[lldb] [server] Support for multiprocess extension

Add a minimal support for the multiprocess extension in lldb-server.
The server indicates support for it via qSupported, and accepts
thread-ids containing a PID.  However, it still does not support
debugging more than one inferior, so any other PID value results
in an error.

Differential Revision: https://reviews.llvm.org/D98482

lldb/source/Utility/StringExtractorGDBRemote.cpp

@@ -11,6 +11,9 @@
 #include <ctype.h>
 #include <string.h>
 
+constexpr lldb::pid_t StringExtractorGDBRemote::AllProcesses;
+constexpr lldb::tid_t StringExtractorGDBRemote::AllThreads;
+
 StringExtractorGDBRemote::ResponseType
 StringExtractorGDBRemote::GetResponseType() const {
   if (m_packet.empty())
@@ -606,3 +609,46 @@ bool StringExtractorGDBRemote::ValidateResponse() const {
   else
     return true; // No validator, so response is valid
 }
+
+llvm::Optional<std::pair<lldb::pid_t, lldb::tid_t>>
+StringExtractorGDBRemote::GetPidTid(lldb::pid_t default_pid) {
+  llvm::StringRef view = llvm::StringRef(m_packet).substr(m_index);
+  size_t initial_length = view.size();
+  lldb::pid_t pid = default_pid;
+  lldb::tid_t tid;
+
+  if (view.consume_front("p")) {
+    // process identifier
+    if (view.consume_front("-1")) {
+      // -1 is a special case
+      pid = AllProcesses;
+    } else if (view.consumeInteger(16, pid) || pid == 0) {
+      // not a valid hex integer OR unsupported pid 0
+      m_index = UINT64_MAX;
+      return llvm::None;
+    }
+
+    // "." must follow if we expect TID too; otherwise, we assume -1
+    if (!view.consume_front(".")) {
+      // update m_index
+      m_index += initial_length - view.size();
+
+      return {{pid, AllThreads}};
+    }
+  }
+
+  // thread identifier
+  if (view.consume_front("-1")) {
+    // -1 is a special case
+    tid = AllThreads;
+  } else if (view.consumeInteger(16, tid) || tid == 0 || pid == AllProcesses) {
+    // not a valid hex integer OR tid 0 OR pid -1 + a specific tid
+    m_index = UINT64_MAX;
+    return llvm::None;
+  }
+
+  // update m_index
+  m_index += initial_length - view.size();
+
+  return {{pid, tid}};
+}