[lldb] Improve logging of failure to get register information from Target XML (#170478)

In
https://discourse.llvm.org/t/does-lldb-qemu-support-dumping-x64-control-registers-such-as-cr3/89031
a user was not seeing certain registers when connected to QEMU. Turns
out their LLDB build did not have libxml2 enabled.

While logging is not the first thing most users will think of, it is
something an expert can ask for to confirm whether they have XML support
enabled.

So in this PR I've shuffled the logic GetGDBServerRegisterInfo to better
report problems in the log.

The key one is when lldb does not have libxml2 but the server did say it
supports qxfer:features. In this case we would have used it if we could,
and the debug session will likely be degraded because we are not able
to.

https://sourceware.org/gdb/current/onlinedocs/gdb.html/General-Query-Packets.html#qXfer-target-description-read

Author: DavidSpickett

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

@@ -439,8 +439,16 @@ 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;
@@ -5135,14 +5143,19 @@ 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
@@ -5159,7 +5172,11 @@ 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,