testing: add TB.Setenv

Add a new method TB.Setenv that'll set environment variables
only for the isolated lifetime of the test, and will clean up
and unset these variables when the test ends.
This method disables the test or benchmark from running in
parallel.

Fixes #41260

Change-Id: I0a18f094ec1c6ec3157b4b12993ea3075e2e9867
GitHub-Last-Rev: 0ca12fa565
GitHub-Pull-Request: golang/go#41857
Reviewed-on: https://go-review.googlesource.com/c/go/+/260577
Trust: Daniel Martí <mvdan@mvdan.cc>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: roger peppe <rogpeppe@gmail.com>

src/testing/testing.go

@@ -667,6 +667,7 @@ var _ TB = (*B)(nil)
 type T struct {
 	common
 	isParallel bool
+	isEnvSet   bool
 	context    *testContext // For running tests and subtests.
 }
 
@@ -964,6 +965,29 @@ func (c *common) TempDir() string {
 	return dir
 }
 
+// Setenv calls os.Setenv(key, value) and uses Cleanup to
+// restore the environment variable to its original value
+// after the test.
+//
+// This cannot be used in parallel tests.
+func (c *common) Setenv(key, value string) {
+	prevValue, ok := os.LookupEnv(key)
+
+	if err := os.Setenv(key, value); err != nil {
+		c.Fatalf("cannot set environment variable: %v", err)
+	}
+
+	if ok {
+		c.Cleanup(func() {
+			os.Setenv(key, prevValue)
+		})
+	} else {
+		c.Cleanup(func() {
+			os.Unsetenv(key)
+		})
+	}
+}
+
 // panicHanding is an argument to runCleanup.
 type panicHandling int
 
@@ -1035,6 +1059,9 @@ func (t *T) Parallel() {
 	if t.isParallel {
 		panic("testing: t.Parallel called multiple times")
 	}
+	if t.isEnvSet {
+		panic("testing: t.Parallel called after t.Setenv; cannot set environment variables in parallel tests")
+	}
 	t.isParallel = true
 
 	// We don't want to include the time we spend waiting for serial tests
@@ -1068,6 +1095,21 @@ func (t *T) Parallel() {
 	t.raceErrors += -race.Errors()
 }
 
+// Setenv calls os.Setenv(key, value) and uses Cleanup to
+// restore the environment variable to its original value
+// after the test.
+//
+// This cannot be used in parallel tests.
+func (t *T) Setenv(key, value string) {
+	if t.isParallel {
+		panic("testing: t.Setenv called after t.Parallel; cannot set environment variables in parallel tests")
+	}
+
+	t.isEnvSet = true
+
+	t.common.Setenv(key, value)
+}
+
 // InternalTest is an internal type but exported because it is cross-package;
 // it is part of the implementation of the "go test" command.
 type InternalTest struct {