper) (string,
 			// a different environment variable.
 			val = os.Expand(defaultValue, func(i string) string {
 				lookuper := l
-				if v, ok := lookuper.(unwrappableLookuper); ok {
+				if v, ok := lookuper.(unwrappableLookuper); ok && unwrapOnExpand {
 					lookuper = v.Unwrap()
 				}
 
@@ -839,13 +846,13 @@ func validateEnvName(s string) bool {
 }
 
 // isLetter returns true if the given rune is a letter between a-z,A-Z. This is
-// different than unicode.IsLetter which includes all L character cases.
+// different from unicode.IsLetter which includes all L character cases.
 func isLetter(r rune) bool {
 	return (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z')
 }
 
 // isNumber returns true if the given run is a number between 0-9. This is
-// different than unicode.IsNumber in that it only allows 0-9.
+// different from unicode.IsNumber in that it only allows 0-9.
 func isNumber(r rune) bool {
 	return r >= '0' && r <= '9'
 }

envconfig_test.go

@@ -233,19 +233,20 @@ func TestProcessWith(t *testing.T) {
 	t.Parallel()
 
 	cases := []struct {
-		name           string
-		target         any
-		lookuper       Lookuper
-		defDelimiter   string
-		defSeparator   string
-		defNoInit      bool
-		defOverwrite   bool
-		defDecodeUnset bool
-		defRequired    bool
-		mutators       []Mutator
-		exp            any
-		err            error
-		errMsg         string
+		name             string
+		target           any
+		lookuper         Lookuper
+		defDelimiter     string
+		defSeparator     string
+		defNoInit        bool
+		defOverwrite     bool
+		defDecodeUnset   bool
+		defRequired      bool
+		keepPrefixExpand bool
+		mutators         []Mutator
+		exp              any
+		err              error
+		errMsg           string
 	}{
 		// nil pointer
 		{
@@ -1311,6 +1312,36 @@ func TestProcessWith(t *testing.T) {
 				"DEFAULT": "value",
 			}))),
 		},
+		{
+			name: "default/expand_keep_prefix",
+			target: &struct {
+				Field string `env:"FIELD,default=$DEFAULT"`
+			}{},
+			exp: &struct {
+				Field string `env:"FIELD,default=$DEFAULT"`
+			}{
+				Field: "value",
+			},
+			lookuper: PrefixLookuper("PREFIX_", MapLookuper(map[string]string{
+				"PREFIX_DEFAULT": "value",
+			})),
+			keepPrefixExpand: true,
+		},
+		{
+			name: "default/expand_keep_prefix_prefix",
+			target: &struct {
+				Field string `env:"FIELD,default=$DEFAULT"`
+			}{},
+			exp: &struct {
+				Field string `env:"FIELD,default=$DEFAULT"`
+			}{
+				Field: "value",
+			},
+			lookuper: PrefixLookuper("OUTER_", PrefixLookuper("INNER_", MapLookuper(map[string]string{
+				"INNER_OUTER_DEFAULT": "value",
+			}))),
+			keepPrefixExpand: true,
+		},
 		{
 			name: "default/slice",
 			target: &struct {
@@ -2873,15 +2904,16 @@ func TestProcessWith(t *testing.T) {
 
 			ctx := context.Background()
 			if err := ProcessWith(ctx, &Config{
-				Target:             tc.target,
-				Lookuper:           tc.lookuper,
-				DefaultDelimiter:   tc.defDelimiter,
-				DefaultSeparator:   tc.defSeparator,
-				DefaultNoInit:      tc.defNoInit,
-				DefaultOverwrite:   tc.defOverwrite,
-				DefaultDecodeUnset: tc.defDecodeUnset,
-				DefaultRequired:    tc.defRequired,
-				Mutators:           tc.mutators,
+				Target:              tc.target,
+				Lookuper:            tc.lookuper,
+				DefaultDelimiter:    tc.defDelimiter,
+				DefaultSeparator:    tc.defSeparator,
+				DefaultNoInit:       tc.defNoInit,
+				DefaultOverwrite:    tc.defOverwrite,
+				DefaultDecodeUnset:  tc.defDecodeUnset,
+				DefaultRequired:     tc.defRequired,
+				KeepPrefixExpansion: tc.keepPrefixExpand,
+				Mutators:            tc.mutators,
 			}); err != nil {
 				if tc.err == nil && tc.errMsg == "" {
 					t.Fatal(err)