limit wrapped-due-to-env special case for `env` to only apply for env.set

In commit 2cb7350d16 we added a special
case for environment() objects to allow skipping `meson --internal exe`
overhead when /usr/bin/env exists and can be used to set environment
variables instead of using a pickled wrapper.

This special case assumed that environment is used for setting
variables, but it is also possible, albeit less common, to
append/prepend to them, in which case `meson --internal exe` will
compute the final value as an offset from whatever the current environment
variables inherited from ninja are. It is not possible to precompute
this when generating command lines for ninja, so using arguments to
/usr/bin/env is not possible. All it can do is set (override) an
environment variable.

In this case, we have to use the python wrapper and cannot optimize it
away. Add a tracking bit to the env object and propagate it to the
backend.

mesonbuild/backend/backends.py

@@ -627,7 +627,7 @@ class Backend:
         # It's also overridden for a few conditions that can't be handled
         # inside a command line
 
-        can_use_env = not force_serialize
+        can_use_env = env.can_use_env and not force_serialize
         force_serialize = force_serialize or bool(reasons)
 
         if capture:

mesonbuild/utils/core.py

@@ -64,6 +64,7 @@ class EnvironmentVariables(HoldableObject):
         # The set of all env vars we have operations for. Only used for self.has_name()
         self.varnames: T.Set[str] = set()
         self.unset_vars: T.Set[str] = set()
+        self.can_use_env = True
 
         if values:
             init_func = getattr(self, init_method)
@@ -95,7 +96,9 @@ class EnvironmentVariables(HoldableObject):
             self.envvars.append((method, name, values, separator))
             if name in self.unset_vars:
                 self.unset_vars.remove(name)
-        self.unset_vars.update(other.unset_vars)
+        if other.unset_vars:
+            self.can_use_env = False
+            self.unset_vars.update(other.unset_vars)
 
     def set(self, name: str, values: T.List[str], separator: str = os.pathsep) -> None:
         if name in self.unset_vars:
@@ -104,17 +107,20 @@ class EnvironmentVariables(HoldableObject):
         self.envvars.append((self._set, name, values, separator))
 
     def unset(self, name: str) -> None:
+        self.can_use_env = False
         if name in self.varnames:
             raise MesonException(f'You cannot unset the {name!r} variable because it is already set')
         self.unset_vars.add(name)
 
     def append(self, name: str, values: T.List[str], separator: str = os.pathsep) -> None:
+        self.can_use_env = False
         if name in self.unset_vars:
             raise MesonException(f'You cannot append to unset variable {name!r}')
         self.varnames.add(name)
         self.envvars.append((self._append, name, values, separator))
 
     def prepend(self, name: str, values: T.List[str], separator: str = os.pathsep) -> None:
+        self.can_use_env = False
         if name in self.unset_vars:
             raise MesonException(f'You cannot prepend to unset variable {name!r}')
         self.varnames.add(name)