context: default and relative BEESHOME

Allow relative paths with BEESHOME.  These paths will be relative
to the root of the dedup target filesystem.

BEESHOME is now optional.  If not specified, '.beeshome' is used.

We don't try to create BEESHOME if it doesn't exist.  BEESHOME might
not be on a btrfs filesystem, so we can't insist it be a subvol.

src/bees-context.cc

@@ -229,12 +229,23 @@ BeesContext::show_progress()
 	}
 }
 
+Fd
+BeesContext::home_fd()
+{
+	const char *base_dir = getenv("BEESHOME");
+	if (!base_dir) {
+		base_dir = ".beeshome";
+	}
+	m_home_fd = openat(root_fd(), base_dir, FLAGS_OPEN_DIR);
+	if (!m_home_fd) {
+		THROW_ERRNO("openat: " << name_fd(root_fd()) << " / " << base_dir);
+	}
+	return m_home_fd;
+}
+
 BeesContext::BeesContext(shared_ptr<BeesContext> parent) :
 	m_parent_ctx(parent)
 {
-	auto base_dir = getenv_or_die("BEESHOME");
-	BEESLOG("BEESHOME = " << base_dir);
-	m_home_fd = open_or_die(base_dir, FLAGS_OPEN_DIR);
 	if (m_parent_ctx) {
 		m_hash_table = m_parent_ctx->hash_table();
 		m_hash_table->set_shared(true);

src/bees.h

@@ -719,7 +719,7 @@ public:
 	void set_root_path(string path);
 
 	Fd root_fd() const { return m_root_fd; }
-	Fd home_fd() const { return m_home_fd; }
+	Fd home_fd();
 	string root_path() const { return m_root_path; }
 	string root_uuid() const { return m_root_uuid; }