allow use of local Terraform executable in tests

If the env var TFEXEC_E2ETEST_TERRAFORM_PATH is set to the path of a valid
local Terraform executable, when e2e tests are run, only tests appropriate toi
that executable's version will be run.

This involves both the runTest() and runTestVersions() codepaths, since both
are used in e2e tests.

tfexec/internal/e2etest/util_test.go

@@ -41,6 +41,32 @@ func runTest(t *testing.T, fixtureName string, cb func(t *testing.T, tfVersion *
 		versions = strings.Split(override, ",")
 	}
 
+	// If the env var TFEXEC_E2ETEST_TERRAFORM_PATH is set to the path of a
+	// valid Terraform executable, only tests appropriate to that
+	// executable's version will be run.
+	if localBinPath := os.Getenv("TFEXEC_E2ETEST_TERRAFORM_PATH"); localBinPath != "" {
+		td, err := ioutil.TempDir("", "tf")
+		if err != nil {
+			t.Fatalf("error creating temporary test directory: %s", err)
+		}
+		t.Cleanup(func() {
+			os.RemoveAll(td)
+		})
+		ltf, err := tfexec.NewTerraform(td, localBinPath)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		ltf.SetAppendUserAgent("tfexec-e2etest")
+
+		lVersion, _, err := ltf.Version(context.Background(), false)
+		if err != nil {
+			t.Fatalf("unable to determine version of Terraform binary at %s: %s", localBinPath, err)
+		}
+
+		versions = []string{lVersion.String()}
+	}
+
 	runTestVersions(t, versions, fixtureName, cb)
 }
 
@@ -63,12 +89,12 @@ func runTestVersions(t *testing.T, versions []string, fixtureName string, cb fun
 				os.RemoveAll(td)
 			})
 
-			// TODO: do this in a cleaner way than string comparison?
 			var execPath string
-			switch {
-			case strings.HasPrefix(tfv, "refs/"):
+			if localBinPath := os.Getenv("TFEXEC_E2ETEST_TERRAFORM_PATH"); localBinPath != "" {
+				execPath = localBinPath
+			} else if strings.HasPrefix(tfv, "refs/") {
 				execPath = tfcache.GitRef(t, tfv)
-			default:
+			} else {
 				execPath = tfcache.Version(t, tfv)
 			}
 
@@ -84,6 +110,17 @@ func runTestVersions(t *testing.T, versions []string, fixtureName string, cb fun
 				t.Fatalf("unable to determine running version (expected %q): %s", tfv, err)
 			}
 
+			// Check that the runningVersion matches the expected
+			// test version. This ensures non-matching tests are
+			// skipped when using a local Terraform executable.
+			testVersion, err := version.NewVersion(tfv)
+			if err != nil {
+				t.Fatalf("unable to parse version %s: %s", testVersion, err)
+			}
+			if !testVersion.Equal(runningVersion) {
+				t.Skipf("test applies to version %s, but local executable is version %s", tfv, runningVersion)
+			}
+
 			if fixtureName != "" {
 				err = copyFiles(filepath.Join(testFixtureDir, fixtureName), td)
 				if err != nil {