fix(config): Move bracketed environment variable substitution to doub…

…le-dollar

config/config.go

@@ -40,13 +40,22 @@ import (
 	"github.com/influxdata/telegraf/plugins/serializers"
 )
 
+// envVarPattern is a regex to determine environment variables in the
+// config file for substitution. Those should start with a dollar signs.
+// Expression modified from
+// https://github.com/compose-spec/compose-go/blob/v1.14.0/template/template.go
+const envVarPattern = `\$(?i:(?P<escaped>\\)|(?P<named>[_a-z][_a-z0-9]*)|\${(?:(?P<braced>[_a-z][_a-z0-9]*(?::?[-+?](.*))?)}|(?P<invalid>)))`
+
 var (
 	httpLoadConfigRetryInterval = 10 * time.Second
 
 	// fetchURLRe is a regex to determine whether the requested file should
 	// be fetched from a remote or read from the filesystem.
 	fetchURLRe = regexp.MustCompile(`^\w+://`)
 
+	// envVarRe is the compiled regex of envVarPattern
+	envVarRe = regexp.MustCompile(envVarPattern)
+
 	// Password specified via command-line
 	Password Secret
 )
@@ -850,12 +859,12 @@ func removeComments(contents []byte) ([]byte, error) {
 
 func substituteEnvironment(contents []byte) ([]byte, error) {
 	envMap := utils.GetAsEqualsMap(os.Environ())
-	retVal, err := template.Substitute(string(contents), func(k string) (string, bool) {
+	retVal, err := template.SubstituteWith(string(contents), func(k string) (string, bool) {
 		if v, ok := envMap[k]; ok {
 			return v, ok
 		}
 		return "", false
-	})
+	}, envVarRe)
 	var invalidTmplError *template.InvalidTemplateError
 	if err != nil && !errors.As(err, &invalidTmplError) {
 		return nil, err

config/config_test.go

@@ -71,7 +71,7 @@ func TestConfig_LoadSingleInputWithEnvVars(t *testing.T) {
 
 	input := inputs.Inputs["memcached"]().(*MockupInputPlugin)
 	input.Servers = []string{"192.168.1.1"}
-	input.Command = `Raw command which may or may not contain # in it
+	input.Command = `Raw command which may or may not contain # or ${var} in it
 # is unique`
 
