Added backtrace primitives.

Using OCaml 3.11+ builtin facilities to record stack frames during
exception raising, we can now recover at runtime the backtrace of
an uncaught toplevel exception and display it to the user, without
the infamous OCaml debugger. The backtrace is displayed when using
the [-debug] flag.

This requires a bit of discipline, as each time we reraise an
exception we need to keep track of those frames we discarded
between the previous raise and the current [try-with] branch.

Currently, only [Anomaly] is handled, but this can be ported to any
exception as long as we add the backtrace info into the exception,
and we provide the corresponding handler to
[Backtracke.register_backtrace_handler].

Hopefully this should not be to costly, as we only do little work
when reraising, and only with the [-debug] flag set.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16166 85f007b7-540e-0410-9357-904b9bb8a0f7

1 files changed, 108 insertions, 0 deletions

diff --git a/lib/backtrace.mli b/lib/backtrace.mli
new file mode 100644
index 0000000000..d107535523
--- /dev/null
+++ b/lib/backtrace.mli
@@ -0,0 +1,108 @@
+(***********************************************************************)
+(*  v      *   The Coq Proof Assistant  /  The Coq Development Team    *)
+(* <O___,, *        INRIA-Rocquencourt  &  LRI-CNRS-Orsay              *)
+(*   \VV/  *************************************************************)
+(*    //   *      This file is distributed under the terms of the      *)
+(*         *       GNU Lesser General Public License Version 2.1       *)
+(***********************************************************************)
+
+(** * Low-level management of OCaml backtraces.
+
+  Currently, OCaml manages its backtraces in a very imperative way. That is to
+  say, it only keeps track of the stack destroyed by the last raised exception.
+  So we have to be very careful when using this module not to do silly things.
+
+  Basically, you need to manually handle the reraising of exceptions. In order
+  to do so, each time the backtrace is lost, you must [push] the stack fragment.
+  This essentially occurs whenever a [with] handler is crossed.
+
+*)
+
+(** {5 Backtrace information} *)
+
+type location = {
+  loc_filename : string;
+  loc_line : int;
+  loc_start : int;
+  loc_end : int;
+}
+(** OCaml debugging information for function calls. *)
+
+type frame = { frame_location : location option; frame_raised : bool; }
+(** A frame contains two informations: its optional physical location, and
+    whether it raised the exception or let it pass through. *)
+
+type t = frame list option
+(** Type of backtraces. They're just stack of frames. [None] indicates that we
+    don't care about recording the backtraces. *)
+
+val empty : t
+(** Empty frame stack. *)
+
+val none : t
+(** Frame stack that will not register anything. *)
+
+val push : t -> t
+(** Add the current backtrace information to a given backtrace. *)
+
+(** {5 Utilities} *)
+
+val print_frame : frame -> string
+(** Represent a frame. *)
+
+(** {5 Exception handling} *)
+
+val register_backtrace_handler : (exn -> exn option) -> unit
+(** Add a handler to enrich backtrace information that may be carried by
+    exceptions. If the handler returns [None], this means that it is not its
+    duty to handle this one. Otherwise, the new exception will be used by the
+    functions thereafter instead of the original one.
+
+    Handlers are called in the reverse order of their registration. If no
+    handler match, the original exception is returned.
+*)
+
+val push_exn : exn -> exn
+(** Add the current backtrace information to the given exception, using the
+    registered handlers.
+
+    The intended use case is of the form: {[
+
+    try foo
+      with
+      | Bar -> bar
+      | err -> let err = push_exn err in baz
+
+    ]}
+
+    WARNING: any intermediate code between the [with] and the handler may
+    modify the backtrace. Yes, that includes [when] clauses. Ideally, what you
+    should do is something like: {[
+
+    try foo
+      with err ->
+        let err = push_exn err in
+        match err with
+        | Bar -> bar
+        | err -> baz
+
+    ]}
+
+    I admit that's a bit heavy, but there is not much to do...
+
+*)
+
+val reraise : exn -> 'a
+(** Convenience function which covers a generic pattern in Coq code.
+    [reraise e] is equivalent to [raise (push_exn e)].
+
+    The intended use case is of the form: {[
+
+    try foo
+      with
+      | Bar -> bar
+      | err -> reraise err
+
+    ]}
+
+*)