Cache go master, use existing go for bootstrap, and tighten some cache keys

.circleci/config.yml

@@ -18,21 +18,35 @@ jobs:
       - run: echo $GOPATH
       - run: which go
       - run: go version
-      - run: cd /usr/local && sudo rm -r go && sudo mkdir go go1.4 && sudo chown `whoami` go go1.4
-      - run: cd /usr/local && git clone git://github.com/golang/go 
-      - run: cd /usr/local/go && git branch -f -t go1.4 origin/release-branch.go1.4 && cd .. && git clone --single-branch -b go1.4 go go1.4
-      - run: cd /usr/local/go1.4/src && ./make.bash 
-      - run: cd /usr/local/go/src && GOROOT_BOOTSTRAP=/usr/local/go1.4 ./make.bash
+      - run: |
+          cd /usr/local
+          sudo mkdir go.master
+          sudo chown `whoami` go.master
+      - restore_cache:
+          key: go-root-
+      - run: |
+          cd /usr/local
+          git clone git://github.com/golang/go go.master || true
+          cd go.master
+          git pull
+          [[ -x bin/go && `git rev-parse HEAD` == `cat anacrolix.built` ]] && exit
+          cd src
+          ./make.bash || exit
+          git rev-parse HEAD > ../anacrolix.built
+      - save_cache:
+          paths: /usr/local/go.master
+          key: go-root-{{ checksum "/usr/local/go.master/anacrolix.built" }}
+      - run: echo 'export PATH="/usr/local/go.master/bin:$PATH"' >> $BASH_ENV
       - run: go version
       - checkout
       - run: sudo apt-get update
       - run: sudo apt install fuse pv
       - restore_cache:
           keys:
-            - go-pkg
+            - go-pkg-
       - restore_cache:
           keys:
-            - go-cache
+            - go-cache-
       - run: go get -d ./...
       - run: go test -v -race ./... -count 2
       - run: go test -bench . ./...
@@ -43,13 +57,13 @@ jobs:
       # - run: GO111MODULE=off go install github.com/anacrolix/godo
       - run: go install ./cmd/torrentfs
       - save_cache:
-          key: go-pkg-{{ epoch }}
+          key: go-pkg-{{ checksum "go.mod" }}
           paths:
             - ~/go/pkg
       - run: sudo modprobe fuse
       - run: fs/test.sh
       - save_cache:
-          key: go-cache-{{ epoch }}
+          key: go-cache-{{ .Revision }}
           paths:
             - ~/.cache/go-build
           when: always