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bug#24617: 26.0.50; Handlers in `condition-case' should have programmati


From: Philipp Stephani
Subject: bug#24617: 26.0.50; Handlers in `condition-case' should have programmatic access to the backtrace
Date: Wed, 28 Dec 2016 20:08:12 +0000



Helmut Eller <address@hidden> schrieb am Mi., 5. Okt. 2016 um 08:18 Uhr:
On Tue, Oct 04 2016, Philipp Stephani wrote:

> Currently a handler in a `condition-case' form doesn't have access to
> the backtrace that was active when the signal was raised.  This makes
> many useful things impossible, such as re-raising signals or returning
> the backtrace to emacsclient.  I suggest either adding true exception
> objects (storing the error symbol, data, and backtrace), or at least
> providing a dynamic variable with the current backtrace.

First, you can copy the backtrace in a signal-hook-function and store it
in global variable.

Yes, that sounds like a good workaround, with the downside that other libraries might override signal-hook-function and disable that functionality.
Do you know why ERT uses a custom debugger instead of signal-hook-function?
 

Second, unconditionally copying the backtrace would be expensive

Are you sure about that? In languages that are typically far more performance-sensitive (e.g. Java), the backtrace is also copied unconditionally.
 
and
would still not allow access to local variables in stack frames or the
value of dynamic variables at the point where the error was signalled
Just because Java or Python do that doesn't make it a great idea.

True, but having the function names and argument values is better than nothing when debugging the cause of a signal.
 

Third, the solution to this problem in Common Lisp is the HANDLER-CASE
macro which is similar to condition-case but the error handlers are
executed without unwinding the stack.  That doesn't require any copying
and gives full access to the stack.

That sounds like an interesting approach that could be made to solve some common use cases (e.g. test runners or rethrowing signals).

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