[Lcms-user] cmsReadRawTag returns different data depending on profile state

[Andrew B. <bae...@gm...>]

The cmsReadRawTag returns different data for the same tag, depending on whether
this tag was converted in internal representation, or not. Following
scenario makes the
problem visible:

  a) open profile:
      all tags are in raw representation at the moment.
  b) read a tag (grayTRC in this test) as raw data:
      returned data has length 14 and contain all required parts of a
tag's data:
      type signature, reserved dword, and curve data.
  c) read the tag using cmsReadTag in order to force a conversion of the tag
      into an internal representation.
  d) read raw data of the tag again:
      Note that now reported data size is 8 bytes shorter and the data does not
      contain the type signature and the reserved dword. It happens because
      in case of 'cooked' tag, raw data is obtained as a result of
serialization of
      an internal structure, and at this codepath tag signature is missed.

Note that the step c) can be done implicitly by, for example, a
transform creation.
Attached test demonstrates the difference in the length of raw tag data.

This problem is present in latest version of sources, available via git.
A suggested fix is to explicitly add a tag signature and reserved dword
to tag raw tag data, in order to comply the ICC spec:

diff --git a/src/cmsio0.c b/src/cmsio0.c
old mode 100644
new mode 100755
index 770e986..c05e5b7
--- a/src/cmsio0.c
+++ b/src/cmsio0.c
@@ -1656,6 +1656,18 @@ cmsInt32Number CMSEXPORT
cmsReadRawTag(cmsHPROFILE hProfile, cmsTagSignature sig
     // Serialize
     TypeHandler ->ContextID  = Icc ->ContextID;
     TypeHandler ->ICCVersion = Icc ->Version;
+
+    // write type signature
+    if (!_cmsWriteUInt32Number(MemIO, TypeHandler->Signature)) {
+        cmsCloseIOhandler(MemIO);
+        return 0;
+    }
+    // write reserved field
+    if (!_cmsWriteUInt32Number(MemIO, 0)) {
+        cmsCloseIOhandler(MemIO);
+        return 0;
+    }
+
     if (!TypeHandler ->WritePtr(TypeHandler, MemIO, Object,
TagDescriptor ->ElemCount)) {
         cmsCloseIOhandler(MemIO);
         return 0;

Thanks,
Andrew