dependencies: allow get_variable to define multiple pkgconfig defines

It was previously impossible to do this:

```
dep.get_pkgconfig_variable(
    'foo',
    define_variable: ['prefix', '/usr', 'datadir', '/usr/share'],
)
```

since get_pkgconfig_variable mandated exactly two (if any) arguments.

However, you could do this:
```
dep.get_variable(
    'foo',
    pkgconfig_define: ['prefix', '/usr', 'datadir', '/usr/share'],
)
```

It would silently do the wrong thing, by defining "prefix" as
`/usr=datadir=/usr/share`, which might not "matter" if only datadir was
used in the "foo" variable as the unmodified value might be adequate.

The actual intention of anyone writing such a meson.build is that they
aren't sure whether the .pc file uses ${prefix} or ${datadir} (or which
one gets used, might have changed between versions of that .pc file,
even).

A recent refactor made this into a hard error, which broke some projects
that were doing this and inadvertently depending on some .pc file that
only used the second variable. (This was "fine" since the result was
essentially meaningful, and even resulted in behavior identical to the
intended behavior if both projects were installed into the same prefix
-- in which case there's nothing to remap.)

Re-allow this. There are two ways we could re-allow this:
- ignore it with a warning
- add a new feature to allow actually doing this

Since the use case which triggered this bug actually has a pretty good
reason to want to do this, it makes sense to add the new feature.

Fixes https://bugs.gentoo.org/916576
Fixes https://github.com/containers/bubblewrap/issues/609

8 files changed, 48 insertions, 9 deletions

diff --git a/docs/markdown/snippets/pkgconfig_define_variable.md b/docs/markdown/snippets/pkgconfig_define_variable.md
new file mode 100644
index 000000000..0f7082b0d
--- /dev/null
+++ b/docs/markdown/snippets/pkgconfig_define_variable.md
@@ -0,0 +1,10 @@
+## pkg-config dependencies can now get a variable with multiple replacements
+
+When using [[dep.get_variable]] and defining a `pkgconfig_define`, it is
+sometimes useful to remap multiple dependency variables. For example, if the
+upstream project changed the variable name that is interpolated and it is
+desirable to support both versions.
+
+It is now possible to pass multiple pairs of variable/value.
+
+The same applies to the compatibility [[dep.get_pkgconfig_variable]] method.
diff --git a/docs/yaml/objects/dep.yaml b/docs/yaml/objects/dep.yaml
index d847690f0..52e28faca 100644
--- a/docs/yaml/objects/dep.yaml
+++ b/docs/yaml/objects/dep.yaml
@@ -38,6 +38,8 @@ methods:
           variable by passing a list to this kwarg
           that can affect the retrieved variable: `['prefix', '/'])`.
 
+          *(Since 1.3.0)* Multiple variables can be specified in pairs.
+
       default:
         type: str
         since: 0.45.0
@@ -174,6 +176,12 @@ methods:
       from the object then `default_value` is returned, if it is not set
       then an error is raised.
 
+    warnings:
+      - Before 1.3.0, specifying multiple pkgconfig_define pairs would silently
+        malform the results. Only the first variable would be redefined, but
+        its value would contain both the second variable name, as well as its
+        value.
+
     optargs:
       varname:
         type: str
diff --git a/mesonbuild/dependencies/pkgconfig.py b/mesonbuild/dependencies/pkgconfig.py
index 93422defd..e86206b88 100644
--- a/mesonbuild/dependencies/pkgconfig.py
+++ b/mesonbuild/dependencies/pkgconfig.py
@@ -139,9 +139,11 @@ class PkgConfigCLI(PkgConfigInterface):
 
     @staticmethod
     def _define_variable_args(define_variable: PkgConfigDefineType) -> T.List[str]:
+        ret = []
         if define_variable:
-            return ['--define-variable=' + '='.join(define_variable)]
-        return []
+            for pair in define_variable:
+                ret.append('--define-variable=' + '='.join(pair))
+        return ret
 
     @lru_cache(maxsize=None)
     def cflags(self, name: str, allow_system: bool = False,
diff --git a/mesonbuild/dependencies/scalapack.py b/mesonbuild/dependencies/scalapack.py
index 158056fa0..a8e20f405 100644
--- a/mesonbuild/dependencies/scalapack.py
+++ b/mesonbuild/dependencies/scalapack.py
@@ -148,5 +148,5 @@ class MKLPkgConfigDependency(PkgConfigDependency):
             # gfortran doesn't appear to look in system paths for INCLUDE files,
             # so don't allow pkg-config to suppress -I flags for system paths
             allow_system = True
-        cflags = self.pkgconfig.cflags(self.name, allow_system, define_variable=('prefix', self.__mklroot.as_posix()))
+        cflags = self.pkgconfig.cflags(self.name, allow_system, define_variable=(('prefix', self.__mklroot.as_posix()),))
         self.compile_args = self._convert_mingw_paths(cflags)
diff --git a/mesonbuild/interpreter/interpreterobjects.py b/mesonbuild/interpreter/interpreterobjects.py
index effeebb20..f13e3ff4a 100644
--- a/mesonbuild/interpreter/interpreterobjects.py
+++ b/mesonbuild/interpreter/interpreterobjects.py
@@ -494,6 +494,9 @@ class DependencyHolder(ObjectHolder[Dependency]):
         from ..dependencies.pkgconfig import PkgConfigDependency
         if not isinstance(self.held_object, PkgConfigDependency):
             raise InvalidArguments(f'{self.held_object.get_name()!r} is not a pkgconfig dependency')
+        if kwargs['define_variable'] and len(kwargs['define_variable']) > 1:
+            FeatureNew.single_use('dependency.get_pkgconfig_variable keyword argument "define_variable"  with more than one pair',
+                                  '1.3.0', self.subproject, location=self.current_node)
         return self.held_object.get_variable(
             pkgconfig=args[0],
             default_value=kwargs['default'],
@@ -536,6 +539,10 @@ class DependencyHolder(ObjectHolder[Dependency]):
         default_varname = args[0]
         if default_varname is not None:
             FeatureNew('Positional argument to dependency.get_variable()', '0.58.0').use(self.subproject, self.current_node)
+        if kwargs['pkgconfig_define'] and len(kwargs['pkgconfig_define']) > 1:
+            FeatureNew.single_use('dependency.get_variable keyword argument "pkgconfig_define" with more than one pair',
+                                  '1.3.0', self.subproject, 'In previous versions, this silently returned a malformed value.',
+                                  self.current_node)
         return self.held_object.get_variable(
             cmake=kwargs['cmake'] or default_varname,
             pkgconfig=kwargs['pkgconfig'] or default_varname,
diff --git a/mesonbuild/interpreter/type_checking.py b/mesonbuild/interpreter/type_checking.py
index 5212a85ba..616f4efbb 100644
--- a/mesonbuild/interpreter/type_checking.py
+++ b/mesonbuild/interpreter/type_checking.py
@@ -4,8 +4,7 @@
 """Helpers for strict type checking."""
 
