restore: Rework extraction pipeline to use xsync.WorkGroup

The pattern where multiple workers are spawned to work on a common task
and where whole work needs to be canceled on first error is now well
understood, with the functionality to broadcast cancel and propagate
errors being wrapped into libraries such as

	https://godoc.org/golang.org/x/sync/errgroup			and
	https://godoc.org/lab.nexedi.com/kirr/go123/xsync#WorkGroup
	(kirr/go123@515a6d14)

Let's streamline the code by using xsync.WorkGroup (it is in our hands,
a bit more well designed (imho), has analog in Pygolang, and can be
changed/enhanced as needed).

The other reason to rework the code is that the workgroup is created
under context (currently always background) and can be canceled by that
context cancel. In the next patch we'll teach all git-backup
subcommands, including restore, to work under context, and by using
xsync.WorkGroup we will automatically handle cancellation from outside,
while without reworking extraction pipeline we would need to
additionally glue ctx cancel to signal to workers to stop.

Compared to previous state both xsync.WorkGroup and errogroup return
only the first error, however it should likely not cause problems in
practice as the first error is usually the most informative one.

git-backup.go

@@ -67,6 +67,7 @@ NOTE the idea of pulling all refs together is similar to git-namespaces
 package main
 
 import (
+	"context"
 	"flag"
 	"fmt"
 	"io/ioutil"
@@ -77,7 +78,6 @@ import (
 	"runtime/debug"
 	"sort"
 	"strings"
-	"sync"
 	"syscall"
 	"time"
 
@@ -87,6 +87,7 @@ import (
 	"lab.nexedi.com/kirr/go123/xerr"
 	"lab.nexedi.com/kirr/go123/xflag"
 	"lab.nexedi.com/kirr/go123/xstrings"
+	"lab.nexedi.com/kirr/go123/xsync"
 
 	git "github.com/libgit2/git2go"
 )
@@ -961,23 +962,18 @@ func cmd_restore_(gb *git.Repository, HEAD_ string, restorespecv []RestoreSpec)
 	repotab = nil
 
 	packxq := make(chan PackExtractReq, 2*njobs) // requests to extract packs
-	errch := make(chan error)                    // errors from workers
-	stopch := make(chan struct{})                // broadcasts restore has to be cancelled
-	wg := sync.WaitGroup{}
+	wg := xsync.NewWorkGroup(context.Background())
 
 	// main worker: walk over specified prefixes restoring files and
 	// scheduling pack extraction requests from *.git -> packxq
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
+	wg.Go(func(ctx context.Context) (err error) {
 		defer close(packxq)
-		// raised err -> errch
+		// raised err -> return
 		here := my.FuncName()
 		defer exc.Catch(func(e *exc.Error) {
-			errch <- exc.Addcallingcontext(here, e)
+			err = exc.Addcallingcontext(here, e)
 		})
 
-	runloop:
 		for _, __ := range restorespecv {
 			prefix, dir := __.prefix, __.dir
 
@@ -1041,33 +1037,32 @@ func cmd_restore_(gb *git.Repository, HEAD_ string, restorespecv []RestoreSpec)
 					repopath: reprefix(prefix, dir, repo.repopath),
 					prefix:   prefix}:
 
-				case <-stopch:
-					break runloop
+				case <-ctx.Done():
+					return ctx.Err()
 				}
 			}
 		}
-	}()
+
+		return nil
+	})
 
 	// pack workers: packxq -> extract packs
 	for i := 0; i < njobs; i++ {
-		wg.Add(1)
-		go func() {
-			defer wg.Done()
-			// raised err -> errch
+		wg.Go(func(ctx context.Context) (err error) {
+			// raised err -> return
 			here := my.FuncName()
 			defer exc.Catch(func(e *exc.Error) {
-				errch <- exc.Addcallingcontext(here, e)
+				err = exc.Addcallingcontext(here, e)
 			})
 
-		runloop:
 			for {
 				select {
-				case <-stopch:
-					break runloop
+				case <-ctx.Done():
+					return ctx.Err()
 
 				case p, ok := <-packxq:
 					if !ok {
-						break runloop
+						return nil
 					}
 					infof("# git  %s\t-> %s", p.prefix, p.repopath)
 
@@ -1129,27 +1124,12 @@ func cmd_restore_(gb *git.Repository, HEAD_ string, restorespecv []RestoreSpec)
 					//         RunWith{stdout: gitprogress(), stderr: gitprogress()})
 				}
 			}
-		}()
-	}
-
-	// wait for workers to finish & collect/reraise their errors
-	go func() {
-		wg.Wait()
-		close(errch)
-	}()
-
-	ev := xerr.Errorv{}
-	for e := range errch {
-		// tell everything to stop on first error
-		if len(ev) == 0 {
-			close(stopch)
-		}
-		ev = append(ev, e)
+		})
 	}
 
-	if len(ev) != 0 {
-		exc.Raise(ev)
-	}
+	// wait for workers to finish & collect/reraise first error, if any
+	err = wg.Wait()
+	exc.Raiseif(err)
 }
 
 // loadBackupRefs loads 'backup.ref' content from a git object.