cross: Be more permissive about not-found exe_wrapper

We used to immediately try to use whatever exe_wrapper was defined in
the cross file, but some people generate the cross file once and use
it for several projects, most of which do not even need an exe wrapper
to build.

Now we're a bit more resilient. We quietly fall back to using
non-exe-wrapper paths for compiler checks and skip the sanity check.
However, if some code needs the exe wrapper, f.ex., if you run a built
executable using custom_target() or run_target(), we will error out
during setup.

Tests will, of course, continue to error out when you run them if the
exe wrapper was not found. We don't want people's tests to silently
"pass" (aka skip) because of a bad CI setup.

Closes https://github.com/mesonbuild/meson/issues/3562

This commit also adds a test for the behaviour of exe_wrapper in these
cases, and refactors the unit tests a bit for it.

3 files changed, 22 insertions, 7 deletions

diff --git a/mesonbuild/backend/backends.py b/mesonbuild/backend/backends.py
index b13aa10..b55d3e0 100644
--- a/mesonbuild/backend/backends.py
+++ b/mesonbuild/backend/backends.py
@@ -276,8 +276,11 @@ class Backend:
                 raise MesonException('Unknown data type in object list.')
         return obj_list
 
-    def serialize_executable(self, exe, cmd_args, workdir, env={},
+    def serialize_executable(self, tname, exe, cmd_args, workdir, env={},
                              extra_paths=None, capture=None):
+        '''
+        Serialize an executable for running with a generator or a custom target
+        '''
         import hashlib
         if extra_paths is None:
             # The callee didn't check if we needed extra paths, so check it here
@@ -302,19 +305,24 @@ class Backend:
         with open(exe_data, 'wb') as f:
             if isinstance(exe, dependencies.ExternalProgram):
                 exe_cmd = exe.get_command()
-                exe_needs_wrapper = False
+                exe_is_native = True
             elif isinstance(exe, (build.BuildTarget, build.CustomTarget)):
                 exe_cmd = [self.get_target_filename_abs(exe)]
-                exe_needs_wrapper = exe.is_cross
+                exe_is_native = not exe.is_cross
             else:
                 exe_cmd = [exe]
-                exe_needs_wrapper = False
-            is_cross_built = exe_needs_wrapper and \
+                exe_is_native = True
+            is_cross_built = (not exe_is_native) and \
                 self.environment.is_cross_build() and \
                 self.environment.cross_info.need_cross_compiler() and \
                 self.environment.cross_info.need_exe_wrapper()
             if is_cross_built:
                 exe_wrapper = self.environment.get_exe_wrapper()
+                if not exe_wrapper.found():
+                    msg = 'The exe_wrapper {!r} defined in the cross file is ' \
+                          'needed by target {!r}, but was not found. Please ' \
+                          'check the command and/or add it to PATH.'
+                    raise MesonException(msg.format(exe_wrapper.name, tname))
             else:
                 exe_wrapper = None
             es = ExecutableSerialisation(basename, exe_cmd, cmd_args, env,
diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index f62bc67..eed40bc 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -541,7 +541,7 @@ int dummy;
             if extra_paths:
                 serialize = True
         if serialize:
-            exe_data = self.serialize_executable(target.command[0], cmd[1:],
+            exe_data = self.serialize_executable(target.name, target.command[0], cmd[1:],
                                                  # All targets are built from the build dir
                                                  self.environment.get_build_dir(),
                                                  extra_paths=extra_paths,
@@ -598,6 +598,11 @@ int dummy;
             if self.environment.is_cross_build():
                 exe_wrap = self.environment.get_exe_wrapper()
                 if exe_wrap:
+                    if not exe_wrap.found():
+                        msg = 'The exe_wrapper {!r} defined in the cross file is ' \
+                              'needed by run target {!r}, but was not found. ' \
+                              'Please check the command and/or add it to PATH.'
+                        raise MesonException(msg.format(exe_wrap.name, target.name))
                     cmd += exe_wrap.get_command()
             cmd.append(abs_exe)
         elif isinstance(texe, dependencies.ExternalProgram):
@@ -1932,6 +1937,7 @@ rule FORTRAN_DEP_HACK%s
             cmdlist = exe_arr + self.replace_extra_args(args, genlist)
             if generator.capture:
                 exe_data = self.serialize_executable(
+                    'generator ' + cmdlist[0],
                     cmdlist[0],
                     cmdlist[1:],
                     self.environment.get_build_dir(),
diff --git a/mesonbuild/backend/vs2010backend.py b/mesonbuild/backend/vs2010backend.py
index d42e91d..cb4f706 100644
--- a/mesonbuild/backend/vs2010backend.py
+++ b/mesonbuild/backend/vs2010backend.py
@@ -133,6 +133,7 @@ class Vs2010Backend(backends.Backend):
                     cmd = exe_arr + self.replace_extra_args(args, genlist)
                     if generator.capture:
                         exe_data = self.serialize_executable(
+                            'generator ' + cmd[0],
                             cmd[0],
                             cmd[1:],
                             self.environment.get_build_dir(),
@@ -489,7 +490,7 @@ class Vs2010Backend(backends.Backend):
         tdir_abs = os.path.join(self.environment.get_build_dir(), self.get_target_dir(target))
         extra_bdeps = target.get_transitive_build_target_deps()
         extra_paths = self.determine_windows_extra_paths(target.command[0], extra_bdeps)
-        exe_data = self.serialize_executable(target.command[0], cmd[1:],
+        exe_data = self.serialize_executable(target.name, target.command[0], cmd[1:],
                                              # All targets run from the target dir
                                              tdir_abs,
                                              extra_paths=extra_paths,