Make BER/CER/DER decodersstreaming and suspendible (#176)

The goal of this change is to make the decoder yielding on input
data starvation and resuming from where it stopped whenever the
caller decides to try again (hopefully making sure that some more
input becomes available).

This change makes it possible for the decoder to operate on streams
of data (meaning that the entire DER blob might not be immediately
available on input).

On top of that, the decoder yields partially reconstructed ASN.1
object on input starvation making it possible for the caller to
inspect what has been decoded so far and possibly consume partial
ASN.1 data.

All these new feature are natively available through
`StreamingDecoder` class. Previously published API is implemented
as a thin wrapper on top of that ensuring backward compatibility.

1 files changed, 50 insertions, 22 deletions

diff --git a/pyasn1/codec/cer/decoder.py b/pyasn1/codec/cer/decoder.py
index 3e86fd0..0a92b26 100644
--- a/pyasn1/codec/cer/decoder.py
+++ b/pyasn1/codec/cer/decoder.py
@@ -5,60 +5,88 @@
 # License: http://snmplabs.com/pyasn1/license.html
 #
 from pyasn1 import error
+from pyasn1.codec.streaming import readFromStream
 from pyasn1.codec.ber import decoder
 from pyasn1.compat.octets import oct2int
 from pyasn1.type import univ
 
-__all__ = ['decode']
+__all__ = ['decode', 'StreamingDecoder']
 
+SubstrateUnderrunError = error.SubstrateUnderrunError
 
-class BooleanDecoder(decoder.AbstractSimpleDecoder):
+
+class BooleanPayloadDecoder(decoder.AbstractSimplePayloadDecoder):
     protoComponent = univ.Boolean(0)
 
     def valueDecoder(self, substrate, asn1Spec,
                      tagSet=None, length=None, state=None,
                      decodeFun=None, substrateFun=None,
                      **options):
-        head, tail = substrate[:length], substrate[length:]
-        if not head or length != 1:
+
+        if length != 1:
             raise error.PyAsn1Error('Not single-octet Boolean payload')
-        byte = oct2int(head[0])
+
+        for chunk in readFromStream(substrate, length, options):
+            if isinstance(chunk, SubstrateUnderrunError):
+                yield chunk
+
+        byte = oct2int(chunk[0])
+
         # CER/DER specifies encoding of TRUE as 0xFF and FALSE as 0x0, while
         # BER allows any non-zero value as TRUE; cf. sections 8.2.2. and 11.1 
         # in https://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf
         if byte == 0xff:
             value = 1
+
         elif byte == 0x00:
             value = 0
+
         else:
             raise error.PyAsn1Error('Unexpected Boolean payload: %s' % byte)
-        return self._createComponent(asn1Spec, tagSet, value, **options), tail
+
+        yield self._createComponent(asn1Spec, tagSet, value, **options)
+
 
 # TODO: prohibit non-canonical encoding
-BitStringDecoder = decoder.BitStringDecoder
-OctetStringDecoder = decoder.OctetStringDecoder
-RealDecoder = decoder.RealDecoder
-
-tagMap = decoder.tagMap.copy()
-tagMap.update(
-    {univ.Boolean.tagSet: BooleanDecoder(),
-     univ.BitString.tagSet: BitStringDecoder(),
-     univ.OctetString.tagSet: OctetStringDecoder(),
-     univ.Real.tagSet: RealDecoder()}
+BitStringPayloadDecoder = decoder.BitStringPayloadDecoder
+OctetStringPayloadDecoder = decoder.OctetStringPayloadDecoder
+RealPayloadDecoder = decoder.RealPayloadDecoder
+
+TAG_MAP = decoder.TAG_MAP.copy()
+TAG_MAP.update(
+    {univ.Boolean.tagSet: BooleanPayloadDecoder(),
+     univ.BitString.tagSet: BitStringPayloadDecoder(),
+     univ.OctetString.tagSet: OctetStringPayloadDecoder(),
+     univ.Real.tagSet: RealPayloadDecoder()}
 )
 
-typeMap = decoder.typeMap.copy()
+TYPE_MAP = decoder.TYPE_MAP.copy()
 
 # Put in non-ambiguous types for faster codec lookup
-for typeDecoder in tagMap.values():
+for typeDecoder in TAG_MAP.values():
     if typeDecoder.protoComponent is not None:
         typeId = typeDecoder.protoComponent.__class__.typeId
-        if typeId is not None and typeId not in typeMap:
-            typeMap[typeId] = typeDecoder
+        if typeId is not None and typeId not in TYPE_MAP:
+            TYPE_MAP[typeId] = typeDecoder
+
+
+class SingleItemDecoder(decoder.SingleItemDecoder):
+    __doc__ = decoder.SingleItemDecoder.__doc__
+
+    TAG_MAP = TAG_MAP
+    TYPE_MAP = TYPE_MAP
+
+
+class StreamingDecoder(decoder.StreamingDecoder):
+    __doc__ = decoder.StreamingDecoder.__doc__
+
+    SINGLE_ITEM_DECODER = SingleItemDecoder
 
 
 class Decoder(decoder.Decoder):
-    pass
+    __doc__ = decoder.Decoder.__doc__
+
+    STREAMING_DECODER = StreamingDecoder
 
 
 #: Turns CER octet stream into an ASN.1 object.
@@ -111,4 +139,4 @@ class Decoder(decoder.Decoder):
 #:    SequenceOf:
 #:     1 2 3
 #:
-decode = Decoder(tagMap, decoder.typeMap)
+decode = Decoder()