diff options
Diffstat (limited to 'vendor/google.golang.org/appengine/blobstore/read.go')
-rw-r--r-- | vendor/google.golang.org/appengine/blobstore/read.go | 160 |
1 files changed, 160 insertions, 0 deletions
diff --git a/vendor/google.golang.org/appengine/blobstore/read.go b/vendor/google.golang.org/appengine/blobstore/read.go new file mode 100644 index 0000000..578b1f5 --- /dev/null +++ b/vendor/google.golang.org/appengine/blobstore/read.go @@ -0,0 +1,160 @@ +// Copyright 2012 Google Inc. All rights reserved. +// Use of this source code is governed by the Apache 2.0 +// license that can be found in the LICENSE file. + +package blobstore + +import ( + "errors" + "fmt" + "io" + "os" + "sync" + + "github.com/golang/protobuf/proto" + "golang.org/x/net/context" + + "google.golang.org/appengine" + "google.golang.org/appengine/internal" + + blobpb "google.golang.org/appengine/internal/blobstore" +) + +// openBlob returns a reader for a blob. It always succeeds; if the blob does +// not exist then an error will be reported upon first read. +func openBlob(c context.Context, blobKey appengine.BlobKey) Reader { + return &reader{ + c: c, + blobKey: blobKey, + } +} + +const readBufferSize = 256 * 1024 + +// reader is a blob reader. It implements the Reader interface. +type reader struct { + c context.Context + + // Either blobKey or filename is set: + blobKey appengine.BlobKey + filename string + + closeFunc func() // is nil if unavailable or already closed. + + // buf is the read buffer. r is how much of buf has been read. + // off is the offset of buf[0] relative to the start of the blob. + // An invariant is 0 <= r && r <= len(buf). + // Reads that don't require an RPC call will increment r but not off. + // Seeks may modify r without discarding the buffer, but only if the + // invariant can be maintained. + mu sync.Mutex + buf []byte + r int + off int64 +} + +func (r *reader) Close() error { + if f := r.closeFunc; f != nil { + f() + } + r.closeFunc = nil + return nil +} + +func (r *reader) Read(p []byte) (int, error) { + if len(p) == 0 { + return 0, nil + } + r.mu.Lock() + defer r.mu.Unlock() + if r.r == len(r.buf) { + if err := r.fetch(r.off + int64(r.r)); err != nil { + return 0, err + } + } + n := copy(p, r.buf[r.r:]) + r.r += n + return n, nil +} + +func (r *reader) ReadAt(p []byte, off int64) (int, error) { + if len(p) == 0 { + return 0, nil + } + r.mu.Lock() + defer r.mu.Unlock() + // Convert relative offsets to absolute offsets. + ab0 := r.off + int64(r.r) + ab1 := r.off + int64(len(r.buf)) + ap0 := off + ap1 := off + int64(len(p)) + // Check if we can satisfy the read entirely out of the existing buffer. + if r.off <= ap0 && ap1 <= ab1 { + // Convert off from an absolute offset to a relative offset. + rp0 := int(ap0 - r.off) + return copy(p, r.buf[rp0:]), nil + } + // Restore the original Read/Seek offset after ReadAt completes. + defer r.seek(ab0) + // Repeatedly fetch and copy until we have filled p. + n := 0 + for len(p) > 0 { + if err := r.fetch(off + int64(n)); err != nil { + return n, err + } + r.r = copy(p, r.buf) + n += r.r + p = p[r.r:] + } + return n, nil +} + +func (r *reader) Seek(offset int64, whence int) (ret int64, err error) { + r.mu.Lock() + defer r.mu.Unlock() + switch whence { + case os.SEEK_SET: + ret = offset + case os.SEEK_CUR: + ret = r.off + int64(r.r) + offset + case os.SEEK_END: + return 0, errors.New("seeking relative to the end of a blob isn't supported") + default: + return 0, fmt.Errorf("invalid Seek whence value: %d", whence) + } + if ret < 0 { + return 0, errors.New("negative Seek offset") + } + return r.seek(ret) +} + +// fetch fetches readBufferSize bytes starting at the given offset. On success, +// the data is saved as r.buf. +func (r *reader) fetch(off int64) error { + req := &blobpb.FetchDataRequest{ + BlobKey: proto.String(string(r.blobKey)), + StartIndex: proto.Int64(off), + EndIndex: proto.Int64(off + readBufferSize - 1), // EndIndex is inclusive. + } + res := &blobpb.FetchDataResponse{} + if err := internal.Call(r.c, "blobstore", "FetchData", req, res); err != nil { + return err + } + if len(res.Data) == 0 { + return io.EOF + } + r.buf, r.r, r.off = res.Data, 0, off + return nil +} + +// seek seeks to the given offset with an effective whence equal to SEEK_SET. +// It discards the read buffer if the invariant cannot be maintained. +func (r *reader) seek(off int64) (int64, error) { + delta := off - r.off + if delta >= 0 && delta < int64(len(r.buf)) { + r.r = int(delta) + return off, nil + } + r.buf, r.r, r.off = nil, 0, off + return off, nil +} |