feat: add merge_strategy attribute to coder_env resource (#489)

Add an optional merge_strategy attribute that controls how environment
variables are combined when multiple coder_env resources define the same
name. Valid values: replace (default), append, prepend, error.

- Default 'replace' preserves backward compatibility
- ForceNew: true ensures strategy changes recreate the resource
- Validated with StringInSlice matching existing codebase patterns
- Added tests for default value, all valid values, and invalid rejection
- Updated docs with merge_strategy description and PATH append example

Author: kacpersaw

docs/resources/env.md

@@ -44,6 +44,7 @@ resource "coder_env" "internal_api_url" {
 
 ### Optional
 
+- `merge_strategy` (String) Controls how this environment variable is merged when multiple coder_env resources define the same name. `replace` (default): last value wins. `append`: appends to existing value with a colon `:` separator. `prepend`: prepends to existing value with a colon `:` separator. `error`: fail the build if another coder_env defines the same name. When multiple resources append or prepend to the same name, they are applied in alphabetical order by Terraform resource address.
 - `value` (String) The value of the environment variable.
 
 ### Read-Only

provider/env.go

@@ -45,6 +45,16 @@ func envResource() *schema.Resource {
 				ForceNew:    true,
 				Optional:    true,
 			},
+			"merge_strategy": {
+				Type:        schema.TypeString,
+				Description: "Controls how this environment variable is merged when multiple coder_env resources define the same name. `replace` (default): last value wins. `append`: appends to existing value with a colon `:` separator. `prepend`: prepends to existing value with a colon `:` separator. `error`: fail the build if another coder_env defines the same name. When multiple resources append or prepend to the same name, they are applied in alphabetical order by Terraform resource address.",
+				ForceNew:    true,
+				Optional:    true,
+				Default:     "replace",
+				ValidateFunc: validation.StringInSlice([]string{
+					"replace", "append", "prepend", "error",
+				}, false),
+			},
 		},
 	}
 }

provider/env_test.go

@@ -117,3 +117,78 @@ func TestEnvNoName(t *testing.T) {
 		}},
 	})
 }
+
+func TestEnvDefaultMergeStrategy(t *testing.T) {
+	t.Parallel()
+	resource.Test(t, resource.TestCase{
+		ProviderFactories: coderFactory(),
+		IsUnitTest:        true,
+		Steps: []resource.TestStep{{
+			Config: `
+			provider "coder" {}
+			resource "coder_env" "example" {
+				agent_id = "king"
+				name = "FOO"
+				value = "bar"
+			}`,
+			Check: func(state *terraform.State) error {
+				require.Len(t, state.Modules, 1)
+				require.Len(t, state.Modules[0].Resources, 1)
+				env := state.Modules[0].Resources["coder_env.example"]
+				require.NotNil(t, env)
+				require.Equal(t, "replace", env.Primary.Attributes["merge_strategy"])
+				return nil
+			},
+		}},
+	})
+}
+
+func TestEnvValidMergeStrategies(t *testing.T) {
+	t.Parallel()
+	for _, strategy := range []string{"replace", "append", "prepend", "error"} {
+		t.Run(strategy, func(t *testing.T) {
+			t.Parallel()
+			resource.Test(t, resource.TestCase{
+				ProviderFactories: coderFactory(),
+				IsUnitTest:        true,
+				Steps: []resource.TestStep{{
+					Config: `
+					provider "coder" {}
+					resource "coder_env" "example" {
+						agent_id = "king"
+						name = "FOO"
+						value = "bar"
+						merge_strategy = "` + strategy + `"
+					}`,
+					Check: func(state *terraform.State) error {
+						require.Len(t, state.Modules, 1)
+						require.Len(t, state.Modules[0].Resources, 1)
+						env := state.Modules[0].Resources["coder_env.example"]
+						require.NotNil(t, env)
+						require.Equal(t, strategy, env.Primary.Attributes["merge_strategy"])
+						return nil
+					},
+				}},
+			})
+		})
+	}
+}
+
+func TestEnvInvalidMergeStrategy(t *testing.T) {
+	t.Parallel()
+	resource.Test(t, resource.TestCase{
+		ProviderFactories: coderFactory(),
+		IsUnitTest:        true,
+		Steps: []resource.TestStep{{
+			Config: `
+			provider "coder" {}
+			resource "coder_env" "example" {
+				agent_id = "king"
+				name = "FOO"
+				value = "bar"
+				merge_strategy = "concat"
+			}`,
+			ExpectError: regexp.MustCompile(`expected merge_strategy to be one of`),
+		}},
+	})
+}