First steps towards reworking the build system... again.

build/protobuf.py

@@ -0,0 +1,65 @@
+from os.path import join
+from build.ab import Rule, Targets, emit, normalrule, filenamesof, flatten
+from build.c import clibrary
+import build.pkg
+from types import SimpleNamespace
+
+emit(
+    """
+PROTOC ?= protoc
+ifeq ($(filter protobuf, $(PACKAGES)),)
+$(error Required package 'protobuf' not installed.)"
+endif
+"""
+)
+
+
+@Rule
+def proto(self, name, srcs: Targets = [], deps: Targets = []):
+    normalrule(
+        replaces=self,
+        ins=srcs,
+        outs=[f"{name}.descriptor"],
+        deps=deps,
+        commands=[
+            "$(PROTOC) --include_source_info --descriptor_set_out={outs[0]} {ins}"
+        ],
+        label="PROTO",
+    )
+    self.proto.srcs = filenamesof(srcs) + flatten(
+        [s.proto.srcs for s in flatten(deps)]
+    )
+
+
+@Rule
+def protocc(self, name, srcs: Targets = [], deps: Targets = []):
+    outs = []
+    protos = []
+    for f in flatten([s.proto.srcs for s in flatten(srcs + deps)]):
+        if f.endswith(".proto"):
+            cc = f.replace(".proto", ".pb.cc")
+            h = f.replace(".proto", ".pb.h")
+            protos += [f]
+            srcs += [f]
+            outs += [cc, h]
+
+    r = normalrule(
+        name=f"{name}_srcs",
+        ins=protos,
+        outs=outs,
+        deps=deps,
+        commands=["$(PROTOC) --cpp_out={self.normalrule.objdir} {ins}"],
+        label="PROTOCC",
+    )
+
+    r.materialise()
+    headers = {
+        f: join(r.normalrule.objdir, f) for f in outs if f.endswith(".pb.h")
+    }
+
+    clibrary(
+        replaces=self,
+        srcs=[f"{name}_srcs"],
+        hdrs=headers,
+        cflags=[f"-I{r.normalrule.objdir}"],
+    )