The Problem:

Suddenly got below errors which killed my daemon:

fatal error: concurrent map writes

goroutine 646 [running]:
runtime.throw(0x75fd38, 0x15)
        /usr/local/go/src/runtime/panic.go:774 +0x72 fp=0xc000315e60 sp=0xc000315e30 pc=0x42ecf2
runtime.mapdelete_fast64(0x6f0800, 0xc00008ad50, 0x2b3e)

goroutine 1 [sleep]:
        /usr/local/go/src/runtime/time.go:105 +0x157

goroutine 6 [IO wait]:
internal/poll.runtime_pollWait(0x7fc308de6f08, 0x72, 0x0)
        /usr/local/go/src/runtime/netpoll.go:184 +0x55
internal/poll.(*pollDesc).wait(0xc000110018, 0x72, 0x0, 0x0, 0x75b00b)
        /usr/local/go/src/internal/poll/fd_poll_runtime.go:87 +0x45
internal/poll.(*FD).Accept(0xc000110000, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
        /usr/local/go/src/internal/poll/fd_unix.go:384 +0x1f8
net.(*netFD).accept(0xc000110000, 0xc000050d50, 0xc000046700, 0x7fc308e426d0)
        /usr/local/go/src/net/fd_unix.go:238 +0x42
net.(*TCPListener).accept(0xc000126000, 0xc000050d80, 0x40dd08, 0x30)
        /usr/local/go/src/net/tcpsock_posix.go:139 +0x32
net.(*TCPListener).Accept(0xc000126000, 0x72f560, 0xc0000f0180, 0x6f4f20, 0x9c00c0)
        /usr/local/go/src/net/tcpsock.go:261 +0x47
net/http.(*Server).Serve(0xc0000f4000, 0x7ccbe0, 0xc000126000, 0x0, 0x0)
        /usr/local/go/src/net/http/server.go:2896 +0x286
net/http.(*Server).ListenAndServe(0xc0000f4000, 0xc0000f4000, 0x8)
        /usr/local/go/src/net/http/server.go:2825 +0xb7

Expected behaviour

In starting for a few seconds it was working smoothly.


Actual behaviour

After few seconds my service got kill with above mentioned error.


Code Overview:

Initialized one global variable with the type ‘map’. Where the key is int and value is channel.

var ActiveInstances = make(map[int](chan string))

Having two functions

  1. go SetValue()
  2. go DeleteValue()

In SetValue()

ActiveInstances[id] = make(chan string, 5)

In DeleteValue()

delete(ActiveInstances, id)

Both functions running in multiple goroutines.


The error itself says concurrent map writes, By which I got the idea that something is wrong with my map variable ActiveInstances. Both functions in multiple goroutines are trying to access the same variable(ActiveInstances) at the same time. Which created race condition. After exploring a few blogs & documentation, I become to know that “Maps are not safe for concurrent use”.

As per golang doc

Map access is unsafe only when updates are occurring. As long as all goroutines are only reading—looking up elements in the map, including iterating through it using a for range loop—and not changing the map by assigning to elements or doing deletions, it is safe for them to access the map concurrently without synchronization.


Here we need to access ActiveInstances synchronously. We want to make sure only one goroutine can access a variable at a time to avoid conflicts, This can be easily achieved by sync.Mutex. This concept is called mutual exclusion which provides methods Lock and Unlock.

We can define a block of code to be executed in mutual exclusion by surrounding it with a call to Lock and Unlock It is as simple as below:

var mutex = &sync.Mutex{}

//my block of code

Code Modifications:

var mutex = &sync.Mutex{}

ActiveInstances[i_id] = make(chan string, 5)
delete(ActiveInstances, id)

This is how we successfully fix this problem.

Code to Reproduce

To reproduce, Comment Mutex related all operation like line no. 12, 30, 32, 44, 46. Mutex is use to prevent race condition which generates this error.