handle monorepo license files specified in project() via ../

We should simply remap these to elide the ../ as it's pretty obviously
the natural expectation of using ../ to fetch files from outside the
project and then drop them *into* the project.

Monorepos will likely have a single license file (or set) under which
the monorepo is licensed. But there will be many components, each of
which may use a different build system, which are "standalone" for the
most part. We already support this case as long as you build from the
monorepo, but the resulting license file gets installed to

```
{licensedir}/../
```

which is silly and unhelpful.

Bug: https://github.com/apache/arrow/issues/36411

4 files changed, 14 insertions, 5 deletions

diff --git a/mesonbuild/backend/backends.py b/mesonbuild/backend/backends.py
index feb002e54..a960537a8 100644
--- a/mesonbuild/backend/backends.py
+++ b/mesonbuild/backend/backends.py
@@ -1354,9 +1354,9 @@ class Backend:
         d.data.append(InstallDataBase(ifilename, ofilename, out_name, None, '',
                                       tag='devel', data_type='depmf'))
         for m in self.build.dep_manifest.values():
-            for ifilename, name in m.license_files:
-                ofilename = os.path.join(odirname, name.relative_name())
-                out_name = os.path.join(out_dir, name.relative_name())
+            for ifilename, name in m.license_mapping():
+                ofilename = os.path.join(odirname, name)
+                out_name = os.path.join(out_dir, name)
                 d.data.append(InstallDataBase(ifilename, ofilename, out_name, None,
                                               m.subproject, tag='devel', data_type='depmf'))
 
diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index a883c3b28..6e233a382 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -223,11 +223,18 @@ class DepManifest:
     license_files: T.List[T.Tuple[str, File]]
     subproject: str
 
+    def license_mapping(self) -> T.List[T.Tuple[str, str]]:
+        ret = []
+        for ifilename, name in self.license_files:
+            fname = os.path.join(*(x for x in pathlib.PurePath(os.path.normpath(name.fname)).parts if x != '..'))
+            ret.append((ifilename, os.path.join(name.subdir, fname)))
+        return ret
+
     def to_json(self) -> T.Dict[str, T.Union[str, T.List[str]]]:
         return {
             'version': self.version,
             'license': self.license,
-            'license_files': [l[1].relative_name() for l in self.license_files],
+            'license_files': [l[1] for l in self.license_mapping()],
         }
 
 
diff --git a/test cases/common/42 subproject/meson.build b/test cases/common/42 subproject/meson.build
index c2c0122f1..bc24adbcc 100644
--- a/test cases/common/42 subproject/meson.build
+++ b/test cases/common/42 subproject/meson.build
@@ -1,7 +1,8 @@
 project('subproj user', 'c',
   version : '2.3.4',
   license : 'mylicense',
-  license_files: 'mylicense.txt',
+  # also grab the meson license to test monorepo support
+  license_files: ['mylicense.txt', '../../../COPYING'],
 )
 
 assert(meson.project_name() == 'subproj user', 'Incorrect project name')
diff --git a/test cases/common/42 subproject/test.json b/test cases/common/42 subproject/test.json
index 949cb79fa..e469052f4 100644
--- a/test cases/common/42 subproject/test.json
+++ b/test cases/common/42 subproject/test.json
@@ -4,6 +4,7 @@
     {"type": "pdb", "file": "usr/bin/user"},
     {"type": "file", "file": "usr/share/sublib/sublib.depmf"},
     {"type": "file", "file": "usr/share/sublib/mylicense.txt"},
+    {"type": "file", "file": "usr/share/sublib/COPYING"},
     {"type": "file", "file": "usr/share/sublib/subprojects/sublib/sublicense1.txt"},
     {"type": "file", "file": "usr/share/sublib/subprojects/sublib/sublicense2.txt"}
   ]