Allow escaping a dollar sign when expanding external labels (#10129)

* Allow escaping a dollar sign when expanding external labels

There is currently no mechanism to natively escape a dollar sign
in the os.Expand function. As a workaround, this commit modifies
the external label expansion logic to treat a double dollar ($$)
as a mechanism for escaping the dollar character.

Signed-off-by: fpetkovski <filip.petkovsky@gmail.com>

config/config.go

@@ -83,6 +83,9 @@ func Load(s string, expandExternalLabels bool, logger log.Logger) (*Config, erro
 
 	for i, v := range cfg.GlobalConfig.ExternalLabels {
 		newV := os.Expand(v.Value, func(s string) string {
+			if s == "$" {
+				return "$"
+			}
 			if v := os.Getenv(s); v != "" {
 				return v
 			}

config/config_test.go

@@ -1480,12 +1480,16 @@ func TestExpandExternalLabels(t *testing.T) {
 	require.Equal(t, labels.Label{Name: "bar", Value: "foo"}, c.GlobalConfig.ExternalLabels[0])
 	require.Equal(t, labels.Label{Name: "baz", Value: "foo${TEST}bar"}, c.GlobalConfig.ExternalLabels[1])
 	require.Equal(t, labels.Label{Name: "foo", Value: "${TEST}"}, c.GlobalConfig.ExternalLabels[2])
+	require.Equal(t, labels.Label{Name: "qux", Value: "foo$${TEST}"}, c.GlobalConfig.ExternalLabels[3])
+	require.Equal(t, labels.Label{Name: "xyz", Value: "foo$$bar"}, c.GlobalConfig.ExternalLabels[4])
 
 	c, err = LoadFile("testdata/external_labels.good.yml", false, true, log.NewNopLogger())
 	require.NoError(t, err)
 	require.Equal(t, labels.Label{Name: "bar", Value: "foo"}, c.GlobalConfig.ExternalLabels[0])
 	require.Equal(t, labels.Label{Name: "baz", Value: "foobar"}, c.GlobalConfig.ExternalLabels[1])
 	require.Equal(t, labels.Label{Name: "foo", Value: ""}, c.GlobalConfig.ExternalLabels[2])
+	require.Equal(t, labels.Label{Name: "qux", Value: "foo${TEST}"}, c.GlobalConfig.ExternalLabels[3])
+	require.Equal(t, labels.Label{Name: "xyz", Value: "foo$bar"}, c.GlobalConfig.ExternalLabels[4])
 
 	os.Setenv("TEST", "TestValue")
 	c, err = LoadFile("testdata/external_labels.good.yml", false, true, log.NewNopLogger())
@@ -1493,6 +1497,8 @@ func TestExpandExternalLabels(t *testing.T) {
 	require.Equal(t, labels.Label{Name: "bar", Value: "foo"}, c.GlobalConfig.ExternalLabels[0])
 	require.Equal(t, labels.Label{Name: "baz", Value: "fooTestValuebar"}, c.GlobalConfig.ExternalLabels[1])
 	require.Equal(t, labels.Label{Name: "foo", Value: "TestValue"}, c.GlobalConfig.ExternalLabels[2])
+	require.Equal(t, labels.Label{Name: "qux", Value: "foo${TEST}"}, c.GlobalConfig.ExternalLabels[3])
+	require.Equal(t, labels.Label{Name: "xyz", Value: "foo$bar"}, c.GlobalConfig.ExternalLabels[4])
 }
 
 func TestEmptyGlobalBlock(t *testing.T) {

config/testdata/external_labels.good.yml

@@ -3,3 +3,5 @@ global:
     bar: foo
     foo: ${TEST}
     baz: foo${TEST}bar
+    qux: foo$${TEST}
+    xyz: foo$$bar

docs/feature_flags.md

@@ -18,6 +18,7 @@ They may be enabled by default in future versions.
 Replace `${var}` or `$var` in the [`external_labels`](configuration/configuration.md#configuration-file)
 values according to the values of the current environment variables. References
 to undefined variables are replaced by the empty string.
+The `$` character can be escaped by using `$$`.
 
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