Re: Scripted clone generating an incomplete, unusable .git/config

From: Jeff King <peff@peff.net>
Date: Nov 12, 2010, 04:24 UTC
Message-ID: <20101112042455.GA20555@sigill.intra.peff.net>
In-Reply-To: <20101112021602.GA10765@burratino>

On Thu, Nov 11, 2010 at 08:16:02PM -0600, Jonathan Nieder wrote:

Show 6 quoted lines

> > I don't think your patch is the right solution, but FWIW, sigchain was
> > explicitly intended to be able to take SIG_DFL and SIG_IGN. Probably
> > sigchain_fun should be removed and we should just use sighandler_t
> > explicitly
> 
> Sorry, that was lazy of me.  The name sighandler_t is a GNU extension[1].

Ah, you're right. ANSI C defines signal() without using a typedef at all. Maybe that is why I didn't use it in the first place. I don't recall.

Show 5 quoted lines

> The following addresses my confusion but I doubt it's worth the
> syntactic ugliness.
>
> -- 8< --
> Subject: sigchain: hide sigchain_fun type

I think it makes the code uglier. Maybe this is a better solution:

-- >8 --
Subject: [PATCH] document sigchain api

It's pretty straightforward, but a stripped-down example never hurts. And we should make clear that it is explicitly OK to use SIG_DFL and SIG_IGN.

Signed-off-by: Jeff King <peff@peff.net>
---
 Documentation/technical/api-sigchain.txt |   41 ++++++++++++++++++++++++++++++
 1 files changed, 41 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/technical/api-sigchain.txt

diff --git a/Documentation/technical/api-sigchain.txt b/Documentation/technical/api-sigchain.txt
new file mode 100644
index 0000000..535cdff
--- /dev/null
+++ b/Documentation/technical/api-sigchain.txt
@@ -0,0 +1,41 @@
+sigchain API
+============
+
+Code often wants to set a signal handler to clean up temporary files or
+other work-in-progress when we die unexpectedly. For multiple pieces of
+code to do this without conflicting, each piece of code must remember
+the old value of the handler and restore it either when:
+
+  1. The work-in-progress is finished, and the handler is no longer
+     necessary. The handler should revert to the original behavior
+     (either another handler, SIG_DFL, or SIG_IGN).
+
+  2. The signal is received. We should then do our cleanup, then chain
+     to the next handler (or die if it is SIG_DFL).
+
+Sigchain is a tiny library for keeping a stack of handlers. Your handler
+and installation code should look something like:
+
+------------------------------------------
+  void clean_foo_on_signal(int sig)
+  {
+	  clean_foo();
+	  sigchain_pop(sig);
+	  raise(sig);
+  }
+
+  void other_func()
+  {
+	  sigchain_push_common(clean_foo_on_signal);
+	  mess_up_foo();
+	  clean_foo();
+  }
+------------------------------------------
+
+Handlers are given the typdef of sigchain_fun. This is the same type
+that is given to signal() or sigaction(). It is perfectly reasonable to
+push SIG_DFL or SIG_IGN onto the stack.
+
+You can sigchain_push and sigchain_pop individual signals. For
+convenience, sigchain_push_common will push the handler onto the stack
+for many common signals.

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