add the doc file about the MacroExclusive app (issue #7366, Steve Davies)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@39683 65c4cc65-6c06-0410-ace0-fbb531ad65f3

doc/macroexclusive.txt

+About the MacroExclusive application
+------------------------------------
+
+Steve Davies <steve@connection-telecom.com
+
+
+The MacroExclusive application was added to solve the problem of
+synchronisation between calls running at the same time.
+
+This is usually an issue when you have calls manipulating global
+variables or the Asterisk database, but may be useful elsewhere.
+
+Consider this example macro, intended to return a "next" number -
+each caller is intended to get a different number:
+
+[macro-next]
+exten => s,1,Set(RESULT=${COUNT})
+exten => s,n,SetGlobalVar(COUNT=$[${COUNT} + 1])
+
+The problem is that in a box with high activity, you can be sure
+that two calls will come along together - both will get the same
+"RESULT", or the "COUNT" value will get mangled.
+
+Calling this Macro via MacroExclusive will use a mutex to make sure
+that only one call executes in the Macro at a time.  This ensures
+that the two lines execute as a unit.
+
+Note that even the s,2 line above has its own race problem.  Two
+calls running that line at once will step on each other and
+the count will end up as +1 rather than +2.
+
+I've also been able to use MacroExclusive where I have two Macros
+that need to be mutually exclusive.
+
+Here's the example:
+
+[macro-push]
+; push value ${ARG2} onto stack ${ARG1}
+exten => s,1,Set(DB(STACK/${ARG1})=${ARG2}^${DB(STACK/${ARG1})})
+
+[macro-pop]
+; pop top value from stack ${ARG1}
+exten => s,1,Set(RESULT=${DB(STACK/${ARG1})})
+exten => s,n,Set(DB(STACK/${ARG1})=${CUT(RESULT,^,2)})
+exten => s,n,Set(RESULT=${CUT(RESULT,^,1)})
+
+All that futzing with the STACK/${ARG1} in the astdb needs protecting
+if this is to work.  But neither push nor pop can run together.
+
+So add this "pattern":
+
+[macro-stack]
+exten => Macro(${ARG1},${ARG2},${ARG3})
+
+... and use it like so:
+
+exten => s,1,MacroExclusive(stack,push,MYSTACK,bananas)
+exten => s,n,MacroExclusive(stack,push,MYSTACK,apples)
+exten => s,n,MacroExclusive(stack,push,MYSTACK,guavas)
+exten => s,n,MacroExclusive(stack,push,MYSTACK,pawpaws)
+exten => s,n,MacroExclusive(stack,pop,MYSTACK) ; RESULT gets pawpaws (yum)
+exten => s,n,MacroExclusive(stack,pop,MYSTACK) ; RESULT gets guavas
+exten => s,n,MacroExclusive(stack,pop,MYSTACK) ; RESULT gets apples
+exten => s,n,MacroExclusive(stack,pop,MYSTACK) ; RESULT gets bananas
+
+We get to the push and pop macros "via" the stack macro.  But only one call
+can execute the stack macro at a time; ergo, only one of push OR pop can
+run at a time.
+
+Hope people find this useful.
+
+Lastly, its worth pointing out that only Macros that access shared data
+will require this MacroExclusive protection.  And Macro's that you call
+with macroExclusive should run quickly or you will clog up your Asterisk
+system.
+
+Regards,
+Steve