[PATCH 1/4] error: save and restore errno

[Jeff King <peff@peff.net>]

It's common to use error() to return from a function, like:

	if (open(...) < 0)
		return error("open failed");

Unfortunately this may clobber the errno from the open()
call. So we often end up with code like this:

        if (open(...) < 0) {
		int saved_errno = errno;
		error("open failed");
		errno = saved_errno;
		return -1;
	}

which is less nice. Let's teach error() to save and restore
errno in each call, so that the original errno is preserved.
This is slightly less efficient for callers which do not
care, but error code paths are generally not performance
critical anyway.

Signed-off-by: Jeff King <peff@peff.net>
---
It's pretty minor to just handle errno in the callers, but I feel like
I've wanted this at least a dozen times, and it seems like it cannot
possibly hurt (i.e., I imagine there are callers where we _should_ be
doing the errno dance but have not realized it).

 usage.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/usage.c b/usage.c
index ed14645..ee44d57 100644
--- a/usage.c
+++ b/usage.c
@@ -142,10 +142,13 @@ void NORETURN die_errno(const char *fmt, ...)
 int error(const char *err, ...)
 {
 	va_list params;
+	int saved_errno = errno;
 
 	va_start(params, err);
 	error_routine(err, params);
 	va_end(params);
+
+	errno = saved_errno;
 	return -1;
 }
 
-- 
2.1.2.596.g7379948