pkg/runtime: 2 sanity checks in the runtime-gdb.py prettyprinters.

Don't try to print obviously corrupt slices or interfaces.
Doesn't actually solve 3047 or 2818, but seems a good idea anyway.

R=rsc, bsiegert
CC=golang-dev
https://golang.org/cl/5708061

src/pkg/runtime/runtime-gdb.py

@@ -58,6 +58,8 @@ class SliceTypePrinter:
 		return str(self.val.type)[6:]  # skip 'struct '
 
 	def children(self):
+		if self.val["len"] > self.val["cap"]:
+			return
 		ptr = self.val["array"]
 		for idx in range(self.val["len"]):
 			yield ('[%d]' % idx, (ptr + idx).dereference())
@@ -184,12 +186,10 @@ def lookup_type(name):
 	except:
 		pass
 
+_rctp_type = gdb.lookup_type("struct runtime.commonType").pointer()
+_rtp_type = gdb.lookup_type("struct runtime._type").pointer()
 
-def iface_dtype(obj):
-	"Decode type of the data field of an eface or iface struct."
-        # known issue: dtype_name decoded from runtime.commonType is "nested.Foo"
-        # but the dwarf table lists it as "full/path/to/nested.Foo"
-
+def iface_commontype(obj):
 	if is_iface(obj):
 		go_type_ptr = obj['tab']['_type']
 	elif is_eface(obj):
@@ -197,12 +197,28 @@ def iface_dtype(obj):
 	else:
 		return
 
-	ct = gdb.lookup_type("struct runtime.commonType").pointer()
-	dynamic_go_type = go_type_ptr['ptr'].cast(ct).dereference()
+	# sanity check: reflection type description ends in a loop.
+	tt = go_type_ptr['_type'].cast(_rtp_type).dereference()['_type']
+	if tt != tt.cast(_rtp_type).dereference()['_type']:
+		return
+	
+	return go_type_ptr['ptr'].cast(_rctp_type).dereference()
+	
+
+def iface_dtype(obj):
+	"Decode type of the data field of an eface or iface struct."
+	# known issue: dtype_name decoded from runtime.commonType is "nested.Foo"
+	# but the dwarf table lists it as "full/path/to/nested.Foo"
+
+	dynamic_go_type = iface_commontype(obj)
+	if dynamic_go_type is None:
+		return
 	dtype_name = dynamic_go_type['string'].dereference()['str'].string()
 
 	dynamic_gdb_type = lookup_type(dtype_name)
-        if dynamic_gdb_type:
+	if dynamic_gdb_type is None:
+		return
+	
 	type_size = int(dynamic_go_type['size'])
 	uintptr_size = int(dynamic_go_type['size'].type.sizeof)	 # size is itself an uintptr
 	if type_size > uintptr_size:
@@ -213,15 +229,9 @@ def iface_dtype(obj):
 def iface_dtype_name(obj):
 	"Decode type name of the data field of an eface or iface struct."
 
-	if is_iface(obj):
-		go_type_ptr = obj['tab']['_type']
-	elif is_eface(obj):
-		go_type_ptr = obj['_type']
-	else:
+	dynamic_go_type = iface_commontype(obj)
+	if dynamic_go_type is None:
 		return
-
-	ct = gdb.lookup_type("struct runtime.commonType").pointer()
-	dynamic_go_type = go_type_ptr['ptr'].cast(ct).dereference()
 	return dynamic_go_type['string'].dereference()['str'].string()
 
 
@@ -244,7 +254,7 @@ class IfacePrinter:
 		except:
 			return "<bad dynamic type>"
 
-                if not dtype:  # trouble looking up, print something reasonable
+		if dtype is None:  # trouble looking up, print something reasonable
 			return "(%s)%s" % (iface_dtype_name(self.val), self.val['data'])
 
 		try:
@@ -267,7 +277,7 @@ goobjfile.pretty_printers.append(ifacematcher)
 class GoLenFunc(gdb.Function):
 	"Length of strings, slices, maps or channels"
 
-	how = ((StringTypePrinter, 'len' ),
+	how = ((StringTypePrinter, 'len'),
 	       (SliceTypePrinter, 'len'),
 	       (MapTypePrinter, 'count'),
 	       (ChanTypePrinter, 'qcount'))
@@ -316,7 +326,7 @@ class DTypeFunc(gdb.Function):
 #  Commands
 #
 
-sts = ( 'idle', 'runnable', 'running', 'syscall', 'waiting', 'moribund', 'dead', 'recovery')
+sts = ('idle', 'runnable', 'running', 'syscall', 'waiting', 'moribund', 'dead', 'recovery')
 
 def linked_list(ptr, linkfield):
 	while ptr:
@@ -403,8 +413,12 @@ class GoIfaceCmd(gdb.Command):
 				print "Can't parse ", obj, ": ", e
 				continue
 
+			if obj['data'] == 0:
+				dtype = "nil"
+			else:
 				dtype = iface_dtype(obj)
-			if not dtype:
+				
+			if dtype is None:
 				print "Not an interface: ", obj.type
 				continue