status: Place upper bounds on concurrency

dep status was handily busting through fd limits on larger projects by
fanning out on all projects without limit. This adds a hardcoded limit
that should still be sufficient to still get parallelism benefits, but
will not exhaust the open files limits for any sane default setting on
any OS.

Fixes #1727
Fixes #1923

Author: sdboyer

cmd/dep/status.go

@@ -955,13 +955,19 @@ func (cmd *statusCommand) runStatusAll(ctx *dep.Ctx, out outputter, p *dep.Proje
 		errListPkgCh := make(chan error, len(slp))
 		errListVerCh := make(chan error, len(slp))
 
+		// Hardcode to a limit of 16 simultaneous projects. This is plenty high
+		// enough to gain concurrency benefits, but low enough that we won't
+		// hit an open fd limit on any OS.
+		sem := make(chan struct{}, 16)
 		var wg sync.WaitGroup
 
 		for i, proj := range slp {
 			wg.Add(1)
 			logger.Printf("(%d/%d) %s\n", i+1, len(slp), proj.Ident().ProjectRoot)
+			sem <- struct{}{}
 
 			go func(proj verify.VerifiableProject) {
+				defer func() { <-sem }()
 				bs := BasicStatus{
 					ProjectRoot:  string(proj.Ident().ProjectRoot),
 					PackageCount: len(proj.Packages()),
@@ -1327,14 +1333,20 @@ func collectConstraints(ctx *dep.Ctx, p *dep.Project, sm gps.SourceManager) (con
 	// Channel for receiving all the errors.
 	errCh := make(chan error, len(lp))
 
+	// Hardcode to a limit of 16 simultaneous projects. This is plenty high
+	// enough to gain concurrency benefits, but low enough that we won't
+	// hit an open fd limit on any OS.
+	sem := make(chan struct{}, 16)
 	var wg sync.WaitGroup
 
 	// Iterate through the locked projects and collect constraints of all the projects.
 	for i, proj := range lp {
 		wg.Add(1)
 		logger.Printf("(%d/%d) %s\n", i+1, len(lp), proj.Ident().ProjectRoot)
+		sem <- struct{}{}
 
 		go func(proj gps.LockedProject) {
+			defer func() { <-sem }()
 			defer wg.Done()
 
 			manifest, _, err := sm.GetManifestAndLock(proj.Ident(), proj.Version(), rootAnalyzer)