Hook for script debugging

[Neil Jerram]

I'd like Guile to support something like the patch below - i.e. some way
of being able to execute arbitrary code before a Guile script starts
running, so as to be able to set up breakpoints (or whatever other
mechanisms are available) to trace or debug the script execution. (And
without having to edit the script.)

Some possible questions...  Is GUILE_BOOT_FORM a good name?  Is it too
general?  Should we instead support a variable GUILE_PRE_SCRIPT_REPL,
which causes us to run a normal REPL before loading the script?  Or
perhaps a more limited or debugging-focussed REPL?

I think the generality of GUILE_BOOT_FORM is useful.  If the only option
is starting a REPL, that makes automated operation less convenient.
Also, imagine a build in which there might be several Guile script
invocations, and you only want to debug one of them.  Then the boot form
could include a check of the script name - (car (command-line)) - and
only start a REPL (or set a breakpoint, or whatever) for that one.

Thoughts?

Regards,
        Neil

diff --git a/module/ice-9/boot-9.scm b/module/ice-9/boot-9.scm
index a1537d1..86ab725 100644
--- a/module/ice-9/boot-9.scm
+++ b/module/ice-9/boot-9.scm
@@ -3514,4 +3514,9 @@ module '(ice-9 q) '(make-q q-length))}."
 (define-module (guile-user)
   #:autoload (system base compile) (compile))
 
+(let ((boot-form (getenv "GUILE_BOOT_FORM")))
+  (if (and (string? boot-form)
+          (not (zero? (string-length boot-form))))
+      (primitive-eval (with-input-from-string boot-form read))))
+
 ;;; boot-9.scm ends here