utils: add environment updating context manager

bb.utils.environment() is a context manager to alter os.environ inside
a specific block, restoring it after the block is closed.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

2 files changed, 34 insertions, 0 deletions

diff --git a/lib/bb/tests/utils.py b/lib/bb/tests/utils.py
index a7ff33db5..4d5e21b99 100644
--- a/lib/bb/tests/utils.py
+++ b/lib/bb/tests/utils.py
@@ -666,3 +666,21 @@ class GetReferencedVars(unittest.TestCase):
 
         layers = [{"SRC_URI"}, {"QT_GIT", "QT_MODULE", "QT_MODULE_BRANCH_PARAM", "QT_GIT_PROTOCOL"}, {"QT_GIT_PROJECT", "QT_MODULE_BRANCH", "BPN"}, {"PN", "SPECIAL_PKGSUFFIX"}]
         self.check_referenced("${SRC_URI}", layers)
+
+
+class EnvironmentTests(unittest.TestCase):
+    def test_environment(self):
+        os.environ["A"] = "this is A"
+        self.assertIn("A", os.environ)
+        self.assertEqual(os.environ["A"], "this is A")
+        self.assertNotIn("B", os.environ)
+
+        with bb.utils.environment(B="this is B"):
+            self.assertIn("A", os.environ)
+            self.assertEqual(os.environ["A"], "this is A")
+            self.assertIn("B", os.environ)
+            self.assertEqual(os.environ["B"], "this is B")
+
+        self.assertIn("A", os.environ)
+        self.assertEqual(os.environ["A"], "this is A")
+        self.assertNotIn("B", os.environ)
diff --git a/lib/bb/utils.py b/lib/bb/utils.py
index e6e82d111..70634910f 100644
--- a/lib/bb/utils.py
+++ b/lib/bb/utils.py
@@ -1681,3 +1681,19 @@ def rename(src, dst):
             shutil.move(src, dst)
         else:
             raise err
+
+@contextmanager
+def environment(**envvars):
+    """
+    Context manager to selectively update the environment with the specified mapping.
+    """
+    backup = dict(os.environ)
+    try:
+        os.environ.update(envvars)
+        yield
+    finally:
+        for var in envvars:
+            if var in backup:
+                os.environ[var] = backup[var]
+            else:
+                del os.environ[var]