Add libbtrfsutil

Currently, users wishing to manage Btrfs filesystems programatically
have to shell out to btrfs-progs and parse the output. This isn't ideal.
The goal of libbtrfsutil is to provide a library version of as many of
the operations of btrfs-progs as possible and to migrate btrfs-progs to
use it.

Rather than simply refactoring the existing btrfs-progs code, the code
has to be written from scratch for a couple of reasons:

* A lot of the btrfs-progs code was not designed with a nice library API
  in mind in terms of reusability, naming, and error reporting.
* libbtrfsutil is licensed under the LGPL, whereas btrfs-progs is under
  the GPL, which makes it dubious to directly copy or move the code.

Eventually, most of the low-level btrfs-progs code should either live in
libbtrfsutil or the shared kernel/userspace filesystem code, and
btrfs-progs will just be the CLI wrapper.

This first commit just includes the build system changes, license,
README, and error reporting helper.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 35 insertions, 0 deletions

diff --git a/libbtrfsutil/README.md b/libbtrfsutil/README.md
new file mode 100644
index 00000000..ee4c6a1d
--- /dev/null
+++ b/libbtrfsutil/README.md
@@ -0,0 +1,35 @@
+libbtrfsutil
+============
+
+libbtrfsutil is a library for managing Btrfs filesystems. It is licensed under
+the LGPL. libbtrfsutil provides interfaces for a subset of the operations
+offered by the `btrfs` command line utility.
+
+Development
+-----------
+
+The [development process for btrfs-progs](../README.md#development) applies.
+
+libbtrfsutil only includes operations that are done through the filesystem and
+ioctl interface, not operations that modify the filesystem directly (e.g., mkfs
+or fsck). This is by design but also a legal necessity, as the filesystem
+implementation is GPL but libbtrfsutil is LGPL. That is also why the
+libbtrfsutil code is a reimplementation of the btrfs-progs code rather than a
+refactoring. Be wary of this when porting functionality.
+
+libbtrfsutil is semantically versioned separately from btrfs-progs. It is the
+maintainers' responsibility to bump the version as needed (at most once per
+release of btrfs-progs).
+
+A few guidelines:
+
+* All interfaces must be documented in `btrfsutil.h` using the kernel-doc style
+* Error codes should be specific about what _exactly_ failed
+* Functions should have a path and an fd variant whenever possible
+* Spell out terms in function names, etc. rather than abbreviating whenever
+  possible
+* Don't require the Btrfs UAPI headers for any interfaces (e.g., instead of
+  directly exposing a type from `linux/btrfs_tree.h`, abstract it away in a
+  type specific to `libbtrfsutil`)
+* Preserve API and ABI compatability at all times (i.e., we don't want to bump
+  the library major version if we don't have to)