environment: Add env.prepend_library_path() method

mesonbuild/interpreter/interpreterobjects.py

@@ -350,6 +350,24 @@ def prepend_method(self, args: T.Tuple[str, T.List[str]], kwargs: 'EnvironmentSe
         self.warn_if_has_name(name)
         self.held_object.prepend(name, values, kwargs['separator'])
 
+    @FeatureNew('environment.prepend_library_path', '1.10.0')
+    @typed_pos_args('environment.prepend_library_path', build.BuildTarget)
+    @noKwargs
+    @InterpreterObject.method('prepend_library_path')
+    def prepend_library_path_method(self, args: T.Tuple[build.BuildTarget], kwargs: TYPE_kwargs) -> None:
+        target = args[0]
+        library_paths: T.Set[str] = set()
+        host_machine = self.interpreter.environment.machines[mesonlib.MachineChoice.HOST]
+        if (host_machine.is_windows() or host_machine.is_cygwin()) and isinstance(target, (build.Executable, build.SharedModule)):
+            # On windows we cannot rely on rpath to run executables from build
+            # directory. We have to add in PATH the location of every DLL needed.
+            library_paths.update(self.interpreter.backend.determine_windows_extra_paths(target, []))
+        tdir = os.path.join(self.interpreter.environment.get_build_dir(), self.interpreter.backend.get_target_dir(target))
+        if isinstance(target, build.SharedLibrary):
+            library_paths.add(tdir)
+        new_env = self.interpreter.environment.get_env_for_paths(library_paths, set())
+        self.held_object.merge(new_env)
+
 
 _CONF_DATA_SET_KWS: KwargInfo[T.Optional[str]] = KwargInfo('description', (str, NoneType))
 

mesonbuild/utils/core.py

@@ -97,8 +97,9 @@ def merge(self, other: EnvironmentVariables) -> None:
             if name in self.unset_vars:
                 self.unset_vars.remove(name)
         if other.unset_vars:
-            self.can_use_env = False
             self.unset_vars.update(other.unset_vars)
+        if not other.can_use_env:
+            self.can_use_env = False
 
     def set(self, name: str, values: T.List[str], separator: str = os.pathsep) -> None:
         if name in self.unset_vars:

test cases/common/288 env lib/meson.build

@@ -0,0 +1,19 @@
+project('library-path', 'c')
+
+snippets = import('snippets')
+snippets.symbol_visibility_header('config.h')
+
+lib = shared_library('mylib', 'mylib.c',
+  c_args: '-DLIBRARY_PATH_COMPILATION',
+)
+
+env = environment()
+env.prepend_library_path(lib)
+
+prog = find_program('prog.py')
+custom_target(
+    output: 'dummy.txt',
+    command: [prog, lib],
+    env: env,
+    build_by_default: true,
+)

test cases/common/288 env lib/mylib.c

@@ -0,0 +1,6 @@
+#include "config.h"
+
+LIBRARY_PATH_API int foo(void)
+{
+    return 42;
+}

test cases/common/288 env lib/prog.py

@@ -0,0 +1,7 @@
+#!/usr/bin/env python3
+
+import ctypes
+import sys
+
+mylib = ctypes.cdll.LoadLibrary(sys.argv[1])
+assert mylib.foo() == 42