[lldb] Issue a warning when Target XML should have been used but we do not have libxml2

lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

@@ -441,8 +441,16 @@ void ProcessGDBRemote::BuildDynamicRegisterInfo(bool force) {
   if (!arch_to_use.IsValid())
     arch_to_use = target_arch;
 
-  if (GetGDBServerRegisterInfo(arch_to_use))
+  llvm::Error register_info_err = GetGDBServerRegisterInfo(arch_to_use);
+  if (!register_info_err) {
+    // We got the registers from target XML.
     return;
+  }
+
+  Log *log = GetLog(GDBRLog::Process);
+  LLDB_LOG_ERROR(log, std::move(register_info_err),
+                 "Failed to read register information from target XML: {0}");
+  LLDB_LOG(log, "Now trying to use qRegisterInfo instead.");
 
   char packet[128];
   std::vector<DynamicRegisterInfo::Register> registers;
@@ -541,7 +549,23 @@ void ProcessGDBRemote::BuildDynamicRegisterInfo(bool force) {
         assert(reg_info.byte_size != 0);
         registers.push_back(reg_info);
       } else {
-        break; // ensure exit before reg_num is incremented
+        // Only warn if we were offered Target XML and could not use it, and
+        // the qRegisterInfo fallback failed. This is something a user could
+        // take action on by getting an lldb with libxml2.
+        //
+        // It's possible we weren't offered Target XML and qRegisterInfo failed,
+        // but there's no much a user can do about that. It may be the intended
+        // way the debug stub works, so we do not warn for that case.
+        if (response_type == StringExtractorGDBRemote::eUnsupported &&
+            m_gdb_comm.GetQXferFeaturesReadSupported() &&
+            !XMLDocument::XMLEnabled()) {
+          Debugger::ReportWarning(
+              "the debug server supports Target Description XML but LLDB does "
+              "not have XML parsing enabled. Using \"qRegisterInfo\" was also "
+              "not possible. Register information may be incorrect or missing.",
+              GetTarget().GetDebugger().GetID());
+        }
+        break;
       }
     } else {
       break;
@@ -5137,14 +5161,19 @@ void ProcessGDBRemote::AddRemoteRegisters(
 
 // query the target of gdb-remote for extended target information returns
 // true on success (got register definitions), false on failure (did not).
-bool ProcessGDBRemote::GetGDBServerRegisterInfo(ArchSpec &arch_to_use) {
-  // Make sure LLDB has an XML parser it can use first
-  if (!XMLDocument::XMLEnabled())
-    return false;
-
-  // check that we have extended feature read support
+llvm::Error ProcessGDBRemote::GetGDBServerRegisterInfo(ArchSpec &arch_to_use) {
+  // If the remote does not offer XML, does not matter if we would have been
+  // able to parse it.
   if (!m_gdb_comm.GetQXferFeaturesReadSupported())
-    return false;
+    return llvm::createStringError(
+        llvm::inconvertibleErrorCode(),
+        "the debug server does not support \"qXfer:features:read\"");
+
+  if (!XMLDocument::XMLEnabled())
+    return llvm::createStringError(
+        llvm::inconvertibleErrorCode(),
+        "the debug server supports \"qXfer:features:read\", but LLDB does not "
+        "have XML parsing enabled (check LLLDB_ENABLE_LIBXML2)");
 
   // These hold register type information for the whole of target.xml.
   // target.xml may include further documents that
@@ -5161,7 +5190,11 @@ bool ProcessGDBRemote::GetGDBServerRegisterInfo(ArchSpec &arch_to_use) {
       !registers.empty())
     AddRemoteRegisters(registers, arch_to_use);
 
-  return m_register_info_sp->GetNumRegisters() > 0;
+  return m_register_info_sp->GetNumRegisters() > 0
+             ? llvm::ErrorSuccess()
+             : llvm::createStringError(
+                   llvm::inconvertibleErrorCode(),
+                   "the debug server did not describe any registers");
 }
 
 llvm::Expected<LoadedModuleInfoList> ProcessGDBRemote::GetLoadedModuleList() {

lldb/source/Plugins/Process/gdb-remote/ProcessGDBRemote.h

@@ -416,7 +416,7 @@ class ProcessGDBRemote : public Process,
   void AddRemoteRegisters(std::vector<DynamicRegisterInfo::Register> &registers,
                           const ArchSpec &arch_to_use);
   // Query remote GDBServer for register information
-  bool GetGDBServerRegisterInfo(ArchSpec &arch);
+  llvm::Error GetGDBServerRegisterInfo(ArchSpec &arch);
 
   lldb::ModuleSP LoadModuleAtAddress(const FileSpec &file,
                                      lldb::addr_t link_map,