internal/coverage: use 128-bit FNV-1a hash instead of MD5

This change replaces the MD5 hash used to identify coverage files with a
128-bit FNV-1a hash. This change is motivated by the fact that MD5
should only be used for legacy cryptographic purposes.

The 128-bit FNV-1a hash is sufficient for the purpose of identifying
coverage files, it having the same theoretical collision resistance as
MD5, but with the added benefit of being faster to compute.

Change-Id: I7b547ce2ea784f8f4071599a10fcb512b87ee469
Reviewed-on: https://go-review.googlesource.com/c/go/+/617360
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/internal/coverage/cfile/emit.go

@@ -9,8 +9,8 @@
 package cfile
 
 import (
-	"crypto/md5"
 	"fmt"
+	"hash/fnv"
 	"internal/coverage"
 	"internal/coverage/encodecounter"
 	"internal/coverage/encodemeta"
@@ -206,7 +206,7 @@ func prepareForMetaEmit() ([]rtcov.CovMetaBlob, error) {
 		}
 	}
 
-	h := md5.New()
+	h := fnv.New128a()
 	tlen := uint64(unsafe.Sizeof(coverage.MetaFileHeader{}))
 	for _, entry := range ml {
 		if _, err := h.Write(entry.Hash[:]); err != nil {

src/internal/coverage/decodemeta/decodefile.go

@@ -12,9 +12,9 @@ package decodemeta
 
 import (
 	"bufio"
-	"crypto/md5"
 	"encoding/binary"
 	"fmt"
+	"hash/fnv"
 	"internal/coverage"
 	"internal/coverage/slicereader"
 	"internal/coverage/stringtab"
@@ -171,8 +171,10 @@ func (r *CoverageMetaFileReader) FileHash() [16]byte {
 func (r *CoverageMetaFileReader) GetPackageDecoder(pkIdx uint32, payloadbuf []byte) (*CoverageMetaDataDecoder, []byte, error) {
 	pp, err := r.GetPackagePayload(pkIdx, payloadbuf)
 	if r.debug {
+		h := fnv.New128a()
+		h.Write(pp)
 		fmt.Fprintf(os.Stderr, "=-= pkidx=%d payload length is %d hash=%s\n",
-			pkIdx, len(pp), fmt.Sprintf("%x", md5.Sum(pp)))
+			pkIdx, len(pp), fmt.Sprintf("%x", h.Sum(nil)))
 	}
 	if err != nil {
 		return nil, nil, err

src/internal/coverage/encodemeta/encode.go

@@ -10,10 +10,10 @@ package encodemeta
 
 import (
 	"bytes"
-	"crypto/md5"
 	"encoding/binary"
 	"fmt"
 	"hash"
+	"hash/fnv"
 	"internal/coverage"
 	"internal/coverage/stringtab"
 	"internal/coverage/uleb128"
@@ -39,7 +39,7 @@ func NewCoverageMetaDataBuilder(pkgpath string, pkgname string, modulepath strin
 	}
 	x := &CoverageMetaDataBuilder{
 		tmp: make([]byte, 0, 256),
-		h:   md5.New(),
+		h:   fnv.New128a(),
 	}
 	x.stab.InitWriter()
 	x.stab.Lookup("")
@@ -188,7 +188,7 @@ func (b *CoverageMetaDataBuilder) Emit(w io.WriteSeeker) ([16]byte, error) {
 // HashFuncDesc computes an md5 sum of a coverage.FuncDesc and returns
 // a digest for it.
 func HashFuncDesc(f *coverage.FuncDesc) [16]byte {
-	h := md5.New()
+	h := fnv.New128a()
 	tmp := make([]byte, 0, 32)
 	hashFuncDesc(h, f, tmp)
 	var r [16]byte

