[lldb/gdb-remote] remove junk bytes, then decode the packet

[borneoa]

Summary

In case of receiving, in a single packet, some junk byte followed
by a complete gdb-remote reply, e.g. "junk+$OK#9a", the current
implementation of CheckForPacket() drops the junk bytes and
returns.
The caller will call again CheckForPacket() only if it receives
another packet. But the reply is already complete and the remote
will not send anything else, so the caller will timeout.

Check for junk bytes and drop them, then decode the packet.

[borneoa]

resend it on top of its dependency
[lldb/gdb-remote] drop all junk bytes in incoming packet

[borneoa]

Sorry, very first time user of arcanist and phabricator.
This revision was actually made by two independent patches that get squashed!
Abandoned, I will try to send the patches separately

[clayborg]

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

650–676

Might be easier to use std::string methods to do this:

const size_t idx = m_bytes.find_first_of("+-\x03%$");
if (idx != 0) {
      LLDB_LOGF(log, "GDBRemoteCommunication::%s tossing %u junk bytes: '%.*s'",
                __FUNCTION__, idx, idx, m_bytes.c_str());
      m_bytes.erase(0, idx);
}

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

if (!m_bytes.empty()) {
     size_t original_packet_size = m_bytes.size();
     if (CompressionIsEnabled()) {
       if (!DecompressPacket()) {
         packet.Clear();
         return GDBRemoteCommunication::PacketType::Standard;
       }
     }
 
+    // Check if we have an unexpected byte and we need to flush all bad data
+    // that is in m_bytes, so we need to find the first byte that is a '+'
+    // (ACK), '-' (NACK), \x03 (CTRL+C interrupt), or '$' character (start of
+    // packet header) or of course, the end of the data in m_bytes...
+    const size_t bytes_len = m_bytes.size();
+    bool done = false;
+    uint32_t idx;
+    for (idx = 0; !done && idx < bytes_len;) {
+      switch (m_bytes[idx]) {
+      case '+':
+      case '-':
+      case '\x03':
+      case '%':
+      case '$':
+        done = true;
+        break;
+
+      default:
+        ++idx;
+        break;
+      }
+    }
+    if (idx) {
+      LLDB_LOGF(log, "GDBRemoteCommunication::%s tossing %u junk bytes: '%.*s'",
+                __FUNCTION__, idx, idx, m_bytes.c_str());
+      m_bytes.erase(0, idx);
+    }
+
     switch (m_bytes[0]) {
     case '+':                            // Look for ack
     case '-':                            // Look for cancel
     case '\x03':                         // ^C to halt target
       content_length = total_length = 1; // The command is one byte long...
       break;
 
     case '%': // Async notify packet
case '$':
           } else {
             // Checksum bytes aren't all here yet
             content_length = std::string::npos;
           }
         }
       }
       break;
 
-    default: {
-      // We have an unexpected byte and we need to flush all bad data that is
-      // in m_bytes, so we need to find the first byte that is a '+' (ACK), '-'
-      // (NACK), \x03 (CTRL+C interrupt), or '$' character (start of packet
-      // header) or of course, the end of the data in m_bytes...
-      const size_t bytes_len = m_bytes.size();
-      bool done = false;
-      uint32_t idx;
-      for (idx = 1; !done && idx < bytes_len; ++idx) {
-        switch (m_bytes[idx]) {
-        case '+':
-        case '-':
-        case '\x03':
-        case '%':
-        case '$':
-          done = true;
-          break;
-
-        default:
-          break;
-        }
-      }
-      LLDB_LOGF(log, "GDBRemoteCommunication::%s tossing %u junk bytes: '%.*s'",
-                __FUNCTION__, idx - 1, idx - 1, m_bytes.c_str());
-      m_bytes.erase(0, idx - 1);
-    } break;
-    }
+      // no more bytes after junk
 
     if (content_length == std::string::npos) {
       packet.Clear();
       return GDBRemoteCommunication::PacketType::Invalid;
     } else if (total_length > 0) {
 
       // We have a valid packet...
       assert(content_length <= m_bytes.size());