avoid double alloc in NewCidV1

We allocate once via "make([]byte, len)",
and again when that buffer is converted to a string.

Thankfully, since Go 1.10 we have strings.Builder,
designed specifically for this use case.

In a downstream benchmark in go-car,
which needs to reconstruct many CID values,
we see small but nice gains:

    name           old time/op    new time/op    delta
    ReadBlocks-16    1.09ms ± 4%    1.06ms ± 5%   -3.33%  (p=0.007 n=11+11)

    name           old speed      new speed      delta
    ReadBlocks-16   478MB/s ± 4%   494MB/s ± 5%   +3.46%  (p=0.007 n=11+11)

    name           old alloc/op   new alloc/op   delta
    ReadBlocks-16    1.30MB ± 0%    1.25MB ± 0%   -3.86%  (p=0.000 n=12+12)

    name           old allocs/op  new allocs/op  delta
    ReadBlocks-16     9.50k ± 0%     8.45k ± 0%  -11.05%  (p=0.000 n=12+12)

.github/workflows/automerge.yml

@@ -33,7 +33,9 @@ jobs:
   automerge:
     needs: automerge-check
     runs-on: ubuntu-latest
-    if: ${{ needs.automerge-check.outputs.status == 'true' }}
+    # The check for the user is redundant here, as this job depends on the automerge-check job,
+    # but it prevents this job from spinning up, just to be skipped shortly after.
+    if: github.event.pull_request.user.login == 'web3-bot' && needs.automerge-check.outputs.status == 'true'
     steps:
     - name: Wait on tests
       uses: lewagon/wait-on-check-action@bafe56a6863672c681c3cf671f5e10b20abf2eaa # v0.2

.github/workflows/go-check.yml

@@ -8,17 +8,28 @@ jobs:
   unit:
     runs-on: ubuntu-latest
     name: All
+    env:
+      RUNGOGENERATE: false
     steps:
       - uses: actions/checkout@v2
         with:
           submodules: recursive
       - uses: actions/setup-go@v2
         with:
-          go-version: "1.16.x"
+          go-version: "1.17.x"
+      - name: Run repo-specific setup
+        uses: ./.github/actions/go-check-setup
+        if: hashFiles('./.github/actions/go-check-setup') != ''
+      - name: Read config
+        if: hashFiles('./.github/workflows/go-check-config.json') != ''
+        run: |
+          if jq -re .gogenerate ./.github/workflows/go-check-config.json; then
+            echo "RUNGOGENERATE=true" >> $GITHUB_ENV
+          fi
       - name: Install staticcheck
-        run: go install honnef.co/go/tools/cmd/staticcheck@434f5f3816b358fe468fa83dcba62d794e7fe04b # 2021.1 (v0.2.0)
+        run: go install honnef.co/go/tools/cmd/staticcheck@df71e5d0e0ed317ebf43e6e59cf919430fa4b8f2 # 2021.1.1 (v0.2.1)
       - name: Check that go.mod is tidy
-        uses: protocol/multiple-go-modules@v1.0
+        uses: protocol/multiple-go-modules@v1.2
         with:
           run: |
             go mod tidy
@@ -37,14 +48,27 @@ jobs:
           fi
       - name: go vet
         if: ${{ success() || failure() }} # run this step even if the previous one failed
-        uses: protocol/multiple-go-modules@v1.0
+        uses: protocol/multiple-go-modules@v1.2
         with:
           run: go vet ./...
       - name: staticcheck
         if: ${{ success() || failure() }} # run this step even if the previous one failed
-        uses: protocol/multiple-go-modules@v1.0
+        uses: protocol/multiple-go-modules@v1.2
         with:
           run: |
             set -o pipefail
             staticcheck ./... | sed -e 's@\(.*\)\.go@./\1.go@g'
+      - name: go generate
+        uses: protocol/multiple-go-modules@v1.2
+        if: (success() || failure()) && env.RUNGOGENERATE == 'true'
+        with:
+          run: |
+            git clean -fd # make sure there aren't untracked files / directories
+            go generate ./...
+            # check if go generate modified or added any files
+            if ! $(git add . && git diff-index HEAD --exit-code --quiet); then
+              echo "go generated caused changes to the repository:"
+              git status --short
+              exit 1
+            fi
 