src/internal/coverage/encodemeta/encodefile.go

@@ -6,9 +6,9 @@ package encodemeta
 
 import (
 	"bufio"
-	"crypto/md5"
 	"encoding/binary"
 	"fmt"
+	"hash/fnv"
 	"internal/coverage"
 	"internal/coverage/stringtab"
 	"io"
@@ -112,7 +112,9 @@ func (m *CoverageMetaFileWriter) Write(finalHash [16]byte, blobs [][]byte, mode
 	// Now emit blobs themselves.
 	for k, blob := range blobs {
 		if m.debug {
-			fmt.Fprintf(os.Stderr, "=+= writing blob %d len %d at off=%d hash %s\n", k, len(blob), off2, fmt.Sprintf("%x", md5.Sum(blob)))
+			h := fnv.New128a()
+			h.Write(blob)
+			fmt.Fprintf(os.Stderr, "=+= writing blob %d len %d at off=%d hash %s\n", k, len(blob), off2, fmt.Sprintf("%x", h.Sum(nil)))
 		}
 		if _, err = m.w.Write(blob); err != nil {
 			return fmt.Errorf("error writing %s: %v", m.mfname, err)

src/internal/coverage/pods/pods_test.go

@@ -5,8 +5,8 @@
 package pods_test
 
 import (
-	"crypto/md5"
 	"fmt"
+	"hash/fnv"
 	"internal/coverage"
 	"internal/coverage/pods"
 	"os"
@@ -35,13 +35,17 @@ func TestPodCollection(t *testing.T) {
 	}
 
 	mkmeta := func(dir string, tag string) string {
-		hash := md5.Sum([]byte(tag))
+		h := fnv.New128a()
+		h.Write([]byte(tag))
+		hash := h.Sum(nil)
 		fn := fmt.Sprintf("%s.%x", coverage.MetaFilePref, hash)
 		return mkfile(dir, fn)
 	}
 
 	mkcounter := func(dir string, tag string, nt int, pid int) string {
-		hash := md5.Sum([]byte(tag))
+		h := fnv.New128a()
+		h.Write([]byte(tag))
+		hash := h.Sum(nil)
 		fn := fmt.Sprintf(coverage.CounterFileTempl, coverage.CounterFilePref, hash, pid, nt)
 		return mkfile(dir, fn)
 	}
@@ -112,16 +116,16 @@ func TestPodCollection(t *testing.T) {
 	}
 
 	expected := []string{
-		`o1/covmeta.ae7be26cdaa742ca148068d5ac90eaca [
-o1/covcounters.ae7be26cdaa742ca148068d5ac90eaca.40.2 o:0
-o1/covcounters.ae7be26cdaa742ca148068d5ac90eaca.41.2 o:0
-o1/covcounters.ae7be26cdaa742ca148068d5ac90eaca.42.1 o:0
-o2/covcounters.ae7be26cdaa742ca148068d5ac90eaca.35.11 o:1
+		`o1/covmeta.0880952782ab1be95aa0733055a4d06b [
+o1/covcounters.0880952782ab1be95aa0733055a4d06b.40.2 o:0
+o1/covcounters.0880952782ab1be95aa0733055a4d06b.41.2 o:0
+o1/covcounters.0880952782ab1be95aa0733055a4d06b.42.1 o:0
+o2/covcounters.0880952782ab1be95aa0733055a4d06b.35.11 o:1
 ]`,
-		`o2/covmeta.aaf2f89992379705dac844c0a2a1d45f [
-o2/covcounters.aaf2f89992379705dac844c0a2a1d45f.36.3 o:1
-o2/covcounters.aaf2f89992379705dac844c0a2a1d45f.37.2 o:1
-o2/covcounters.aaf2f89992379705dac844c0a2a1d45f.38.1 o:1
+		`o2/covmeta.0880952783ab1be95aa0733055a4d1a6 [
+o2/covcounters.0880952783ab1be95aa0733055a4d1a6.36.3 o:1
+o2/covcounters.0880952783ab1be95aa0733055a4d1a6.37.2 o:1
+o2/covcounters.0880952783ab1be95aa0733055a4d1a6.38.1 o:1
 ]`,
 	}
 	for k, exp := range expected {