utils: reduceErrs returns and validates quorum errors. (#3300)

This is needed as explained by @krisis

Lets say we have following errors.

```
[]error{nil, errFileNotFound, errDiskAccessDenied, errDiskAccesDenied}
```

Since the last two errors are filtered, the maximum is nil,
depending on map order.

Let's say we get nil from reduceErr. Clearly at this point
we don't have quorum nodes agreeing about the data and since
GetObject only requires N/2 (Read quorum) and isDiskQuorum
would have returned true. This is problematic and can lead to
undersiable consequences.

Fixes #3298

cmd/xl-v1-utils.go

@@ -31,8 +31,7 @@ import (
 // golang's map orders keys. This doesn't affect correctness as long as quorum
 // value is greater than or equal to simple majority, since none of the equally
 // maximal values would occur quorum or more number of times.
-
-func reduceErrs(errs []error, ignoredErrs []error) error {
+func reduceErrs(errs []error, ignoredErrs []error) (maxCount int, maxErr error) {
 	errorCounts := make(map[error]int)
 	errs = errorsCause(errs)
 	for _, err := range errs {
@@ -42,14 +41,45 @@ func reduceErrs(errs []error, ignoredErrs []error) error {
 		errorCounts[err]++
 	}
 	max := 0
-	var errMax error
 	for err, count := range errorCounts {
 		if max < count {
 			max = count
-			errMax = err
+			maxErr = err
 		}
 	}
-	return traceError(errMax, errs...)
+	return max, maxErr
+}
+
+// reduceQuorumErrs behaves like reduceErrs by only for returning
+// values of maximally occurring errors validated against a generic
+// quorum number can be read or write quorum depending on usage.
+// additionally a special error is provided as well to be returned
+// in case quorum is not satisfied.
+func reduceQuorumErrs(errs []error, ignoredErrs []error, quorum int, quorumErr error) (maxErr error) {
+	maxCount, maxErr := reduceErrs(errs, ignoredErrs)
+	if maxErr == nil && maxCount >= quorum {
+		// Success in quorum.
+		return nil
+	}
+	if maxErr != nil && maxCount >= quorum {
+		// Errors in quorum.
+		return traceError(maxErr, errs...)
+	}
+	// No quorum satisfied.
+	maxErr = traceError(quorumErr, errs...)
+	return
+}
+
+// reduceReadQuorumErrs behaves like reduceErrs but only for returning
+// values of maximally occurring errors validated against readQuorum.
+func reduceReadQuorumErrs(errs []error, ignoredErrs []error, readQuorum int) (maxErr error) {
+	return reduceQuorumErrs(errs, ignoredErrs, readQuorum, errXLReadQuorum)
+}
+
+// reduceWriteQuorumErrs behaves like reduceErrs but only for returning
+// values of maximally occurring errors validated against writeQuorum.
+func reduceWriteQuorumErrs(errs []error, ignoredErrs []error, writeQuorum int) (maxErr error) {
+	return reduceQuorumErrs(errs, ignoredErrs, writeQuorum, errXLWriteQuorum)
 }
 
 // List of all errors which are ignored while verifying quorum.