[lldb/qemu] Set qemu's "ld prefix" based on the platform sysroot

Both serve the same purpose (finding shared libraries) and allow one to
launch a dynamically linked executable by just specifying the platform
sysroot.

Author: labath

lldb/source/Plugins/Platform/QemuUser/PlatformQemuUser.cpp

@@ -191,6 +191,8 @@ lldb::ProcessSP PlatformQemuUser::DebugProcess(ProcessLaunchInfo &launch_info,
   launch_info.SetArguments(args, true);
 
   Environment emulator_env = Host::GetEnvironment();
+  if (ConstString sysroot = GetSDKRootDirectory())
+    emulator_env["QEMU_LD_PREFIX"] = sysroot.GetStringRef().str();
   for (const auto &KV : GetGlobalProperties().GetEmulatorEnvVars())
     emulator_env[KV.first()] = KV.second;
   launch_info.GetEnvironment() = ComputeLaunchEnvironment(

lldb/test/API/qemu/TestQemuLaunch.py

@@ -249,3 +249,11 @@ def test_arg0(self):
 
         self.assertEqual(state["program"], self.getBuildArtifact())
         self.assertEqual(state["0"], "ARG0")
+
+    def test_sysroot(self):
+        sysroot = self.getBuildArtifact("sysroot")
+        self.runCmd("platform select qemu-user --sysroot %s" % sysroot)
+        state = self._run_and_get_state()
+        self.assertEqual(state["environ"]["QEMU_LD_PREFIX"], sysroot)
+        self.assertIn("QEMU_LD_PREFIX",
+                state["environ"]["QEMU_UNSET_ENV"].split(","))