cmd/go: make 'mod verify' use multiple CPUs

'go mod verify' checksums one module zip at a time, which is
CPU-intensive on most modern machines with fast disks. As a result, one
can see a CPU bottleneck when running the command on, for example, a
module where 'go list -m all' lists ~440 modules:

	$ /usr/bin/time go mod verify
	all modules verified
	11.47user 0.77system 0:09.41elapsed 130%CPU (0avgtext+0avgdata 24284maxresident)k
	0inputs+0outputs (0major+4156minor)pagefaults 0swaps

Instead, verify up to GOMAXPROCS zips at once, which should line up
pretty well with the amount of processors we can use on a machine. The
results below are obtained via 'benchcmd -n 5 GoModVerify go mod verify'
on the same large module.

	name         old time/op         new time/op         delta
	GoModVerify          9.35s ± 1%          3.03s ± 2%  -67.60%  (p=0.008 n=5+5)

	name         old user-time/op    new user-time/op    delta
	GoModVerify          11.2s ± 1%          16.3s ± 3%  +45.38%  (p=0.008 n=5+5)

	name         old sys-time/op     new sys-time/op     delta
	GoModVerify          841ms ± 9%          865ms ± 8%     ~     (p=0.548 n=5+5)

	name         old peak-RSS-bytes  new peak-RSS-bytes  delta
	GoModVerify         27.8MB ±13%         50.7MB ±27%  +82.01%  (p=0.008 n=5+5)

The peak memory usage nearly doubles, and there is some extra overhead,
but it seems clearly worth the tradeoff given that we see a ~3x speedup
on my laptop with 4 physical cores. The vast majority of developer
machines nowadays should have 2-4 cores at least.

No test or benchmark is included; one can benchmark 'go mod verify'
directly, as I did above. The existing tests also cover correctness,
including any data races via -race.

Fixes #38623.

Change-Id: I45d8154687a6f3a6a9fb0e2b13da4190f321246c
Reviewed-on: https://go-review.googlesource.com/c/go/+/229817
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modcmd/verify.go

@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"os"
+	"runtime"
 
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
@@ -52,17 +53,41 @@ func runVerify(cmd *base.Command, args []string) {
 			base.Fatalf("go: cannot find main module; see 'go help modules'")
 		}
 	}
+
+	// Only verify up to GOMAXPROCS zips at once.
+	type token struct{}
+	sem := make(chan token, runtime.GOMAXPROCS(0))
+
+	// Use a slice of result channels, so that the output is deterministic.
+	mods := modload.LoadBuildList()[1:]
+	errsChans := make([]<-chan []error, len(mods))
+
+	for i, mod := range mods {
+		sem <- token{}
+		errsc := make(chan []error, 1)
+		errsChans[i] = errsc
+		mod := mod // use a copy to avoid data races
+		go func() {
+			errsc <- verifyMod(mod)
+			<-sem
+		}()
+	}
+
 	ok := true
-	for _, mod := range modload.LoadBuildList()[1:] {
-		ok = verifyMod(mod) && ok
+	for _, errsc := range errsChans {
+		errs := <-errsc
+		for _, err := range errs {
+			base.Errorf("%s", err)
+			ok = false
+		}
 	}
 	if ok {
 		fmt.Printf("all modules verified\n")
 	}
 }
 
-func verifyMod(mod module.Version) bool {
-	ok := true
+func verifyMod(mod module.Version) []error {
+	var errs []error
 	zip, zipErr := modfetch.CachePath(mod, "zip")
 	if zipErr == nil {
 		_, zipErr = os.Stat(zip)
@@ -73,10 +98,10 @@ func verifyMod(mod module.Version) bool {
 		if zipErr != nil && errors.Is(zipErr, os.ErrNotExist) &&
 			dirErr != nil && errors.Is(dirErr, os.ErrNotExist) {
 			// Nothing downloaded yet. Nothing to verify.
-			return true
+			return nil
 		}
-		base.Errorf("%s %s: missing ziphash: %v", mod.Path, mod.Version, err)
-		return false
+		errs = append(errs, fmt.Errorf("%s %s: missing ziphash: %v", mod.Path, mod.Version, err))
+		return errs
 	}
 	h := string(bytes.TrimSpace(data))
 
@@ -85,11 +110,10 @@ func verifyMod(mod module.Version) bool {
 	} else {
 		hZ, err := dirhash.HashZip(zip, dirhash.DefaultHash)
 		if err != nil {
-			base.Errorf("%s %s: %v", mod.Path, mod.Version, err)
-			return false
+			errs = append(errs, fmt.Errorf("%s %s: %v", mod.Path, mod.Version, err))
+			return errs
 		} else if hZ != h {
-			base.Errorf("%s %s: zip has been modified (%v)", mod.Path, mod.Version, zip)
-			ok = false
+			errs = append(errs, fmt.Errorf("%s %s: zip has been modified (%v)", mod.Path, mod.Version, zip))
 		}
 	}
 	if dirErr != nil && errors.Is(dirErr, os.ErrNotExist) {
@@ -98,13 +122,12 @@ func verifyMod(mod module.Version) bool {
 		hD, err := dirhash.HashDir(dir, mod.Path+"@"+mod.Version, dirhash.DefaultHash)
 		if err != nil {
 
-			base.Errorf("%s %s: %v", mod.Path, mod.Version, err)
-			return false
+			errs = append(errs, fmt.Errorf("%s %s: %v", mod.Path, mod.Version, err))
+			return errs
 		}
 		if hD != h {
-			base.Errorf("%s %s: dir has been modified (%v)", mod.Path, mod.Version, dir)
-			ok = false
+			errs = append(errs, fmt.Errorf("%s %s: dir has been modified (%v)", mod.Path, mod.Version, dir))
 		}
 	}
-	return ok
+	return errs
 }