handler/PUT: Handle signature verification through a custom reader. (#2066)

Change brings in a new signVerifyReader which provides a io.Reader
compatible reader, additionally implements Verify() function.

Verify() function validates the signature present in the incoming
request. This approach is choosen to avoid complexities involved
in using io.Pipe().

Thanks to Krishna for his inputs on this.

Fixes #2058
Fixes #2054
Fixes #2087

pkg/objcache/objcache.go

@@ -20,6 +20,7 @@ package objcache
 
 import (
 	"errors"
+	"fmt"
 	"io"
 	"sync"
 	"time"
@@ -85,10 +86,21 @@ func (c *Cache) Size(key string) int64 {
 }
 
 // Create validates and returns an in memory writer referencing entry.
-func (c *Cache) Create(key string, size int64) (io.Writer, error) {
+func (c *Cache) Create(key string, size int64) (writer io.Writer, err error) {
 	c.mutex.Lock()
 	defer c.mutex.Unlock()
 
+	// Recovers any panic generated and return errors appropriately.
+	defer func() {
+		if r := recover(); r != nil {
+			var ok bool
+			err, ok = r.(error)
+			if !ok {
+				err = fmt.Errorf("objcache: %v", r)
+			}
+		}
+	}() // Do not crash the server.
+
 	valueLen := uint64(size)
 	if c.maxSize > 0 {
 		// Check if the size of the object is not bigger than the capacity of the cache.
@@ -105,8 +117,8 @@ func (c *Cache) Create(key string, size int64) (io.Writer, error) {
 	return c.entries[key], nil
 }
 
-// Open - open the in-memory file, returns an memory reader.
-// returns error ErrNotFoundInCache if fsPath does not exist.
+// Open - open the in-memory file, returns an in memory read seeker.
+// returns an error ErrNotFoundInCache, if the key does not exist.
 func (c *Cache) Open(key string) (io.ReadSeeker, error) {
 	c.mutex.RLock()
 	defer c.mutex.RUnlock()