[gRPC][discovery] panic: JSON decoder out of sync - data changing underfoot?

[kittaakos]

Bug Report

Current behavior

I am using the CLI via gRPC from a JS-based application. My client code performs mainly the following requests:

• PlatformListReq,
• PlatformSearchReq, and
• BoardListReq.

Due to the async nature of the client, I am working on, and these requests might run in parallel which from time to time results in a panic error from the discovery code. After the failure, the CLI crashes, and the daemon process terminates.

panic: JSON decoder out of sync - data changing underfoot?
goroutine 42 [running]:
encoding/json.(*decodeState).value(0xc000130188, 0x46e4da0, 0xc001ffa390, 0x16, 0xa, 0xa)
    /usr/local/go/src/encoding/json/decode.go:367 +0x23f
encoding/json.(*decodeState).unmarshal(0xc000130188, 0x46e4da0, 0xc001ffa390, 0x0, 0xc0004b2480)
    /usr/local/go/src/encoding/json/decode.go:179 +0x209
encoding/json.(*Decoder).Decode(0xc000130160, 0x46e4da0, 0xc001ffa390, 0xc001ff6055, 0xc000eba9c0)
    /usr/local/go/src/encoding/json/stream.go:73 +0x187
github.com/arduino/arduino-cli/arduino/discovery.(*Discovery).List(0xc000eba9c0, 0xc001ffa330, 0xc0000cc1c0, 0x1, 0x1, 0x4e8a7c0)
    /Users/akos.kitta/git/arduino-cli/arduino/discovery/discovery.go:105 +0x230
github.com/arduino/arduino-cli/commands/board.List(0x4933f80, 0xc001ffa300, 0xc001ffa330, 0x48064e0, 0xc001ff8100, 0x49412c0)
    /Users/akos.kitta/git/arduino-cli/commands/board/list.go:38 +0x609
github.com/arduino/arduino-cli/commands/daemon.(*ArduinoCoreServerImpl).BoardList(0xc000208200, 0x4933f80, 0xc001ffa300, 0xc001ffa330, 0xc000208200, 0xc001ffa300, 0xc000075ba8)
    /Users/akos.kitta/git/arduino-cli/commands/daemon/daemon.go:52 +0x3f
github.com/arduino/arduino-cli/rpc/commands._ArduinoCore_BoardList_Handler(0x48064e0, 0xc000208200, 0x4933f80, 0xc001ffa300, 0xc001ff81e0, 0x0, 0x4933f80, 0xc001ffa300, 0xc001ff604c, 0x4)
    /Users/akos.kitta/git/arduino-cli/rpc/commands/commands.pb.go:1551 +0x23e
google.golang.org/grpc.(*Server).processUnaryRPC(0xc00009a780, 0x493bd00, 0xc00009ad80, 0xc00021a800, 0xc0001fc900, 0x4e90ea0, 0x0, 0x0, 0x0)
    /Users/akos.kitta/go/pkg/mod/google.golang.org/grpc@v1.21.1/server.go:998 +0x470
google.golang.org/grpc.(*Server).handleStream(0xc00009a780, 0x493bd00, 0xc00009ad80, 0xc00021a800, 0x0)
    /Users/akos.kitta/go/pkg/mod/google.golang.org/grpc@v1.21.1/server.go:1278 +0xda6
google.golang.org/grpc.(*Server).serveStreams.func1.1(0xc0000e9320, 0xc00009a780, 0x493bd00, 0xc00009ad80, 0xc00021a800)
    /Users/akos.kitta/go/pkg/mod/google.golang.org/grpc@v1.21.1/server.go:717 +0x9f
created by google.golang.org/grpc.(*Server).serveStreams.func1
    /Users/akos.kitta/go/pkg/mod/google.golang.org/grpc@v1.21.1/server.go:715 +0xa1

After making my client code a bit more relaxed, the error happens less frequently, but the daemon produces the following error:

Error getting port list from discovery /Users/akos.kitta/xxx/data/packages/builtin/tools/serial-discovery/0.5.0/serial-discovery: sending LIST command to discovery: write |1: file already closed
Error getting port list from discovery /Users/akos.kitta/xxx/data/packages/builtin/tools/serial-discovery/0.5.0/serial-discovery: sending LIST command to discovery: write |1: file already closed

This could lead to unpredictable behavior.

Question: is the CLI capable of processing concurrent requests via gRPC?

Expected behavior

The CLI does not crash when it has to process multiple gRPC requests concurrently.

Environment

• CLI version (output of arduino-cli version):
• Go version (if building from sources):
• OS version:
  macOS 10.14.4 (18E226)

Additional context

[masci]

I was able to reproduce, the problematic command is BoardList, any concurrent execution (even with just 2 goroutines) provides the same output you get