fix(provisioner/terraform/tfparse): evaluate coder_parameter defaults with variables (#15800)

- adds support for dynamic default values in coder_parameter data source

provisioner/terraform/tfparse/tfparse.go

@@ -172,7 +172,7 @@ func (p *Parser) WorkspaceTagDefaults(ctx context.Context) (map[string]string, e
 	if err != nil {
 		return nil, xerrors.Errorf("load variable defaults: %w", err)
 	}
-	paramsDefaults, err := p.CoderParameterDefaults(ctx)
+	paramsDefaults, err := p.CoderParameterDefaults(ctx, varsDefaults)
 	if err != nil {
 		return nil, xerrors.Errorf("load parameter defaults: %w", err)
 	}
@@ -268,7 +268,7 @@ func (p *Parser) VariableDefaults(ctx context.Context) (map[string]string, error
 
 // CoderParameterDefaults returns the default values of all coder_parameter data sources
 // in the parsed module.
-func (p *Parser) CoderParameterDefaults(ctx context.Context) (map[string]string, error) {
+func (p *Parser) CoderParameterDefaults(ctx context.Context, varsDefaults map[string]string) (map[string]string, error) {
 	defaultsM := make(map[string]string)
 	var (
 		skipped []string
@@ -316,6 +316,7 @@ func (p *Parser) CoderParameterDefaults(ctx context.Context) (map[string]string,
 			}
 
 			if _, ok := resContent.Attributes["default"]; !ok {
+				p.logger.Warn(ctx, "coder_parameter data source does not have a default value", slog.F("name", dataResource.Name))
 				defaultsM[dataResource.Name] = ""
 			} else {
 				expr := resContent.Attributes["default"].Expr
@@ -323,7 +324,20 @@ func (p *Parser) CoderParameterDefaults(ctx context.Context) (map[string]string,
 				if err != nil {
 					return nil, xerrors.Errorf("can't preview the resource file: %v", err)
 				}
-				defaultsM[dataResource.Name] = strings.Trim(value, `"`)
+				// Issue #15795: the "default" value could also be an expression we need
+				// to evaluate.
+				// TODO: should we support coder_parameter default values that reference other coder_parameter data sources?
+				evalCtx := buildEvalContext(varsDefaults, nil)
+				val, diags := expr.Value(evalCtx)
+				if diags.HasErrors() {
+					return nil, xerrors.Errorf("failed to evaluate coder_parameter %q default value %q: %s", dataResource.Name, value, diags.Error())
+				}
+				// Do not use "val.AsString()" as it can panic
+				strVal, err := ctyValueString(val)
+				if err != nil {
+					return nil, xerrors.Errorf("failed to marshal coder_parameter %q default value %q as string: %s", dataResource.Name, value, err)
+				}
+				defaultsM[dataResource.Name] = strings.Trim(strVal, `"`)
 			}
 		}
 	}

provisioner/terraform/tfparse/tfparse_test.go

@@ -114,6 +114,73 @@ func Test_WorkspaceTagDefaultsFromFile(t *testing.T) {
 			expectTags:  map[string]string{"platform": "kubernetes", "cluster": "developers", "region": "us", "az": "a"},
 			expectError: "",
 		},
+		{
+			name: "main.tf with parameter that has default value from dynamic value",
+			files: map[string]string{
+				"main.tf": `
+					provider "foo" {}
+					resource "foo_bar" "baz" {}
+					variable "region" {
+						type    = string
+						default = "us"
+					}
+					variable "az" {
+						type    = string
+						default = "${""}${"a"}"
+					}
+					data "base" "ours" {
+						all = true
+					}
+					data "coder_parameter" "az" {
+						name = "az"
+						type = "string"
+						default = var.az
+					}
+					data "coder_workspace_tags" "tags" {
+						tags = {
+							"platform" = "kubernetes",
+							"cluster"  = "${"devel"}${"opers"}"
+							"region"   = var.region
+							"az"       = data.coder_parameter.az.value
+						}
+					}`,
+			},
+			expectTags:  map[string]string{"platform": "kubernetes", "cluster": "developers", "region": "us", "az": "a"},
+			expectError: "",
+		},
+		{
+			name: "main.tf with parameter that has default value from another parameter",
+			files: map[string]string{
+				"main.tf": `
+					provider "foo" {}
+					resource "foo_bar" "baz" {}
+					variable "region" {
+						type    = string
+						default = "us"
+					}
+					data "base" "ours" {
+						all = true
+					}
+					data "coder_parameter" "az" {
+						type    = string
+						default = "${""}${"a"}"
+					}
+					data "coder_parameter" "az2" {
+					  name = "az"
+						type = "string"
+						default = data.coder_parameter.az.value
+					}
+					data "coder_workspace_tags" "tags" {
+						tags = {
+							"platform" = "kubernetes",
+							"cluster"  = "${"devel"}${"opers"}"
+							"region"   = var.region
+							"az"       = data.coder_parameter.az2.value
+						}
+					}`,
+			},
+			expectError: "Unknown variable; There is no variable named \"data\".",
+		},
 		{
 			name: "main.tf with multiple valid workspace tags",
 			files: map[string]string{