Use QLibrary to find librustc_demangle at runtime

Don't link to librustc_demangle at compile time, just try to find it
at runtime via QLibrary and then use it. This makes it much easier
to enable/disable support for rust demangling for packagers.

Fixes: https://github.com/KDAB/hotspot/issues/269
Change-Id: Ib33e2699780ed6d827bddeea75f82137daeacc9d
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

4 files changed, 55 insertions, 15 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index b892b27..d379096 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -8,10 +8,12 @@ endif()
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 find_package(Zstd)
 
-set(RUSTC_DEMANGLE_INCLUDE_DIR "" CACHE STRING "Path to the folder containing rustc_demangle.h from https://github.com/alexcrichton/rustc-demangle")
-set(RUSTC_DEMANGLE_LIBRARY "" CACHE STRING "Path to the librustc_demangle.so library from https://github.com/alexcrichton/rustc-demangle")
-option(WITH_RUSTC_DEMANGLE "Build rust demangle support" OFF)
-add_feature_info("Rust demangler for perfparser" WITH_RUSTC_DEMANGLE "Requires RUSTC_DEMANGLE_INCLUDE_DIR and RUSTC_DEMANGLE_LIBRARY to be specified")
+find_package(LibRustcDemangle)
+set_package_properties(LibRustcDemangle PROPERTIES
+        DESCRIPTION "Demangling for Rust symbols, written in Rust."
+        PURPOSE "Demangling of Rust symbols"
+        URL "https://github.com/alexcrichton/rustc-demangle"
+        TYPE RUNTIME)
 
 add_subdirectory(app)
 add_subdirectory(tests)
diff --git a/app/CMakeLists.txt b/app/CMakeLists.txt
index 746583e..b324907 100644
--- a/app/CMakeLists.txt
+++ b/app/CMakeLists.txt
@@ -30,12 +30,6 @@ if (Zstd_FOUND)
     target_compile_definitions(perfparser_lib PRIVATE HAVE_ZSTD=1)
 endif()
 
-if (WITH_RUSTC_DEMANGLE)
-    target_include_directories(perfparser_lib PRIVATE ${RUSTC_DEMANGLE_INCLUDE_DIR})
-    target_link_libraries(perfparser_lib PRIVATE ${RUSTC_DEMANGLE_LIBRARY})
-    target_compile_definitions(perfparser_lib PRIVATE HAVE_RUSTC_DEMANGLE=1)
-endif()
-
 add_qtc_executable(perfparser
   DEPENDS
     perfparser_lib
diff --git a/app/perfdwarfdiecache.cpp b/app/perfdwarfdiecache.cpp
index 4823646..4b70c0b 100644
--- a/app/perfdwarfdiecache.cpp
+++ b/app/perfdwarfdiecache.cpp
@@ -22,11 +22,35 @@
 
 #include <dwarf.h>
 
-#ifdef HAVE_RUSTC_DEMANGLE
-#include <rustc_demangle.h>
-#endif
+#include <QLibrary>
+#include <QDebug>
 
 namespace {
+bool rustc_demangle(const char *symbol, char *buffer, size_t bufferSize)
+{
+    using demangler_t = int (*) (const char*, char *, size_t);
+    static const auto demangler = []() -> demangler_t {
+        QLibrary lib(QStringLiteral("rustc_demangle"));
+        if (!lib.load()) {
+            qDebug() << "failed to load rustc_demangle library, rust demangling is support not available."
+                     << lib.errorString();
+            return nullptr;
+        }
+        const auto rawSymbol = lib.resolve("rustc_demangle");
+        if (!rawSymbol) {
+            qDebug() << "failed to resolve rustc_demangle function in library"
+                     << lib.fileName() << lib.errorString();
+            return nullptr;
+        }
+        return reinterpret_cast<demangler_t>(rawSymbol);
+    }();
+
+    if (demangler)
+        return demangler(symbol, buffer, bufferSize);
+    else
+        return false;
+}
+
 enum class WalkResult
 {
     Recurse,
@@ -227,10 +251,8 @@ QByteArray demangle(const QByteArray &mangledName)
         static size_t demangleBufferLength = 1024;
         static char *demangleBuffer = reinterpret_cast<char *>(eu_compat_malloc(demangleBufferLength));
 
-#ifdef HAVE_RUSTC_DEMANGLE
         if (rustc_demangle(mangledName.constData(), demangleBuffer, demangleBufferLength))
             return demangleBuffer;
-#endif
 
         // Require GNU v3 ABI by the "_Z" prefix.
         if (mangledName[0] == '_' && mangledName[1] == 'Z') {
diff --git a/cmake/FindLibRustcDemangle.cmake b/cmake/FindLibRustcDemangle.cmake
new file mode 100644
index 0000000..c247e16
--- /dev/null
+++ b/cmake/FindLibRustcDemangle.cmake
@@ -0,0 +1,22 @@
+if (LIBRUSTC_DEMANGLE_LIBRARIES)
+  set (LibRustcDemangle_FIND_QUIETLY TRUE)
+endif()
+
+find_library(LIBRUSTC_DEMANGLE_LIBRARIES
+    NAMES
+      rustc_demangle
+    PATHS
+      /usr/lib
+      /usr/local/lib
+      /opt/local/lib
+      /sw/lib
+      ENV LIBRARY_PATH
+      ENV LD_LIBRARY_PATH)
+
+include (FindPackageHandleStandardArgs)
+
+# handle the QUIETLY and REQUIRED arguments and set LIBRUSTC_DEMANGLE_FOUND to TRUE if all listed variables are TRUE
+find_package_handle_standard_args(LibRustcDemangle DEFAULT_MSG
+    LIBRUSTC_DEMANGLE_LIBRARIES)
+
+mark_as_advanced(LIBRUSTC_DEMANGLE_LIBRARIES)