Add madmin package context support (#9172)

This is to improve responsiveness for all
admin API operations and allowing callers
to cancel any on-going admin operations,
if they happen to be waiting too long.

pkg/madmin/retry.go

@@ -18,6 +18,7 @@
 package madmin
 
 import (
+	"context"
 	"math/rand"
 	"net"
 	"net/http"
@@ -68,7 +69,7 @@ func (r *lockedRandSource) Seed(seed int64) {
 
 // newRetryTimer creates a timer with exponentially increasing
 // delays until the maximum retry attempts are reached.
-func (adm AdminClient) newRetryTimer(maxRetry int, unit time.Duration, cap time.Duration, jitter float64, doneCh chan struct{}) <-chan int {
+func (adm AdminClient) newRetryTimer(ctx context.Context, maxRetry int, unit time.Duration, cap time.Duration, jitter float64) <-chan int {
 	attemptCh := make(chan int)
 
 	// computes the exponential backoff duration according to
@@ -96,14 +97,15 @@ func (adm AdminClient) newRetryTimer(maxRetry int, unit time.Duration, cap time.
 	go func() {
 		defer close(attemptCh)
 		for i := 0; i < maxRetry; i++ {
-			select {
 			// Attempts start from 1.
-			case attemptCh <- i + 1:
-			case <-doneCh:
+			attemptCh <- i + 1
+
+			select {
+			case <-time.After(exponentialBackoffWait(i)):
+			case <-ctx.Done():
 				// Stop the routine.
 				return
 			}
-			time.Sleep(exponentialBackoffWait(i))
 		}
 	}()
 	return attemptCh