[lldb/cmake] Plugin layering enforcement mechanism

lldb/CMakeLists.txt

@@ -37,6 +37,7 @@ endif()
 
 include(LLDBConfig)
 include(AddLLDB)
+include(LLDBLayeringCheck)
 
 set(LLDB_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/include)
 
@@ -127,6 +128,8 @@ add_subdirectory(source)
 add_subdirectory(tools)
 add_subdirectory(docs)
 
+check_lldb_plugin_layering()
+
 if (LLDB_ENABLE_PYTHON)
   if(LLDB_BUILD_FRAMEWORK)
     set(lldb_python_target_dir "${LLDB_FRAMEWORK_ABSOLUTE_BUILD_DIR}/LLDB.framework/Resources/Python/lldb")

lldb/cmake/modules/LLDBLayeringCheck.cmake

@@ -0,0 +1,68 @@
+define_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND)
+define_property(TARGET PROPERTY LLDB_PLUGIN_KIND INHERITED)
+
+define_property(DIRECTORY PROPERTY LLDB_ACCEPTABLE_PLUGIN_DEPENDENCIES)
+define_property(TARGET PROPERTY LLDB_ACCEPTABLE_PLUGIN_DEPENDENCIES INHERITED)
+
+define_property(DIRECTORY PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES)
+define_property(TARGET PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES INHERITED)
+
+option(LLDB_GENERATE_PLUGIN_DEP_GRAPH OFF)
+
+function(check_lldb_plugin_layering)
+  get_property(plugins GLOBAL PROPERTY LLDB_PLUGINS)
+  foreach(plugin ${plugins})
+    get_property(plugin_kind TARGET ${plugin} PROPERTY LLDB_PLUGIN_KIND)
+    get_property(acceptable_deps TARGET ${plugin}
+      PROPERTY LLDB_ACCEPTABLE_PLUGIN_DEPENDENCIES)
+    get_property(tolerated_deps TARGET ${plugin}
+      PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES)
+
+    # A plugin is always permitted to depend on its own kind for the purposes
+    # subclassing. Ideally the intra-kind dependencies should not form a loop,
+    # but we're not checking that here.
+    list(APPEND acceptable_deps ${plugin_kind})
+
+    list(APPEND all_plugin_kinds ${plugin_kind})
+
+    get_property(link_libs TARGET ${plugin} PROPERTY LINK_LIBRARIES)
+    foreach(link_lib ${link_libs})
+      if(link_lib IN_LIST plugins)
+        get_property(lib_kind TARGET ${link_lib} PROPERTY LLDB_PLUGIN_KIND)
+        if (lib_kind)
+          if (lib_kind IN_LIST acceptable_deps)
+            set(dep_kind green)
+          elseif (lib_kind IN_LIST tolerated_deps)
+            set(dep_kind yellow)
+          else()
+            set(dep_kind red)
+            message(SEND_ERROR "Plugin ${plugin} cannot depend on ${lib_kind} "
+              "plugin ${link_lib}")
+          endif()
+          list(APPEND dep_${dep_kind}_${plugin_kind}_${lib_kind} ${plugin})
+        endif()
+      endif()
+    endforeach()
+  endforeach()
+
+  if (LLDB_GENERATE_PLUGIN_DEP_GRAPH)
+    set(dep_graph "digraph Plugins {\n")
+    list(REMOVE_DUPLICATES all_plugin_kinds)
+    foreach (from ${all_plugin_kinds})
+      foreach (to ${all_plugin_kinds})
+        foreach (dep_kind green yellow red)
+          if (dep_${dep_kind}_${from}_${to})
+            list(REMOVE_DUPLICATES dep_${dep_kind}_${from}_${to})
+            string(REGEX REPLACE "lldbPlugin|${from}" "" short_deps
+              "${dep_${dep_kind}_${from}_${to}}")
+            string(JOIN "\n" plugins ${short_deps})
+            string(APPEND dep_graph
+              "  ${from}->${to}[color=\"${dep_kind}\" label=\"${plugins}\"];\n")
+          endif()
+        endforeach()
+      endforeach()
+    endforeach()
+    string(APPEND dep_graph "}\n")
+    file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/lldb-plugin-deps.dot" "${dep_graph}")
+  endif()
+endfunction()

lldb/docs/resources/contributing.rst

@@ -56,6 +56,56 @@ subset of LLDB tests (the API tests) use a different system. Refer to the
 `lldb/test <https://github.com/llvm/llvm-project/tree/main/lldb/test>`_ folder
 for examples.
 
