Fix Fetch -> FetchStreaming conversion

As blob.fetcherToSeekerWrapper.Fetch erroneously asserts that FetchStreaming
returns a ReadSeekCloser everytime, it had to be changed.

Move MaxBlobSize from blobserver to constants (new package).

Change-Id: I4b4f22c302cbec84d77d21454e0c9e8aebdf73e5

pkg/blob/fetcher.go

@@ -27,6 +27,7 @@ import (
 	"strings"
 	"sync"
 
+	"camlistore.org/pkg/constants"
 	"camlistore.org/pkg/osutil"
 	"camlistore.org/pkg/types"
 )
@@ -64,8 +65,26 @@ func (w *fetcherToSeekerWrapper) Fetch(r Ref) (file types.ReadSeekCloser, size i
 	if err != nil {
 		return
 	}
-	file = rc.(types.ReadSeekCloser)
-	return
+	file, ok := rc.(types.ReadSeekCloser)
+	if ok {
+		return
+	}
+	// we must make it seekable
+	var slurp bytes.Buffer
+	n, err := io.CopyN(&slurp, rc, constants.MaxBlobSize+1)
+	if err != nil && err != io.EOF {
+		return nil, 0, err
+	}
+	if n > constants.MaxBlobSize {
+		return nil, 0, fmt.Errorf("blob %v too big", r)
+	}
+	return struct {
+		io.ReadSeeker
+		io.Closer
+	}{
+		bytes.NewReader(slurp.Bytes()),
+		ioutil.NopCloser(nil),
+	}, n, err
 }
 
 // StreamingFetcher is the minimal interface for retrieving a blob from storage.

pkg/blobserver/interface.go

@@ -24,15 +24,12 @@ import (
 	"time"
 
 	"camlistore.org/pkg/blob"
+	"camlistore.org/pkg/constants"
 	"camlistore.org/pkg/context"
 )
 
 // MaxBlobSize is the size of a single blob in Camlistore.
-//
-// TODO: formalize this in the specs. This value of 16 MB is less than
-// App Engine's 32 MB request limit, much more than Venti's limit, and
-// much more than the ~64 KB & 256 KB chunks that the FileWriter make
-const MaxBlobSize = 16 << 20
+const MaxBlobSize = constants.MaxBlobSize
 
 var ErrCorruptBlob = errors.New("corrupt blob; digest doesn't match")
 

pkg/constants/constants.go

@@ -0,0 +1,27 @@
+/*
+Copyright 2014 the Camlistore authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package constants contains Camlistore constants.
+//
+// This is a leaf package, without dependencies.
+package constants
+
+// MaxBlobSize is the size of a single blob in Camlistore.
+//
+// TODO: formalize this in the specs. This value of 16 MB is less than
+// App Engine's 32 MB request limit, much more than Venti's limit, and
+// much more than the ~64 KB & 256 KB chunks that the FileWriter make
+const MaxBlobSize = 16 << 20