require-executable should ignore empty list

examples/require-executable/main.tf

@@ -7,3 +7,14 @@ module "require_executables" {
   required_executables = ["${var.required_executables}"]
   error_message        = "${var.error_message}"
 }
+
+# Conditional checking example
+module "conditional_require_executables" {
+  # When using these modules in your own templates, you will need to use a Git URL with a ref attribute that pins you
+  # to a specific version of the modules, such as the following example:
+  # source = "git::git@github.com:gruntwork-io/package-terraform-utilities.git//modules/require-executable?ref=v1.0.8"
+  source = "../../modules/require-executable"
+
+  required_executables = ["${var.validate_bad_executable ? "this-executable-should-not-exist" : ""}"]
+  error_message        = "${var.bad_executable_error_message}"
+}

examples/require-executable/variables.tf

@@ -11,3 +11,13 @@ variable "required_executables" {
 variable "error_message" {
   description = "Error message to show if the required executable is not found. This is printed for each executable that was not found. The module will make the following substitutions in the string: `__EXECUTABLE_NAME__` will become the name of the executable that was not found."
 }
+
+variable "validate_bad_executable" {
+  description = "Whether or not to validate the existence of a bad executable."
+  default     = false
+}
+
+variable "bad_executable_error_message" {
+  description = "Error message to show for bad_executable check."
+  default     = ""
+}

modules/require-executable/README.md

@@ -10,8 +10,21 @@ which executables were missing.
 This module uses Python under the hood, so Python must be installed and available on the OS.
 
 
-
-
 ## Example code
 
 See the [require-executable example](/examples/require-executable) for working sample code.
+
+
+## Conditional check
+
+Sometimes you might want to guard the check for a required executable on a condition (e.g only check if an executable
+exists if a particular input flag is set). For this reason, this module will accept and noop on empty strings. For
+example, suppose you want to check if `go` is installed based on the condition `validate_go`. You can achieve this with
+the following terraform code:
+
+```hcl
+module "require_executables" {
+  source = "git::git@github.com:gruntwork-io/package-terraform-utilities.git//modules/require-executable?ref=v1.0.8"
+  required_executables = ["${var.validate_go ? "go" : ""}"]
+}
+```

modules/require-executable/require_executable.py

@@ -54,6 +54,10 @@ def main():
     found = {}
     not_found = []
     for executable in required_executables:
+        # Ignore empty string
+        if not executable.strip():
+            continue
+
         maybe_executable = spawn.find_executable(executable)
         if not maybe_executable:
             not_found.append(executable)

test/require_executable_test.go

@@ -40,3 +40,25 @@ func TestRequireExecutableFailsForMissingExecutable(t *testing.T) {
 	assert.Error(t, err)
 	assert.True(t, strings.Contains(out, randomString))
 }
+
+func TestConditionalRequireExecutable(t *testing.T) {
+	t.Parallel()
+
+	randomString := random.UniqueId()
+	terratestOptions := createBaseTerratestOptions(t, "../examples/require-executable")
+	terratestOptions.Vars = map[string]interface{}{
+		"required_executables":         []string{},
+		"error_message":                "",
+		"validate_bad_executable":      "1",
+		"bad_executable_error_message": randomString,
+	}
+	defer terraform.Destroy(t, terratestOptions)
+
+	out, err := terraform.InitAndApplyE(t, terratestOptions)
+	assert.Error(t, err)
+	assert.True(t, strings.Contains(out, randomString))
+
+	terratestOptions.Vars["validate_bad_executable"] = "0"
+	out = terraform.InitAndApply(t, terratestOptions)
+	assert.False(t, strings.Contains(out, randomString))
+}