Optional power-user treatment to reduce syscalls during startup

This commit provides the possibility to pass system library search
paths as well as some compiler include paths to ROOT as environment
variables.
This has the advantage that ROOT will not spawn sub-processes
and we can do the setup only once, instead of doing it for every
single executable that is linked to ROOT.

core/unix/src/TUnixSystem.cxx

@@ -4603,6 +4603,20 @@ static const char *DynamicPath(const char *newpath = 0, Bool_t reset = kFALSE)
       dynpath += "/usr/local/lib:/usr/X11R6/lib:/usr/lib:/lib:";
       dynpath += "/lib/x86_64-linux-gnu:/usr/local/lib64:/usr/lib64:/lib64:";
 #else
+      // ALICE: Let's see if an outside entity gave us the system path.
+      // This is a power-user feature with the goal to prevent repeated sys-calls (popen + sh) and to
+      // having to search repeatetly through potentially long strings (when the LD_LIBRARY_PATH of the software stack
+      // is very large).
+      // The power-user might define this during build or during setup of the software stack.
+      auto ldsyspath = getenv("ROOT_LDSYSPATH");
+      if (ldsyspath != nullptr) {
+         dynpath += ldsyspath;
+         // some code duplication here in order to keep patch
+         // minimal and easily applicable
+         dynpath.ReplaceAll("::", ":");
+         return dynpath;
+      }
+
       // trick to get the system search path
       std::string cmd("LD_DEBUG=libs LD_PRELOAD=DOESNOTEXIST ls 2>&1");
       FILE *pf = popen(cmd.c_str (), "r");

interpreter/cling/lib/Interpreter/CIFactory.cpp

@@ -107,6 +107,22 @@ namespace {
                                        llvm::SmallVectorImpl<char>& Buf,
                                        AdditionalArgList& Args,
                                        bool Verbose) {
+    // ALICE: let's see if the path is available as a predefined env variable
+    // to save repeated system calls when ROOT is initialized for each process
+    // in a many process system
+    auto PrefCppSystemIncl = getenv("ROOT_CPPSYSINCL");
+    if (PrefCppSystemIncl != nullptr) {
+      llvm::StringRef PathsString(PrefCppSystemIncl);
+      llvm::SmallVector<StringRef, 10> Paths;
+      PathsString.split(Paths, ":");
+      for (auto& P : Paths) {
+        P = P.trim();
+        Args.addArgument("-cxx-isystem", P.str());
+      }
+      return;
+    }
+
+    // execute default dynamic search
     std::string CppInclQuery("LC_ALL=C ");
     CppInclQuery.append(Compiler);
 

interpreter/cling/lib/Utils/PlatformPosix.cpp

@@ -182,6 +182,19 @@ bool GetSystemLibraryPaths(llvm::SmallVectorImpl<std::string>& Paths) {
   Paths.push_back("/lib64/");
  #endif
 #else
+  // see if the result if this query is cached/provided in env variable
+  // power-user mode to avoid sys-calls and spawning processes
+  auto ldsyspath = getenv("ROOT_LDSYSPATH");
+  if (ldsyspath != nullptr) {
+    std::string SysPath(ldsyspath);
+    llvm::SmallVector<llvm::StringRef, 10> CurPaths;
+    SplitPaths(SysPath, CurPaths);
+    for (const auto& Path : CurPaths) {
+      Paths.push_back(Path.str());
+    }
+    return true;
+  }
+
   llvm::SmallString<1024> Buf;
   platform::Popen("LD_DEBUG=libs LD_PRELOAD=DOESNOTEXIST ls", Buf, true);
   const llvm::StringRef Result = Buf.str();