Use the dyld logic if we don't have library dependency information av…

…ailable.

In cases where ACLiC builds a library without a rootmap file or a modulemap file
we have no reasonable way to detect what are the library dependencies.
Occasionally when loading a shared object the dynamic linker will tell us if
something is missing (via dlerror) but only on the intialization path.

This patch fixes ROOT-10637.

core/metacling/src/TCling.cxx

@@ -3050,6 +3050,12 @@ Bool_t TCling::HasPCMForLibrary(const char *libname) const
    ModuleName = llvm::sys::path::stem(ModuleName);
    ModuleName.consume_front("lib");
 
+   // FIXME: In case when the modulemap is not yet loaded we will return the
+   // wrong result. Consider a call to HasPCMForLibrary(../test/libEvent.so)
+   // We will only load the modulemap for libEvent.so after we dlopen libEvent
+   // which may happen after calling this interface. Maybe we should also check
+   // if there is a Event.pcm file and a module.modulemap, load it and return
+   // true.
    clang::ModuleMap &moduleMap = fInterpreter->getCI()->getPreprocessor().getHeaderSearchInfo().getModuleMap();
    clang::Module *M = moduleMap.findModule(ModuleName);
    return M && !M->IsMissingRequirement && M->getASTFile();
@@ -7176,6 +7182,31 @@ static std::string GetSharedLibImmediateDepsSlow(std::string lib,
 }
 
 
+static bool hasPrecomputedLibraryDeps(llvm::StringRef lib) {
+   // FIXME: Provide more robust discovery of rootmap files when `lib` is not
+   // with its full path.
+   if (gCling->HasPCMForLibrary(lib.data()))
+      return true;
+
+   // Check if we have parsed a rootmap file.
+   llvm::SmallString<256> rootmapName;
+   if (!lib.startswith("lib"))
+      rootmapName.append("lib");
+
+   rootmapName.append(llvm::sys::path::filename(lib));
+   llvm::sys::path::replace_extension(rootmapName, "rootmap");
+
+   if (gCling->GetRootMapFiles()->FindObject(rootmapName.c_str()))
+      return true;
+
+   // Perform a last resort by dropping the lib prefix.
+   llvm::StringRef rootmapNameNoLib = rootmapName.str();
+   if (rootmapNameNoLib.consume_front("lib"))
+      return gCling->GetRootMapFiles()->FindObject(rootmapNameNoLib.data());
+
+   return false;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 /// Get the list a libraries on which the specified lib depends. The
 /// returned string contains as first element the lib itself.
@@ -7186,6 +7217,15 @@ static std::string GetSharedLibImmediateDepsSlow(std::string lib,
 
 const char* TCling::GetSharedLibDeps(const char* lib, bool useDyld/* = false*/)
 {
+   if (!useDyld && !hasPrecomputedLibraryDeps(lib)) {
+      // FIXME: As a future performance optimization see HasPCMForLibrary's FIXME.
+      useDyld = true;
+      if (gDebug > 2)
+         Warning("TCling::GetSharedLibDeps",
+                 "Trying to load %s but its dependencies are unresolved.",
+                 lib);
+   }
+
    if (useDyld) {
       std::string libs = GetSharedLibImmediateDepsSlow(lib, GetInterpreterImpl());
       if (!libs.empty()) {