+
+LLDB plugins and their dependencies
+-----------------------------------
+
+LLDB has a concept of *plugins*, which are used to provide abstraction
+boundaries over functionality that is specific to a certain architecture,
+operating system, programming language, etc. A plugin implements an abstract
+base class (rarely, a set of related base classes), which is a part of LLDB
+core. This setup allows the LLDB core to remain generic while making it possible
+to support for new architectures, languages, and so on. For this to work, all
+code needs to obey certain rules.
+
+The principal rule is that LLDB core (defined as: everything under lldb/source
+*minus* lldb/source/Plugins) must not depend on any specific plugin. The only
+way it can interact with them is through the abstract interface. Explicit
+dependencies such as casting the base class to the plugin type are not permitted
+and neither are more subtle dependencies like checking the name plugin or or
+other situations where some code in LLDB core is tightly coupled to the
+implementation details of a specific plugin.
+
+The rule for interaction between different plugins is more nuanced. We recognize
+that some cross-plugin dependencies are unavoidable or even desirable. For
+example, a plugin may want to extend a plugin of the same kind to
+add/override/refine some functionality (e.g., Android is a "kind of" Linux, but
+it handles some things differently). Alternatively, a plugin of one kind may
+want to build on the functionality offered by a specific plugin of another kind
+(ELFCore Process plugin uses ELF ObjectFile plugin to create a process out of an
+ELF core file).
+
+In cases such as these, direct dependencies are acceptable. However, to keep the
+dependency graph manageable, we still have some rules to govern these
+relationships:
+
+* All dependencies between plugins of the same kind must flow in the same
+  direction (if plugin `A1` depends on plugin `B1`, then `B2` must not depend on
+  `A2`)
+* Dependency graph of plugin kinds must not contain loops (dependencies like
+  `A1->B1`, `B2->C2` and `C3->A3` are forbidden because they induce a cycle in
+  the plugin kind graph even though the plugins themselves are acyclical)
+
+
+The first of these rules is checked via CMake scripts (using the
+`LLDB_ACCEPTABLE_PLUGIN_DEPENDENCIES` property). Dependencies in this category
+are expected and permitted (subject to other constraints such as that dependency
+making sense for the particular pair of plugins). Unfortunately, due to historic
+reasons, not all plugin dependencies follow this rule, which is why we have
+another category called `LLDB_TOLERATED_PLUGIN_DEPENDENCIES`. New dependencies
+are forbidden (even though they are accepted by CMake) and existing ones should
+be removed whereever possible.
+
 .. _Error handling:
 
 Error handling and use of assertions in LLDB

lldb/source/Plugins/ABI/CMakeLists.txt

@@ -1,3 +1,9 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND ABI)
+set_property(DIRECTORY PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES
+  ProcessUtility
+  TypeSystem
+)
+
 foreach(target AArch64 ARM ARC Hexagon LoongArch Mips MSP430 PowerPC RISCV SystemZ X86)
   if (${target} IN_LIST LLVM_TARGETS_TO_BUILD)
     add_subdirectory(${target})

lldb/source/Plugins/Architecture/CMakeLists.txt

@@ -1,3 +1,5 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND Architecture)
+
 add_subdirectory(Arm)
 add_subdirectory(Mips)
 add_subdirectory(PPC64)

lldb/source/Plugins/Disassembler/CMakeLists.txt

@@ -1 +1,3 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND Disassembler)
+
 add_subdirectory(LLVMC)

