Bug 991983 - Don't pass template paths to contexts. r=gps

When doing a template call, what's relevant for relative paths is the source
path of the caller moz.build, not where the template is defined.

python/mozbuild/mozbuild/frontend/reader.py

@@ -431,7 +431,7 @@ class MozbuildSandbox(Sandbox):
             for k, v in inspect.getcallargs(func, *args, **kwargs).items():
                 sandbox[k] = v
 
-            sandbox.exec_source(code, path)
+            sandbox.exec_source(code, path, becomes_current_path=False)
 
             # This is gross, but allows the merge to happen. Eventually, the
             # merging will go away and template contexts emitted independently.

python/mozbuild/mozbuild/frontend/sandbox.py

@@ -151,7 +151,7 @@ class Sandbox(dict):
 
         self.exec_source(source, path)
 
-    def exec_source(self, source, path=''):
+    def exec_source(self, source, path='', becomes_current_path=True):
         """Execute Python code within a string.
 
         The passed string should contain Python code to be executed. The string
@@ -161,7 +161,7 @@ class Sandbox(dict):
         does not perform extra path normalization. This can cause relative
         paths to behave weirdly.
         """
-        if path:
+        if path and becomes_current_path:
             self._context.push_source(path)
 
         old_sandbox = self._context._sandbox
@@ -193,16 +193,24 @@ class Sandbox(dict):
 
             if self._last_name_error is not None:
                 actual = self._last_name_error
-
-            raise SandboxExecutionError(self._context.source_stack,
-                type(actual), actual, sys.exc_info()[2])
+            source_stack = self._context.source_stack
+            if not becomes_current_path:
+                # Add current file to the stack because it wasn't added before
+                # sandbox execution.
+                source_stack.append(path)
+            raise SandboxExecutionError(source_stack, type(actual), actual,
+                                        sys.exc_info()[2])
 
         except Exception as e:
             # Need to copy the stack otherwise we get a reference and that is
             # mutated during the finally.
             exc = sys.exc_info()
-            raise SandboxExecutionError(self._context.source_stack, exc[0],
-                exc[1], exc[2])
+            source_stack = self._context.source_stack
+            if not becomes_current_path:
+                # Add current file to the stack because it wasn't added before
+                # sandbox execution.
+                source_stack.append(path)
+            raise SandboxExecutionError(source_stack, exc[0], exc[1], exc[2])
         finally:
             self._context._sandbox = old_sandbox
             if path:

python/mozbuild/mozbuild/test/frontend/test_sandbox.py

@@ -382,7 +382,7 @@ SOURCES += ['hoge.cpp']
 
         self.assertEqual(sandbox2._context, {
             'SOURCES': ['qux.cpp', 'bar.cpp', 'foo.cpp', 'hoge.cpp'],
-            'DIRS': [sandbox.normalize_path('foo')],
+            'DIRS': [sandbox2.normalize_path('foo')],
         })
 
         sandbox2 = self.sandbox(metadata={'templates': sandbox.templates})