roots: reimplement transid_max_nocache using extent tree root

ROOT_TREE contains the ROOT_ITEM for EXTENT_TREE.  Every modification
(that we care about) to a btrfs must go through EXTENT_TREE, and must
modify the page in ROOT_TREE pointing to the root of EXTENT_TREE...
which makes that a very good source for the filesystem transid.

Remove the loop and the root lookups, and just look at one item for
max_transid.

Also note that every caller of transid_max_nocache() immediately
feeds the return value to m_transid_re.update(), so don't do that
inside transid_max_nocache().

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees-roots.cc

@@ -219,6 +219,7 @@ BeesRoots::transid_max_nocache()
 
 	while (true) {
 		sk.nr_items = 1024;
+		BEESTRACE("transid_max search sk " << sk);
 		sk.do_ioctl(m_ctx->root_fd());
 
 		if (sk.m_result.empty()) {
@@ -415,13 +416,15 @@ BeesRoots::crawl_thread()
 	BEESNOTE("tracking transid");
 	auto last_count = m_transid_re.count();
 	while (true) {
-		// Measure current transid
+		BEESTRACE("Measure current transid");
 		catch_all([&]() {
+			BEESTRACE("calling transid_max_nocache");
 			m_transid_re.update(transid_max_nocache());
 		});
 
-		// Make sure we have a full complement of crawlers
+		BEESTRACE("Make sure we have a full complement of crawlers");
 		catch_all([&]() {
+			BEESTRACE("calling insert_new_crawl");
 			insert_new_crawl();
 		});
 
@@ -489,19 +492,24 @@ BeesRoots::insert_new_crawl()
 	unique_lock<mutex> lock(m_mutex);
 	set<uint64_t> excess_roots;
 	for (auto i : m_root_crawl_map) {
+		BEESTRACE("excess_roots.insert(" << i.first << ")");
 		excess_roots.insert(i.first);
 	}
 	lock.unlock();
 
 	while (new_bcs.m_root) {
+		BEESTRACE("excess_roots.erase(" << new_bcs.m_root << ")");
 		excess_roots.erase(new_bcs.m_root);
+		BEESTRACE("insert_root(" << new_bcs << ")");
 		insert_root(new_bcs);
 		BEESCOUNT(crawl_create);
+		BEESTRACE("next_root(" << new_bcs.m_root << ")");
 		new_bcs.m_root = next_root(new_bcs.m_root);
 	}
 
 	for (auto i : excess_roots) {
 		new_bcs.m_root = i;
+		BEESTRACE("crawl_state_erase(" << new_bcs << ")");
 		crawl_state_erase(new_bcs);
 	}
 }