Do not request 0-length skips; sanity-check return.

I noticed that my skip callback was always being invoked with a request of
0.  This is a bit wasteful since skip callbacks commonly involve a syscall
like lseek().

Also, it seems good to error out when the skip callback is buggy, and claims
to skip more than requested.

Test Plan:

```
autoreconf -ivf && ./configure && make && make check
```

The same tests fail as before, with the same error messages. If interested,
both failure logs are here:

https://github.com/snarkmaster/libarchive/commit/00c9751cde6cc888fb844b7a1fcc0f82dbaaedb1

These are on Ubuntu 14.04.

diff --git a/libarchive/archive_read.c b/libarchive/archive_read.c
index d649e9aae1de3b6073f321a5d961991ffbd81294..2cd9726a6e844bb0e179ef867c87117e7b190465 100644 (file)
--- a/libarchive/archive_read.c
+++ b/libarchive/archive_read.c
@@ -195,10 +195,12 @@ client_skip_proxy(struct archive_read_filter *self, int64_t request)
                                ask = skip_limit;
                        get = (self->archive->client.skipper)
                                (&self->archive->archive, self->data, ask);
-                       if (get == 0)
+                       total += get;
+                       if (get == 0 || get == request)
                                return (total);
+                       if (get > request)
+                               return ARCHIVE_FATAL;
                        request -= get;
-                       total += get;
                }
        } else if (self->archive->client.seeker != NULL
                && request > 64 * 1024) {