[PATCH] Add errno property to condition objects

[Mario Domenech Goulart]

Hi,

The attached patch adds errno to condition objects produced by
procedures that change it.

Motivation for this patch: at the moment, it is difficult to write
race condition-free code in some cases.  For example, trying to delete
a file that might not exist.  The naive approach would be:

  (when (file-exists? the-file)
    (delete-file the-file))

This is subject to race conditions, as by the time delete-file is
called, the result of file-exists? might not be valid anymore (e.g.,
code in another thread or process might have deleted it).  Another
approach would be trying to delete the file unconditionally and
checking errno:

  (handle-exceptions exn
    (unless (= (errno) errno/noent)
      (signal exn))
    (delete-file the-file))

This is also subject to race conditions, as by the time (errno) is
called, it might reflect the result of an error produced by a
procedure executed on another thread.

The attached patch addresses those use-cases by injecting errno into
condition objects at the time they are created, where errno is
guaranteed to have been set by procedures whose underlying C calls set
it, as units that provide those procedures are compiled with
disable-interrupts.  With this, something like the following snippet
can be used for race-free code to delete files that might not exist:

  (handle-exceptions exn
    (unless (= (get-condition-property exn 'exn 'errno) errno/noent)
      (signal exn))
    (delete-file the-file))

The patch is rather large, but mostly straightforward.  It basically
adds two new internal procedures, ##sys#error/errno and
##sys#signal-hook/errno (analogous to ##sys#error and ##sys#signal-hook,
respectively) which are now used by procedures which call
##sys#update-errno.

General notes about the availability of errno in condition objects
have been added to the documentation of modules which provide
procedures that update errno.

All the best.
Mario
-- 
http://parenteses.org/mario

0001-Add-errno-property-to-condition-objects.patch
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