Tcl Source Code

Script to reproduce:
-------------
proc LTrace {args} {
puts "LTrace: $args"
#Hardcoded:
trace remove execution test leave LTrace
}

proc GTrace {args} {puts "This is global trace: $args"}

proc localproc {pname arglist body} {
set caller [lindex [info level 1] 0]
trace add execution $caller leave LTrace
puts "Active traces on $caller: [trace info execution $caller]"
proc $pname $arglist "uplevel 1 [list $body]"
return
}

proc test {a b} {
localproc lproc {} {puts "a=$a, b=$b"}
lproc
set a "A" ; set b "B"
lproc
}

#trace add execution test leave GTrace
test a 4
puts "---------"
test a 5
------------------
In tcl 8.4.19 it gives:

Active traces on test: {leave LTrace}
a=a, b=4
a=A, b=B
LTrace: {test a 4} 0 {} leave
---------
Active traces on test: {leave LTrace}
a=a, b=5
a=A, b=B
LTrace: {test a 5} 0 {} leave

But in tcl 8.6b2 and 8.5.10:

Active traces on test: {leave LTrace}
a=a, b=4
a=A, b=B
---------
Active traces on test: {leave LTrace} {leave LTrace}
a=a, b=5
a=A, b=B
LTrace: {test a 5} 0 {} leave
-----
So, LTrace isn't called first time.

Also, if I uncomment the line "#trace add execution test leave GTrace", then in all tcl versions result becomes:
---
Active traces on test: {leave LTrace} {leave GTrace}
a=a, b=4
a=A, b=B
This is global trace: {test a 4} 0 {} leave
LTrace: {test a 4} 0 {} leave
---------
Active traces on test: {leave LTrace} {leave GTrace}
a=a, b=5
a=A, b=B
This is global trace: {test a 5} 0 {} leave
LTrace: {test a 5} 0 {} leave

The real issue is: when a command call occurs, are there any execution traces active in the interpreter or for that specific command?

1. if not, then *that* command call will ignore leave traces until it returns
2. if yes, leave traces will be called - whatever they are when the command is done. That is, a *change* during execution will be taken into account, but creation of traces on an untraced-at-invocation command will not.

Even more confusing: if instead of uncommenting that line you replace it with
    trace add execution test enter GTrace
then ... the leave trace fires too! The important thing is: was the command traced on invocation or not?

The docs are possibly unclear about this (granted, somewhat confusing) behaviour. Suggestions?

Ok, but if it's so, then why uncommenting a line:
'trace add execution test leave GTrace'
makes  LTrace active?

Isn't this covered in the changes file?

2007-06-21 (feature change)[1740962] leave traces created during execution
of traced command do not fire (sofer)