mulint

mulint is a program/lint to check potential dead locks on go programs. The documentation for sync.RWMutex says:

If a goroutine holds a RWMutex for reading and another goroutine might call Lock, no goroutine should expect to be able to acquire a read lock until the initial read lock is released. In particular, this prohibits recursive read locking. This is to ensure that the lock eventually becomes available; a blocked Lock call excludes new readers from acquiring the lock.

As this is stated on the documentation, but is not enforced by the language nor the library, this program tries to detect some recursive read locks on the same goroutine.

Install

As usual on Go environment, to install, run the following command:

go get -u github.com/gnieto/mulint

This should install the lint on your $GOPATH.

Running

To run the lint, you just need to:

mulint ./...

Note that this lint uses golang.org/x/tools, so all the usual wildcards to filter or select packages should work.

On the following lines, you can see an example of output of this lint:

mixed_locks.go:27: Mutex lock is acquired on this line: m.Test()
	mixed_locks.go:24: But the same lock was acquired here: m.m.Lock()

The first line indicates where the recursive lock is acquired (so, the second Lock on the same goroutine), the file and line where it occurs. The second one, indicate where the first Lock occurs

Exit code

If the program detects some error, it will return a 1 as error code

Limitations

At the moment, it can only detect some basic scenarios when a Mutex or RWMutex is part of a struct and all the Lock and Unlock operations are executed inside the scopes of the mentioned struct. So, it may detect a recursive call like the following one:

type someStruct struct {
  m sync.RWMutex
}

func (s *someStruct) A() {
  s.m.RLock()
  s.B()
  s.m.RUnlock()
}

func (s *someStruct) B() {
  s.m.RLock() // This is a recursive lock and it should be detected by this tool
  s.m.RUnlock()
}

But it's not able to detect a recursive lock when a mutex is moved to another scope, as it may happen with funcs

var m sync.RWMutex

A(m)

func A(m *sync.RWMutex) {
  m.RLock()
  B(m)
  m.RUnlock()
}

func B(m *sync.RWMutex) {
  m.RLock()
  m.RUnlock()
}

There is no technical limitation (as far as I am aware) to track when a mutex is moved, but I left this cases for now. Feel free to do the change and open a PR to include this case.

Finally, all the analysis is done per package. If there's some recursive RLock which operates among distinct packages, it won't be detected by this lint.

Attribution

• I have checked the errcheck code (https://github.com/kisielk/errcheck) as an example of how the AST is walked on Go
• Lint uses gosec (https://github.com/securego/gosec), just for a method that refused to re-writte myself (GetCallInfo)