 from __future__ import annotations
-import os
-import re
+import itertools, os, re
 import typing as T
 
 from .. import compilers
@@ -30,7 +29,7 @@ if T.TYPE_CHECKING:
     from ..mesonlib import EnvInitValueType
 
     _FullEnvInitValueType = T.Union[EnvironmentVariables, T.List[str], T.List[T.List[str]], EnvInitValueType, str, None]
-    PkgConfigDefineType = T.Optional[T.Tuple[str, str]]
+    PkgConfigDefineType = T.Optional[T.Tuple[T.Tuple[str, str], ...]]
     SourcesVarargsType = T.List[T.Union[str, File, CustomTarget, CustomTargetIndex, GeneratedList, StructuredSources, ExtractedObjects, BuildTarget]]
 
 
@@ -841,10 +840,16 @@ BUILD_TARGET_KWS = [
     )
 ]
 
+def _pkgconfig_define_convertor(x: T.List[str]) -> PkgConfigDefineType:
+    if x:
+        keys = itertools.islice(x, 0, None, 2)
+        vals = itertools.islice(x, 1, None, 2)
+        return tuple(zip(keys, vals))
+    return None
+
 PKGCONFIG_DEFINE_KW: KwargInfo = KwargInfo(
     'pkgconfig_define',
     ContainerTypeInfo(list, str, pairs=True),
     default=[],
-    validator=lambda x: 'must be of length 2 or empty' if len(x) not in {0, 2} else None,
-    convertor=lambda x: tuple(x) if x else None
+    convertor=_pkgconfig_define_convertor,
 )
diff --git a/mesonbuild/mdevenv.py b/mesonbuild/mdevenv.py
index b60d7a572..9f3d1b973 100644
--- a/mesonbuild/mdevenv.py
+++ b/mesonbuild/mdevenv.py
@@ -87,7 +87,7 @@ def bash_completion_files(b: build.Build, install_data: 'InstallData') -> T.List
         datadir = b.environment.coredata.get_option(OptionKey('datadir'))
         assert isinstance(datadir, str), 'for mypy'
         datadir_abs = os.path.join(prefix, datadir)
-        completionsdir = dep.get_variable(pkgconfig='completionsdir', pkgconfig_define=('datadir', datadir_abs))
+        completionsdir = dep.get_variable(pkgconfig='completionsdir', pkgconfig_define=(('datadir', datadir_abs),))
         assert isinstance(completionsdir, str), 'for mypy'
         completionsdir_path = Path(completionsdir)
         for f in install_data.data:
diff --git a/test cases/linuxlike/1 pkg-config/meson.build b/test cases/linuxlike/1 pkg-config/meson.build
index ca48e9bd0..b09630a16 100644
--- a/test cases/linuxlike/1 pkg-config/meson.build
+++ b/test cases/linuxlike/1 pkg-config/meson.build
@@ -22,6 +22,13 @@ assert(dep.get_pkgconfig_variable('nonexisting') == '', 'Value of unknown variab
 assert(dep.get_pkgconfig_variable('nonexisting', default: 'foo') == 'foo', 'Value of unknown variable is not defaulted.')
 # pkg-config is able to replace variables
 assert(dep.get_pkgconfig_variable('prefix', define_variable: ['prefix', '/tmp']) == '/tmp', 'prefix variable has not been replaced.')
+assert(dep.get_variable('prefix', pkgconfig_define: ['prefix', '/tmp']) == '/tmp', 'prefix variable has not been replaced.')
+# pkg-config can replace multiple variables at once
+assert(dep.get_variable('prefix', pkgconfig_define: ['prefix', '/tmp', 'libdir', '/bad/libdir']) == '/tmp', 'first variable has not been replaced.')
+assert(dep.get_variable('prefix', pkgconfig_define: ['libdir', '/bad/libdir', 'prefix', '/tmp']) == '/tmp', 'second variable has not been replaced.')
+assert(dep.get_pkgconfig_variable('prefix', define_variable: ['prefix', '/tmp', 'libdir', '/bad/libdir']) == '/tmp', 'first variable has not been replaced.')
+assert(dep.get_pkgconfig_variable('prefix', define_variable: ['libdir', '/bad/libdir', 'prefix', '/tmp']) == '/tmp', 'second variable has not been replaced.')
+
 
 # Test that dependencies of dependencies work.
 dep2 = declare_dependency(dependencies : dep)