extend install scripts to allow specific directory variable exports

This is useful for internal scripts that want to know about something
other than MESON_INSTALL_PREFIX and MESON_INSTALL_DESTDIR_PREFIX, which
is very specific to the prefix.

mesonbuild/backend/backends.py

@@ -499,7 +499,8 @@ class Backend:
             feed: T.Optional[bool] = None,
             env: T.Optional[build.EnvironmentVariables] = None,
             tag: T.Optional[str] = None,
-            verbose: bool = False) -> 'ExecutableSerialisation':
+            verbose: bool = False,
+            installdir_map: T.Optional[T.Dict[str, str]] = None) -> 'ExecutableSerialisation':
 
         # XXX: cmd_args either need to be lowered to strings, or need to be checked for non-string arguments, right?
         exe, *raw_cmd_args = cmd
@@ -560,7 +561,7 @@ class Backend:
         workdir = workdir or self.environment.get_build_dir()
         return ExecutableSerialisation(exe_cmd + cmd_args, env,
                                        exe_wrapper, workdir,
-                                       extra_paths, capture, feed, tag, verbose)
+                                       extra_paths, capture, feed, tag, verbose, installdir_map)
 
     def as_meson_exe_cmdline(self, exe: T.Union[str, mesonlib.File, build.BuildTarget, build.CustomTarget, programs.ExternalProgram],
                              cmd_args: T.Sequence[T.Union[str, mesonlib.File, build.BuildTarget, build.CustomTarget, programs.ExternalProgram]],

mesonbuild/minstall.py

@@ -672,8 +672,6 @@ class Installer:
     def run_install_script(self, d: InstallData, destdir: str, fullprefix: str) -> None:
         env = {'MESON_SOURCE_ROOT': d.source_dir,
                'MESON_BUILD_ROOT': d.build_dir,
-               'MESON_INSTALL_PREFIX': d.prefix,
-               'MESON_INSTALL_DESTDIR_PREFIX': fullprefix,
                'MESONINTROSPECT': ' '.join([shlex.quote(x) for x in d.mesonintrospect]),
                }
         if self.options.quiet:
@@ -684,6 +682,15 @@ class Installer:
         for i in d.install_scripts:
             if not self.should_install(i):
                 continue
+
+            if i.installdir_map is not None:
+                mapp = i.installdir_map
+            else:
+                mapp = {'prefix': d.prefix}
+            localenv = env.copy()
+            localenv.update({'MESON_INSTALL_'+k.upper(): os.path.join(d.prefix, v) for k, v in mapp.items()})
+            localenv.update({'MESON_INSTALL_DESTDIR_'+k.upper(): get_destdir_path(destdir, fullprefix, v) for k, v in mapp.items()})
+
             name = ' '.join(i.cmd_args)
             if i.skip_if_destdir and destdir:
                 self.log(f'Skipping custom install script because DESTDIR is set {name!r}')
@@ -691,7 +698,7 @@ class Installer:
             self.did_install_something = True  # Custom script must report itself if it does nothing.
             self.log(f'Running custom install script {name!r}')
             try:
-                rc = self.run_exe(i, env)
+                rc = self.run_exe(i, localenv)
             except OSError:
                 print(f'FAILED: install script \'{name}\' could not be run, stopped')
                 # POSIX shells return 127 when a command could not be found

mesonbuild/utils/core.py

@@ -152,6 +152,7 @@ class ExecutableSerialisation:
     feed: T.Optional[bool] = None
     tag: T.Optional[str] = None
     verbose: bool = False
+    installdir_map: T.Optional[T.Dict[str, str]] = None
 
     def __post_init__(self) -> None:
         self.pickled = False