lldb/source/Plugins/DynamicLoader/CMakeLists.txt

@@ -1,3 +1,10 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND DynamicLoader)
+set_property(DIRECTORY PROPERTY LLDB_ACCEPTABLE_PLUGIN_DEPENDENCIES ObjectFile)
+set_property(DIRECTORY PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES
+  Process # part of a loop (Process<->DynamicLoader).
+  TypeSystem
+)
+
 add_subdirectory(Darwin-Kernel)
 add_subdirectory(FreeBSD-Kernel)
 add_subdirectory(MacOSX-DYLD)

lldb/source/Plugins/ExpressionParser/CMakeLists.txt

@@ -1 +1,3 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND ExpressionParser)
+
 add_subdirectory(Clang)

lldb/source/Plugins/Instruction/CMakeLists.txt

@@ -1,3 +1,5 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND Instruction)
+
 add_subdirectory(ARM)
 add_subdirectory(ARM64)
 add_subdirectory(LoongArch)

lldb/source/Plugins/InstrumentationRuntime/CMakeLists.txt

@@ -1,3 +1,5 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND InstrumentationRuntime)
+
 add_subdirectory(ASan)
 add_subdirectory(ASanLibsanitizers)
 add_subdirectory(MainThreadChecker)

lldb/source/Plugins/JITLoader/CMakeLists.txt

@@ -1 +1,4 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND JITLoader)
+set_property(DIRECTORY PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES ObjectFile)
+
 add_subdirectory(GDB)

lldb/source/Plugins/Language/CMakeLists.txt

@@ -1,3 +1,9 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND Language)
+set_property(DIRECTORY PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES
+  LanguageRuntime
+  TypeSystem
+)
+
 add_subdirectory(ClangCommon)
 add_subdirectory(CPlusPlus)
 add_subdirectory(ObjC)

lldb/source/Plugins/LanguageRuntime/CMakeLists.txt

@@ -1,2 +1,5 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND LanguageRuntime)
+set_property(DIRECTORY PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES TypeSystem)
+
 add_subdirectory(CPlusPlus)
 add_subdirectory(ObjC)

lldb/source/Plugins/MemoryHistory/CMakeLists.txt

@@ -1 +1,3 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND MemoryHistory)
+
 add_subdirectory(asan)

lldb/source/Plugins/ObjectContainer/CMakeLists.txt

@@ -1,3 +1,5 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND ObjectContainer)
+
 add_subdirectory(BSD-Archive)
 add_subdirectory(Universal-Mach-O)
 add_subdirectory(Mach-O-Fileset)

lldb/source/Plugins/ObjectFile/CMakeLists.txt

@@ -1,3 +1,5 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND ObjectFile)
+
 add_subdirectory(Breakpad)
 add_subdirectory(COFF)
 add_subdirectory(ELF)

lldb/source/Plugins/OperatingSystem/CMakeLists.txt

@@ -1,3 +1,5 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND OperatingSystem)
+
 if (LLDB_ENABLE_PYTHON)
   add_subdirectory(Python)
 endif()

lldb/source/Plugins/Platform/CMakeLists.txt

@@ -1,3 +1,10 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND Platform)
+set_property(DIRECTORY PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES
+  DynamicLoader
+  ObjectContainer
+  Process
+)
+
 add_subdirectory(AIX)
 add_subdirectory(Android)
 add_subdirectory(FreeBSD)

lldb/source/Plugins/Process/CMakeLists.txt

@@ -1,3 +1,8 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND Process)
+set_property(DIRECTORY PROPERTY LLDB_ACCEPTABLE_PLUGIN_DEPENDENCIES ObjectFile)
+# This dependency is part of a loop (Process<->DynamicLoader).
+set_property(DIRECTORY PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES DynamicLoader)
+
 if (CMAKE_SYSTEM_NAME MATCHES "Linux|Android")
   add_subdirectory(Linux)
   add_subdirectory(POSIX)

