[lldb] Warn when we fail to find dwo/dwp files

[labath]

Summary

This ensures that the user is aware that many commands will not work
correctly.

We print the warning only once (per module) to avoid spamming the user
with potentially thousands of error messages.

[JDevlieghere]

LGTM.

FWIW I'm planning some changes to the error reporting "soonish". The gist is using error/warning events which Xcode (or VSCode) can subscribe to. I'll send out an RFC with more details as I get closer to working on this.

[labath]

This code can be invoked concurrently (during indexing). Use atomic test-and-set
to ensure correctness.

[JDevlieghere]

Might a llvm::once_flag + llvm::call_once be simpler?

[labath]

In D120892#3357424, @JDevlieghere wrote:

FWIW I'm planning some changes to the error reporting "soonish". The gist is using error/warning events which Xcode (or VSCode) can subscribe to. I'll send out an RFC with more details as I get closer to working on this.

Sounds like a good idea.

[labath]

In D120892#3357780, @JDevlieghere wrote:

Might a llvm::once_flag + llvm::call_once be simpler?

Simpler... maybe.. depends how you look at it.

But, synchronization-wise, it is overkill. once_flag ensures that none of the racing threads can make progress until the action terminates. Most of the time, that's exactly what we want, but here the correct operation of the other threads is not affected by this action, so it is sufficient that the warning gets printed "eventually".

[JDevlieghere]

Cool, thanks for the explanation. Ship it.

lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.h

protected:
   /// DWARF does not provide a good way for traditional (concatenating) linkers
   /// to invalidate debug info describing dead-stripped code. These linkers will
   /// keep the debug info but resolve any addresses referring to such code as
   /// zero (BFD) or a small positive integer (zero + relocation addend -- GOLD).
   /// Try to filter out this debug info by comparing it to the lowest code
   /// address in the module.
   lldb::addr_t m_first_code_address = LLDB_INVALID_ADDRESS;
   lldb_private::StatsDuration m_parse_time;
+  std::atomic_flag m_dwo_warning_issued = ATOMIC_FLAG_INIT;
 };
 
 #endif // LLDB_SOURCE_PLUGINS_SYMBOLFILE_DWARF_SYMBOLFILEDWARF_H

lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp

if (dwo_file.IsRelative()) {
       // launched.
       dwo_file.PrependPathComponent(
           m_objfile_sp->GetFileSpec().GetDirectory().GetStringRef());
     }
     FileSystem::Instance().Resolve(dwo_file);
     dwo_file.AppendPathComponent(dwo_name);
   }
 
-  if (!FileSystem::Instance().Exists(dwo_file))
+  if (!FileSystem::Instance().Exists(dwo_file)) {
+    if (m_dwo_warning_issued.test_and_set(std::memory_order_relaxed) == false) {
+      GetObjectFile()->GetModule()->ReportWarning(
+          "unable to locate separate debug file (dwo, dwp). Debugging will be "
+          "degraded.");
+    }
     return nullptr;
+  }
 
   const lldb::offset_t file_offset = 0;
   DataBufferSP dwo_file_data_sp;
   lldb::offset_t dwo_file_data_offset = 0;
   ObjectFileSP dwo_obj_file = ObjectFile::FindPlugin(
       GetObjectFile()->GetModule(), &dwo_file, file_offset,
       FileSystem::Instance().GetByteSize(dwo_file), dwo_file_data_sp,
       dwo_file_data_offset);

lldb/test/Shell/SymbolFile/DWARF/x86/dwo-not-found-warning.cpp

+// RUN: %clang --target=x86_64-pc-linux -g -gsplit-dwarf -c %s -o %t.o
+// RUN: rm %t.dwo
+// RUN: %lldb %t.o -o "br set -n main" -o exit 2>&1 | FileCheck %s
+
+// CHECK: warning: {{.*}} unable to locate separate debug file (dwo, dwp). Debugging will be degraded.
+
+int main() { return 47; }