[PATCH 3/7] read_in_full: reset errno before reading

[Jeff King <peff@peff.net>]

Many callers of read_in_full() complain when we do not read
their full byte-count. But a check like:

  if (read_in_full(fd, buf, len) != len)
	  return error_errno("unable to read");

conflates two problem conditions:

  1. A real error from read().

  2. There were fewer than "len" bytes available.

In the first case, showing the user strerror(errno) is
useful. But in the second, we may see a random errno that
was set by some previous system call.

In an ideal world, callers would always distinguish between
these cases and give a useful message for each. But as an
easy way to make our imperfect world better, let's reset
errno to a known value. The best we can do is "0", which
will yield something like:

  unable to read: Success

That's not great, but at least it's deterministic and makes
it clear that we didn't see an error from read().

Signed-off-by: Jeff King <peff@peff.net>
---
 wrapper.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/wrapper.c b/wrapper.c
index 61aba0b5c1..f55debc92d 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -314,6 +314,7 @@ ssize_t read_in_full(int fd, void *buf, size_t count)
 	char *p = buf;
 	ssize_t total = 0;
 
+	errno = 0;
 	while (count > 0) {
 		ssize_t loaded = xread(fd, p, count);
 		if (loaded < 0)
-- 
2.14.1.1148.ga2561536a1