cmd/go/internal: remove some users of par.Work

par.Work is used in a number of places as a parallel
work queue. This change replaces it with goroutines
and channels in a number of simpler places where it's
used.

This is the same CL as golang.org/cl/240062 and golang.org/cl/248326
except for the following changes in convert.go (all line numbers
from this CL), as well as fixing up imports in download.go:
- On line 44, the "*" before modules.Versions is removed (we were
trying to assign to a nil value on lines 72 and 73).
- Line 64 is new, and ensures that we receive on the semaphore
channel once the goroutine function exits. (The previous versions
of this CL only received at the end of the function, ignoring
the return point in the branch in the middle of the function.)
- The semaphore channel receive right before line 74 is gone,
replaced with the deferred receive above.
- The if block at line 83 is new, accounting for cases where
modfetch.ImportRepoRev returned an error in the goroutine,
so that versions[i] is ignored.

Change-Id: I0e33670bb2eb0a1e4d7a5fa693a471e61ffbc8b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/249020
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 24 insertions, 13 deletions

diff --git a/src/cmd/go/internal/modload/list.go b/src/cmd/go/internal/modload/list.go
index 4768516e90..8db4d64706 100644
--- a/src/cmd/go/internal/modload/list.go
+++ b/src/cmd/go/internal/modload/list.go
@@ -9,12 +9,12 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"runtime"
 	"strings"
 
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/modinfo"
-	"cmd/go/internal/par"
 	"cmd/go/internal/search"
 
 	"golang.org/x/mod/module"
@@ -22,24 +22,35 @@ import (
 
 func ListModules(ctx context.Context, args []string, listU, listVersions bool) []*modinfo.ModulePublic {
 	mods := listModules(ctx, args, listVersions)
+
+	type token struct{}
+	sem := make(chan token, runtime.GOMAXPROCS(0))
 	if listU || listVersions {
-		var work par.Work
 		for _, m := range mods {
-			work.Add(m)
+			add := func(m *modinfo.ModulePublic) {
+				sem <- token{}
+				go func() {
+					if listU {
+						addUpdate(m)
+					}
+					if listVersions {
+						addVersions(m)
+					}
+					<-sem
+				}()
+			}
+
+			add(m)
 			if m.Replace != nil {
-				work.Add(m.Replace)
+				add(m.Replace)
 			}
 		}
-		work.Do(10, func(item interface{}) {
-			m := item.(*modinfo.ModulePublic)
-			if listU {
-				addUpdate(m)
-			}
-			if listVersions {
-				addVersions(m)
-			}
-		})
 	}
+	// Fill semaphore channel to wait for all tasks to finish.
+	for n := cap(sem); n > 0; n-- {
+		sem <- token{}
+	}
+
 	return mods
 }