Fatal error when requesting multiple challenges at the same time for the same token

[aeyoll]

Hello,

If I do the following:

curl -H "Authorization: Bearer 163b9d3cfab1ad8407a9e0f4bc984a81" -X POST "http://127.0.0.1:2370/GetChallenges?difficultyLevel=5" & curl -H "Authorization: Bearer 163b9d3cfab1ad8407a9e0f4bc984a81" -X POST "http://127.0.0.1:2370/GetChallenges?difficultyLevel=5"

It can result in a fatal error:

❯ ./pow-captcha
2023/10/17 11:15:43 💥  PoW! Captcha server listening on port 2370
fatal error: concurrent map writes
fatal error: concurrent map writes

goroutine 26 [running]:
runtime.throw(0x704aac, 0x15)
        /usr/lib/go-1.15/src/runtime/panic.go:1116 +0x72 fp=0xc0001ab878 sp=0xc0001ab848 pc=0x437272
runtime.mapassign_faststr(0x6b5ba0, 0xc000115140, 0xc0000276e0, 0x5c, 0xc000117c28)
        /usr/lib/go-1.15/src/runtime/map_faststr.go:291 +0x3d8 fp=0xc0001ab8e0 sp=0xc0001ab878 pc=0x415c58
main.main.func7(0x75e7e0, 0xc0001820e0, 0xc00055e000, 0x0)
        /tmp/pow-captcha/main.go:307 +0x510 fp=0xc0001abb20 sp=0xc0001ab8e0 pc=0x6847f0
main.myHTTPHandleFunc.func1(0x75e7e0, 0xc0001820e0, 0xc00055e000)
        /tmp/pow-captcha/main.go:430 +0x7a fp=0xc0001abb68 sp=0xc0001abb20 pc=0x6860da
net/http.HandlerFunc.ServeHTTP(0xc00012c400, 0x75e7e0, 0xc0001820e0, 0xc00055e000)                                                                                                                                                                    /usr/lib/go-1.15/src/net/http/server.go:2042 +0x44 fp=0xc0001abb90 sp=0xc0001abb68 pc=0x64b6c4
net/http.(*ServeMux).ServeHTTP(0x8e96a0, 0x75e7e0, 0xc0001820e0, 0xc00055e000)
        /usr/lib/go-1.15/src/net/http/server.go:2417 +0x1ad fp=0xc0001abbf0 sp=0xc0001abb90 pc=0x64d6ed
net/http.serverHandler.ServeHTTP(0xc000182000, 0x75e7e0, 0xc0001820e0, 0xc00055e000)
        /usr/lib/go-1.15/src/net/http/server.go:2843 +0xa3 fp=0xc0001abc20 sp=0xc0001abbf0 pc=0x64e663
net/http.(*conn).serve(0xc000138280, 0x75ed60, 0xc000558040)
        /usr/lib/go-1.15/src/net/http/server.go:1925 +0x8ad fp=0xc0001abfc8 sp=0xc0001abc20 pc=0x64a70d
runtime.goexit()
        /usr/lib/go-1.15/src/runtime/asm_amd64.s:1374 +0x1 fp=0xc0001abfd0 sp=0xc0001abfc8 pc=0x46b7a1
created by net/http.(*Server).Serve
        /usr/lib/go-1.15/src/net/http/server.go:2969 +0x36c