Heal only when atleast one of the disk is unformatted (#5866)

Current healing has an issue when disks are healed
even when they are offline without knowing if disk
is unformatted. This can lead to issues of pre-maturely
removing the disk from the set just because it was
temporarily offline.

There is an increasing number of `mc admin heal` usage
on a cron or regular basis. It is possible that if healing
code saw disk is offline it might prematurely take it down,
this causes availability issues.

Fixes #5826

cmd/format-xl.go

@@ -288,10 +288,10 @@ func formatXLMigrateV2ToV3(export string) error {
 	return ioutil.WriteFile(formatPath, b, 0644)
 }
 
-// Returns true, if one of the errors is non-nil.
-func hasAnyErrors(errs []error) bool {
+// Returns true, if one of the errors is non-nil and is Unformatted disk.
+func hasAnyErrorsUnformatted(errs []error) bool {
 	for _, err := range errs {
-		if err != nil {
+		if err != nil && err == errUnformattedDisk {
 			return true
 		}
 	}

cmd/xl-sets.go

@@ -998,8 +998,9 @@ func (s *xlSets) ReloadFormat(ctx context.Context, dryRun bool) (err error) {
 	return nil
 }
 
-// HealFormat - heals missing `format.json` on freshly or corrupted
-// disks (missing format.json but does have erasure coded data in it).
+// HealFormat - heals missing `format.json` on fresh unformatted disks.
+// TODO: In future support corrupted disks missing format.json but has erasure
+// coded data in it.
 func (s *xlSets) HealFormat(ctx context.Context, dryRun bool) (res madmin.HealResultItem, err error) {
 	// Acquire lock on format.json
 	formatLock := s.getHashedSet(formatConfigFile).nsMutex.NewNSLock(minioMetaBucket, formatConfigFile)
@@ -1051,11 +1052,6 @@ func (s *xlSets) HealFormat(ctx context.Context, dryRun bool) (res madmin.HealRe
 		}
 	}
 
-	// no errors found, no healing is required.
-	if !hasAnyErrors(sErrs) {
-		return res, errNoHealRequired
-	}
-
 	for index, sErr := range sErrs {
 		if sErr != nil {
 			// Look for acceptable heal errors, for any other
@@ -1066,6 +1062,12 @@ func (s *xlSets) HealFormat(ctx context.Context, dryRun bool) (res madmin.HealRe
 		}
 	}
 
+	if !hasAnyErrorsUnformatted(sErrs) {
+		// No unformatted disks found disks are either offline
+		// or online, no healing is required.
+		return res, errNoHealRequired
+	}
+
 	// All disks are unformatted, return quorum error.
 	if shouldInitXLDisks(sErrs) {
 		return res, errXLReadQuorum