signhandler: inject the claimdate (if present) in the signrequest

This allows to set a signature time (instead of using time.Now()),
so that identical files will result in identical permanodes.

Change-Id: I315c06bd30e8ac5e4d36b0e3f758483c22a31063

pkg/jsonsign/signhandler/sig.go

@@ -255,5 +255,13 @@ func (h *Handler) Sign(bb *schema.Builder) (string, error) {
 		ServerMode:        true,
 		SecretKeyringPath: h.secretRing,
 	}
+	claimTime, err := bb.Blob().ClaimDate()
+	if err != nil {
+		if !schema.IsMissingField(err) {
+			return "", err
+		}
+	} else {
+		sreq.SignatureTime = claimTime
+	}
 	return sreq.Sign()
 }

pkg/schema/blob.go

@@ -27,6 +27,19 @@ import (
 	"camlistore.org/pkg/blobref"
 )
 
+// A MissingFieldError represents a missing JSON field in a schema blob.
+type MissingFieldError string
+
+func (e MissingFieldError) Error() string {
+	return fmt.Sprintf("schema: missing field %q", string(e))
+}
+
+// IsMissingField returns whether error is of type MissingFieldError.
+func IsMissingField(err error) bool {
+	_, ok := err.(MissingFieldError)
+	return ok
+}
+
 // AnyBlob represents any type of schema blob.
 type AnyBlob interface {
 	Blob() *Blob
@@ -73,6 +86,17 @@ func (b *Blob) FileName() string {
 	return b.ss.FileNameString()
 }
 
+// ClaimDate returns the "claimDate" field.
+// If there is no claimDate, the error will be a MissingFieldError.
+func (b *Blob) ClaimDate() (time.Time, error) {
+	var ct time.Time
+	claimDate := b.ss.ClaimDate
+	if claimDate == "" {
+		return ct, MissingFieldError("claimDate")
+	}
+	return time.Parse(time.RFC3339, claimDate)
+}
+
 // ByteParts returns the "parts" field. The caller owns the returned
 // slice.
 func (b *Blob) ByteParts() []BytesPart {