.github/workflows/go-test.yml

@@ -10,7 +10,9 @@ jobs:
       fail-fast: false
       matrix:
         os: [ "ubuntu", "windows", "macos" ]
-        go: [ "1.15.x", "1.16.x" ]
+        go: [ "1.16.x", "1.17.x" ]
+    env:
+      COVERAGES: ""
     runs-on: ${{ matrix.os }}-latest
     name: ${{ matrix.os}} (go ${{ matrix.go }})
     steps:
@@ -24,24 +26,30 @@ jobs:
         run: |
           go version
           go env
+      - name: Run repo-specific setup
+        uses: ./.github/actions/go-test-setup
+        if: hashFiles('./.github/actions/go-test-setup') != ''
       - name: Run tests
-        uses: protocol/multiple-go-modules@v1.0
+        uses: protocol/multiple-go-modules@v1.2
         with:
-          run: go test -v -coverprofile coverage.txt ./...
+          run: go test -v -coverprofile module-coverage.txt ./...
       - name: Run tests (32 bit)
         if: ${{ matrix.os != 'macos' }} # can't run 32 bit tests on OSX.
-        uses: protocol/multiple-go-modules@v1.0
+        uses: protocol/multiple-go-modules@v1.2
         env:
           GOARCH: 386
         with:
           run: go test -v ./...
       - name: Run tests with race detector
         if: ${{ matrix.os == 'ubuntu' }} # speed things up. Windows and OSX VMs are slow
-        uses: protocol/multiple-go-modules@v1.0
+        uses: protocol/multiple-go-modules@v1.2
         with:
           run: go test -v -race ./...
+      - name: Collect coverage files
+        shell: bash
+        run: echo "COVERAGES=$(find . -type f -name 'module-coverage.txt' | tr -s '\n' ',' | sed 's/,$//')" >> $GITHUB_ENV
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@a1ed4b322b4b38cb846afb5a0ebfa17086917d27 # v1.5.0
+        uses: codecov/codecov-action@51d810878be5422784e86451c0e7c14e5860ec47 # v2.0.2
         with:
-          file: coverage.txt
+          files: '${{ env.COVERAGES }}'
           env_vars: OS=${{ matrix.os }}, GO=${{ matrix.go }}

cid.go

@@ -22,6 +22,7 @@ package cid
 import (
 	"bytes"
 	"encoding"
+	"encoding/binary"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -173,16 +174,24 @@ func NewCidV0(mhash mh.Multihash) Cid {
 // Panics if the multihash is invalid.
 func NewCidV1(codecType uint64, mhash mh.Multihash) Cid {
 	hashlen := len(mhash)
-	// two 8 bytes (max) numbers plus hash
-	buf := make([]byte, 1+varint.UvarintSize(codecType)+hashlen)
-	n := varint.PutUvarint(buf, 1)
-	n += varint.PutUvarint(buf[n:], codecType)
-	cn := copy(buf[n:], mhash)
+
+	// Two 8 bytes (max) numbers plus hash.
+	// We use strings.Builder to only allocate once.
+	var b strings.Builder
+	b.Grow(1 + varint.UvarintSize(codecType) + hashlen)
+
+	b.WriteByte(1)
+
+	var buf [binary.MaxVarintLen64]byte
+	n := varint.PutUvarint(buf[:], codecType)
+	b.Write(buf[:n])
+
+	cn, _ := b.Write(mhash)
 	if cn != hashlen {
 		panic("copy hash length is inconsistent")
 	}
 
-	return Cid{string(buf[:n+hashlen])}
+	return Cid{b.String()}
 }
 
 var (

cid_fuzz.go

@@ -1,3 +1,4 @@
+//go:build gofuzz
 // +build gofuzz
 
 package cid

go.mod

@@ -7,4 +7,4 @@ require (
 	golang.org/x/crypto v0.0.0-20210506145944-38f3c27a63bf // indirect
 )
 
-go 1.15
+go 1.16