lldb/source/Plugins/Process/Utility/CMakeLists.txt

@@ -1,3 +1,6 @@
+# TODO: Clean up this directory and its dependencies
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND ProcessUtility)
+
 add_lldb_library(lldbPluginProcessUtility
   AuxVector.cpp
   FreeBSDSignals.cpp

lldb/source/Plugins/REPL/CMakeLists.txt

@@ -1 +1,4 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND REPL)
+set_property(DIRECTORY PROPERTY LLDB_ACCEPTABLE_PLUGIN_DEPENDENCIES TypeSystem)
+
 add_subdirectory(Clang)

lldb/source/Plugins/RegisterTypeBuilder/CMakeLists.txt

@@ -1,3 +1,5 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND RegisterTypeBuilder)
+
 add_lldb_library(lldbPluginRegisterTypeBuilderClang PLUGIN
   RegisterTypeBuilderClang.cpp
 

lldb/source/Plugins/ScriptInterpreter/CMakeLists.txt

@@ -1,3 +1,5 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND ScriptInterpreter)
+
 add_subdirectory(None)
 if (LLDB_ENABLE_PYTHON)
   add_subdirectory(Python)

lldb/source/Plugins/StructuredData/CMakeLists.txt

@@ -1,2 +1,4 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND StructuredData)
+
 add_subdirectory(DarwinLog)
 

lldb/source/Plugins/SymbolFile/CMakeLists.txt

@@ -1,3 +1,10 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND SymbolFile)
+set_property(DIRECTORY PROPERTY LLDB_ACCEPTABLE_PLUGIN_DEPENDENCIES ObjectFile)
+set_property(DIRECTORY PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES
+  Language
+  TypeSystem  # part of a loop (TypeSystem<->SymbolFile).
+)
+
 add_subdirectory(Breakpad)
 add_subdirectory(CTF)
 add_subdirectory(DWARF)

lldb/source/Plugins/SymbolLocator/CMakeLists.txt

@@ -1,3 +1,5 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND SymbolLocator)
+
 # Order matters here: the first symbol locator prevents further searching.
 # For DWARF binaries that are both stripped and split, the Default plugin
 # will return the stripped binary when asked for the ObjectFile, which then

lldb/source/Plugins/SymbolVendor/CMakeLists.txt

@@ -1,3 +1,6 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND SymbolVendor)
+set_property(DIRECTORY PROPERTY LLDB_ACCEPTABLE_PLUGIN_DEPENDENCIES ObjectFile)
+
 add_subdirectory(ELF)
 
 if (CMAKE_SYSTEM_NAME MATCHES "Darwin")

lldb/source/Plugins/SystemRuntime/CMakeLists.txt

@@ -1 +1,4 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND SystemRuntime)
+set_property(DIRECTORY PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES TypeSystem)
+
 add_subdirectory(MacOSX)

lldb/source/Plugins/Trace/CMakeLists.txt

@@ -1,3 +1,5 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND Trace)
+
 option(LLDB_BUILD_INTEL_PT "Enable Building of Intel(R) Processor Trace Tool" OFF)
 
 add_subdirectory(common)

lldb/source/Plugins/TraceExporter/CMakeLists.txt

@@ -1,2 +1,4 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND TraceExporter)
+
 add_subdirectory(common)
 add_subdirectory(ctf)

lldb/source/Plugins/TypeSystem/CMakeLists.txt

@@ -1 +1,5 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND TypeSystem)
+# This dependency is part of a loop (TypeSystem<->SymbolFile).
+set_property(DIRECTORY PROPERTY LLDB_TOLERATED_PLUGIN_DEPENDENCIES SymbolFile)
+
 add_subdirectory(Clang)

lldb/source/Plugins/UnwindAssembly/CMakeLists.txt

@@ -1,2 +1,4 @@
+set_property(DIRECTORY PROPERTY LLDB_PLUGIN_KIND UnwindAssembly)
+
 add_subdirectory(InstEmulation)
 add_subdirectory(x86)