Prevent a potential race condition and crash when hanging up a channel by removing the channel from the channel list before begining channel tear down.

This fix may potentially cause problems with CDR backends that access the channel a CDR is associated with via the channel list.  This fix makes the channel unavabile at the time when the CDR backend is invoked.  This has been documented in include/asterisk/cdr.h.

(closes issue #15316)
Reported by: vmarrone
Tested by: mnicholson

Review: https://reviewboard.asterisk.org/r/362/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@219136 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/cdr.h

@@ -112,6 +112,11 @@ int ast_cdr_serialize_variables(struct ast_cdr *cdr, char *buf, size_t size, cha
 void ast_cdr_free_vars(struct ast_cdr *cdr, int recur);
 int ast_cdr_copy_vars(struct ast_cdr *to_cdr, struct ast_cdr *from_cdr);
 
+/*!\brief CDR backend callback
+ * \warning CDR backends should NOT attempt to access the channel associated
+ * with a CDR record.  This channel is not guaranteed to exist when the CDR
+ * backend is invoked.
+ */
 typedef int (*ast_cdrbe)(struct ast_cdr *cdr);
 
 /*! \brief Allocate a CDR record 

include/asterisk/channel.h

@@ -515,6 +515,8 @@ enum {
 	 *  bridge terminates, this will allow the hangup in the pbx loop to be run instead.
 	 *  */
 	AST_FLAG_BRIDGE_HANGUP_DONT = (1 << 17),
+	/*! This flag indicates whether the channel is in the channel list or not. */
+	AST_FLAG_IN_CHANNEL_LIST = (1 << 19),
 };
 
 /*! \brief ast_bridge_config flags */