GetRegions: stop process and release resources when the connection is disconnected.

[AndreMouche]

Enhancement Task

As we have observed, the GetRegions request will return information about all regions. If there are a large number of regions, it will occupy a significant amount of memory in a short period of time.

3: 2013265920 [15: 10066329600] @ 0xedfb76 0xedf50f 0xedf706 0xf4f2ce 0xf50194 0xf51c15 0xf51727 0xf50194 0xf51c15 0xf51727 0xf50194 0xf51fde 0xf4dbb8 0xf4d6ba 0xf59ede 0x1cfb777 0x1cfb5c8 0x1cfdc30 0x1cfe3c5 0x237bdb1 0x1dcdd71 0x1dcd9f3 0x1dcdaf6 0x2375b7e 0x1dcdaf6 0x23752a3 0x1dcdaf6 0x2374a3a 0x1dcdaf6 0x1dce0c5 0x2374daa 0x11db10f
#       0xedfb75        bytes.growSlice+0x95                                                    /usr/local/go/src/bytes/buffer.go:240
#       0xedf50e        bytes.(*Buffer).grow+0x14e                                              /usr/local/go/src/bytes/buffer.go:142
#       0xedf705        bytes.(*Buffer).Write+0x65                                              /usr/local/go/src/bytes/buffer.go:170
#       0xf4f2cd        encoding/json.uintEncoder+0xed                                          /usr/local/go/src/encoding/json/encode.go:566
#       0xf50193        encoding/json.structEncoder.encode+0x1f3                                /usr/local/go/src/encoding/json/encode.go:760
#       0xf51c14        encoding/json.arrayEncoder.encode+0xd4                                  /usr/local/go/src/encoding/json/encode.go:915
#       0xf51726        encoding/json.sliceEncoder.encode+0x366                                 /usr/local/go/src/encoding/json/encode.go:888
#       0xf50193        encoding/json.structEncoder.encode+0x1f3                                /usr/local/go/src/encoding/json/encode.go:760
#       0xf51c14        encoding/json.arrayEncoder.encode+0xd4                                  /usr/local/go/src/encoding/json/encode.go:915
#       0xf51726        encoding/json.sliceEncoder.encode+0x366                                 /usr/local/go/src/encoding/json/encode.go:888
#       0xf50193        encoding/json.structEncoder.encode+0x1f3                                /usr/local/go/src/encoding/json/encode.go:760
#       0xf51fdd        encoding/json.ptrEncoder.encode+0x25d                                   /usr/local/go/src/encoding/json/encode.go:944
#       0xf4dbb7        encoding/json.(*encodeState).reflectValue+0x77                          /usr/local/go/src/encoding/json/encode.go:359
#       0xf4d6b9        encoding/json.(*encodeState).marshal+0xf9                               /usr/local/go/src/encoding/json/encode.go:331
#       0xf59edd        encoding/json.(*Encoder).Encode+0x7d                                    /usr/local/go/src/encoding/json/stream.go:206
#       0x1cfb776       github.com/unrolled/render.JSON.renderStreamingJSON+0x136               /go/pkg/mod/github.com/unrolled/render@v1.0.1/engine.go:147
#       0x1cfb5c7       github.com/unrolled/render.JSON.Render+0x347                            /go/pkg/mod/github.com/unrolled/render@v1.0.1/engine.go:105
#       0x1cfdc2f       github.com/unrolled/render.(*Render).Render+0x4f                        /go/pkg/mod/github.com/unrolled/render@v1.0.1/render.go:341
#       0x1cfe3c4       github.com/unrolled/render.(*Render).JSON+0x184                         /go/pkg/mod/github.com/unrolled/render@v1.0.1/render.go:408
#       0x237bdb0       github.com/tikv/pd/server/api.(*regionsHandler).GetRegions+0x90         /pd/server/api/region.go:334
#       0x1dcdd70       github.com/urfave/negroni.WrapFunc.func1+0x30                           /go/pkg/mod/github.com/urfave/negroni@v0.3.0/negroni.go:56
#       0x1dcd9f2       github.com/urfave/negroni.HandlerFunc.ServeHTTP+0x32                    /go/pkg/mod/github.com/urfave/negroni@v0.3.0/negroni.go:29
#       0x1dcdaf5       github.com/urfave/negroni.middleware.ServeHTTP+0xb5                     /go/pkg/mod/github.com/urfave/negroni@v0.3.0/negroni.go:38
#       0x2375b7d       github.com/tikv/pd/server/api.(*rateLimitMiddleware).ServeHTTP+0x39d    /pd/server/api/middleware.go:170
#       0x1dcdaf5       github.com/urfave/negroni.middleware.ServeHTTP+0xb5                     /go/pkg/mod/github.com/urfave/negroni@v0.3.0/negroni.go:38
#       0x23752a2       github.com/tikv/pd/server/api.(*auditMiddleware).ServeHTTP+0x3a2        /pd/server/api/middleware.go:150
#       0x1dcdaf5       github.com/urfave/negroni.middleware.ServeHTTP+0xb5                     /go/pkg/mod/github.com/urfave/negroni@v0.3.0/negroni.go:38
#       0x2374a39       github.com/tikv/pd/server/api.(*requestInfoMiddleware).ServeHTTP+0x2f9  /pd/server/api/middleware.go:76
#       0x1dcdaf5       github.com/urfave/negroni.middleware.ServeHTTP+0xb5                     /go/pkg/mod/github.com/urfave/negroni@v0.3.0/negroni.go:38
#       0x1dce0c4       github.com/urfave/negroni.(*Negroni).ServeHTTP+0x124                    /go/pkg/mod/github.com/urfave/negroni@v0.3.0/negroni.go:96
#       0x2374da9       github.com/tikv/pd/server/api.clusterMiddleware.Middleware.func1+0x1a9  /pd/server/api/middleware.go:99
#       0x11db10e       net/http.HandlerFunc.ServeHTTP+0x2e                                     /usr/local/go/src/net/http/server.go:2109

Currently, we have found that after the connection is disconnected, GetRegions will continue to be processed and occupy a large amount of memory until it is completely handled before the memory is released.This is not a very good design, it will cause resource waste. We hope that after the connection is disconnected, PD can immediately stop processing requests and release resources.

[AndreMouche]

@lhy1024 Please take a look, thanks.