chore: prefer chan struct{} over chan bool for signalling (#733)

Prefer using `chan struct{}` since:
- We are not interested in value being passed to channel, it is just
used for signaling
- It occupies zero memory (bool:64bit)
- We used `chan struct{}` and `chan bool` interchangeably. To maintain a
standard in the codebase, let's only use chan struct.

command/cp.go

@@ -107,12 +107,12 @@ Examples:
 
  22. Upload a file to S3 with a content-type and content-encoding header
  > s5cmd --content-type "text/css" --content-encoding "br" myfile.css.br s3://bucket/
- 
+
  23. Download the specific version of a remote object to working directory
  > s5cmd {{.HelpName}} --version-id VERSION_ID s3://bucket/prefix/object .
 
- 24. Pass arbitrary metadata to the object during upload or copy 
- > s5cmd {{.HelpName}} --metadata "camera=Nixon D750" --metadata "imageSize=6032x4032" flowers.png s3://bucket/prefix/flowers.png 
+ 24. Pass arbitrary metadata to the object during upload or copy
+ > s5cmd {{.HelpName}} --metadata "camera=Nixon D750" --metadata "imageSize=6032x4032" flowers.png s3://bucket/prefix/flowers.png
 `
 
 func NewSharedFlags() []cli.Flag {
@@ -425,7 +425,7 @@ func (c Copy) Run(ctx context.Context) error {
  var (
  merrorWaiter error
  merrorObjects error
- errDoneCh = make(chan bool)
+ errDoneCh = make(chan struct{})
  )
 
  go func() {

command/run.go

@@ -87,7 +87,7 @@ func (r Run) Run(ctx context.Context) error {
 
  waiter := parallel.NewWaiter()
 
- var errDoneCh = make(chan bool)
+ var errDoneCh = make(chan struct{})
  var merrorWaiter error
  go func() {
  defer close(errDoneCh)

command/select.go

@@ -28,15 +28,15 @@ Options:
  {{range .VisibleFlags}}{{.}}
  {{end}}
 Examples:
- 01. Select the average price of the avocado and amount sold, set the output format as csv 
+ 01. Select the average price of the avocado and amount sold, set the output format as csv
  > s5cmd select csv -use-header USE --query "SELECT s.avg_price, s.quantity FROM S3Object s WHERE s.item='avocado'" "s3://bucket/prices.csv"
 
- 02. Query TSV files 
+ 02. Query TSV files
  > s5cmd select csv --delimiter=\t --use-header USE --query "SELECT s.avg_price, s.quantity FROM S3Object s WHERE s.item='avocado'" "s3://bucket/prices.tsv"
 
- 03. Select a specific field in a JSON document 
+ 03. Select a specific field in a JSON document
  > s5cmd select json --structure document --query "SELECT s.tracking_id FROM s3object[*]['metadata']['.zattrs'] s" "s3://bucket/metadata.json"
- 
+
  04. Query files that contain lines of JSON objects
  > s5cmd select json --query "SELECT s.id FROM s3object s WHERE s.lineNumber = 1"
 `
@@ -310,8 +310,8 @@ func (s Select) Run(ctx context.Context) error {
  )
 
  waiter := parallel.NewWaiter()
- errDoneCh := make(chan bool)
- writeDoneCh := make(chan bool)
+ errDoneCh := make(chan struct{})
+ writeDoneCh := make(chan struct{})
  resultCh := make(chan json.RawMessage, 128)
 
  go func() {

command/sync.go

@@ -66,7 +66,7 @@ Examples:
 
  10. Sync all files to S3 bucket but exclude the ones with txt and gz extension
  > s5cmd {{.HelpName}} --exclude "*.txt" --exclude "*.gz" dir/ s3://bucket
- 
+
  11. Sync all files to S3 bucket but include the only ones with txt and gz extension
  > s5cmd {{.HelpName}} --include "*.txt" --include "*.gz" dir/ s3://bucket
 `
@@ -211,7 +211,7 @@ func (s Sync) Run(c *cli.Context) error {
  waiter := parallel.NewWaiter()
  var (
  merrorWaiter error
- errDoneCh = make(chan bool)
+ errDoneCh = make(chan struct{})
  )
 
  go func() {

parallel/parallel.go

@@ -15,7 +15,7 @@ type Task func() error
 // Manager is a structure for running tasks in parallel.
 type Manager struct {
  wg *sync.WaitGroup
- semaphore chan bool
+ semaphore chan struct{}
 }
 
 // New creates a new parallel.Manager.
@@ -30,13 +30,13 @@ func New(workercount int) *Manager {
 
  return &Manager{
  wg: &sync.WaitGroup{},
- semaphore: make(chan bool, workercount),
+ semaphore: make(chan struct{}, workercount),
  }
 }
 
 // acquire limits concurrency by trying to acquire the semaphore.
 func (p *Manager) acquire() {
- p.semaphore <- true
+ p.semaphore <- struct{}{}
  p.wg.Add(1)
 }
 

storage/s3.go

@@ -1061,7 +1061,7 @@ func (s *S3) MultiDelete(ctx context.Context, urlch <-chan *url.URL) <-chan *Obj
  resultch := make(chan *Object)
 
  go func() {
- sem := make(chan bool, 10)
+ sem := make(chan struct{}, 10)
  defer close(sem)
  defer close(resultch)
 
@@ -1072,7 +1072,7 @@ func (s *S3) MultiDelete(ctx context.Context, urlch <-chan *url.URL) <-chan *Obj
  chunk := chunk
 
  wg.Add(1)
- sem <- true
+ sem <- struct{}{}
 
  go func() {
  defer wg.Done()