 	filter := models.Filter{

config/internal_test.go

@@ -25,25 +25,25 @@ func TestEnvironmentSubstitution(t *testing.T) {
 				t.Setenv("TEST_ENV1", "VALUE1")
 				t.Setenv("TEST_ENV2", "VALUE2")
 			},
-			contents: "A string with ${TEST_ENV1}, $TEST_ENV2 and $TEST_ENV1 as repeated",
+			contents: "A string with $${TEST_ENV1}, $TEST_ENV2 and $TEST_ENV1 as repeated",
 			expected: "A string with VALUE1, VALUE2 and VALUE1 as repeated",
 		},
 		{
 			name:     "Env not set",
-			contents: "Env variable ${NOT_SET} will be empty",
+			contents: "Env variable $${NOT_SET} will be empty",
 			expected: "Env variable  will be empty", // Two spaces present
 		},
 		{
 			name:     "Env not set, fallback to default",
-			contents: "Env variable ${THIS_IS_ABSENT:-Fallback}",
+			contents: "Env variable $${THIS_IS_ABSENT:-Fallback}",
 			expected: "Env variable Fallback",
 		},
 		{
 			name: "No fallback",
 			setEnv: func(t *testing.T) {
 				t.Setenv("MY_ENV1", "VALUE1")
 			},
-			contents: "Env variable ${MY_ENV1:-Fallback}",
+			contents: "Env variable $${MY_ENV1:-Fallback}",
 			expected: "Env variable VALUE1",
 		},
 		{
@@ -52,17 +52,17 @@ func TestEnvironmentSubstitution(t *testing.T) {
 				t.Setenv("MY_VAR", "VALUE")
 				t.Setenv("MY_VAR2", "VALUE2")
 			},
-			contents: "Env var ${MY_VAR} is set, with $MY_VAR syntax and default on this ${MY_VAR1:-Substituted}, no default on this ${MY_VAR2:-NoDefault}",
+			contents: "Env var $${MY_VAR} is set, with $MY_VAR syntax and default on this $${MY_VAR1:-Substituted}, no default on this $${MY_VAR2:-NoDefault}",
 			expected: "Env var VALUE is set, with VALUE syntax and default on this Substituted, no default on this VALUE2",
 		},
 		{
 			name:     "Default has special chars",
-			contents: `Not recommended but supported ${MY_VAR:-Default with special chars Supported#$\"}`,
+			contents: `Not recommended but supported $${MY_VAR:-Default with special chars Supported#$\"}`,
 			expected: `Not recommended but supported Default with special chars Supported#$\"`, // values are escaped
 		},
 		{
 			name:         "unset error",
-			contents:     "Contains ${THIS_IS_NOT_SET?unset-error}",
+			contents:     "Contains $${THIS_IS_NOT_SET?unset-error}",
 			wantErr:      true,
 			errSubstring: "unset-error",
 		},
@@ -71,7 +71,7 @@ func TestEnvironmentSubstitution(t *testing.T) {
 			setEnv: func(t *testing.T) {
 				t.Setenv("ENV_EMPTY", "")
 			},
-			contents:     "Contains ${ENV_EMPTY:?empty-error}",
+			contents:     "Contains $${ENV_EMPTY:?empty-error}",
 			wantErr:      true,
 			errSubstring: "empty-error",
 		},
@@ -80,9 +80,17 @@ func TestEnvironmentSubstitution(t *testing.T) {
 			setEnv: func(t *testing.T) {
 				t.Setenv("FALLBACK", "my-fallback")
 			},
-			contents: "Should output ${NOT_SET:-${FALLBACK}}",
+			contents: "Should output $${NOT_SET:-${FALLBACK}}",
 			expected: "Should output my-fallback",
 		},
+		{
+			name: "leave alone single dollar expressions #13432",
+			setEnv: func(t *testing.T) {
+				t.Setenv("MYVAR", "my-variable")
+			},
+			contents: "Should output ${MYVAR}",
+			expected: "Should output ${MYVAR}",
+		},
 	}
 
 	for _, tt := range tests {

config/testdata/single_plugin_env_vars.toml

@@ -10,27 +10,26 @@
 # file would generate.
 #
 # Environment variables can be used anywhere in this config file, simply surround
-# them with ${}. For strings the variable must be within quotes (ie, "${STR_VAR}"),
-# for numbers and booleans they should be plain (ie, ${INT_VAR}, ${BOOL_VAR})
+# them with $${}. For strings the variable must be within quotes (ie, "$${STR_VAR}"),
+# for numbers and booleans they should be plain (ie, $${INT_VAR}, $${BOOL_VAR})
 
 [[inputs.memcached]]
   # this comment line will be ignored by the parser
-  servers = ["$MY_TEST_SERVER"] 
-  namepass = ["metricname1", "ip_${MY_TEST_SERVER}_name"] # this comment will be ignored as well
+  servers = ["$MY_TEST_SERVER"]
+  namepass = ["metricname1", "ip_$${MY_TEST_SERVER}_name"] # this comment will be ignored as well
   namedrop = ["metricname2"]
   fieldpass = ["some", "strings"]
   fielddrop = ["other", "stuff"]
   interval = "$TEST_INTERVAL"
    ##### this input is provided to test multiline strings
   command = """
-Raw command which may or may not contain # in it
+Raw command which may or may not contain # or ${var} in it
 # is unique""" # Multiline comment black starting with #
   [inputs.memcached.tagpass]
-    goodtag = ["mytag", """tagwith#value""", 
+    goodtag = ["mytag", """tagwith#value""",
       # comment in between array items
       # should ignore "quotes" in comments
       '''TagWithMultilineSyntax''', ## ignore this comment
     ] # hastag
   [inputs.memcached.tagdrop]
     badtag = ["othertag"]
-