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

@@ -207,43 +207,37 @@ uint64_t
 BeesRoots::transid_max_nocache()
 {
 	uint64_t rv = 0;
-	uint64_t root = BTRFS_FS_TREE_OBJECTID;
-	BEESNOTE("Calculating transid_max (" << rv << " as of root " << root << ")");
-	BEESTRACE("Calculating transid_max...");
-
-	rv = btrfs_get_root_transid(root);
-
-	// XXX:  Do we need any of this?  Or is
-	// m_transid_re.update(btrfs_get_root_transid(BTRFS_FS_TREE_OBJECTID)) good enough?
+	BEESNOTE("Calculating transid_max");
+	BEESTRACE("Calculating transid_max");
 
+	// We look for the root of the extent tree and read its transid.
+	// Should run in O(1) time and be fairly reliable.
 	BtrfsIoctlSearchKey sk;
 	sk.tree_id = BTRFS_ROOT_TREE_OBJECTID;
-	sk.min_type = sk.max_type = BTRFS_ROOT_BACKREF_KEY;
-	sk.min_objectid = root;
+	sk.min_type = sk.max_type = BTRFS_ROOT_ITEM_KEY;
+	sk.min_objectid = sk.max_objectid = BTRFS_EXTENT_TREE_OBJECTID;
 
 	while (true) {
 		sk.nr_items = 1024;
+		BEESTRACE("transid_max search sk " << sk);
 		sk.do_ioctl(m_ctx->root_fd());
 
 		if (sk.m_result.empty()) {
 			break;
 		}
 
+		// We are just looking for the highest transid on the filesystem.
+		// We don't care which object it comes from.
 		for (auto i : sk.m_result) {
 			sk.next_min(i);
-			if (i.type == BTRFS_ROOT_BACKREF_KEY) {
-				if (i.transid > rv) {
-					BEESLOGDEBUG("transid_max root " << i.objectid << " parent " << i.offset << " transid " << i.transid);
-					BEESCOUNT(transid_max_miss);
-				}
-				root = i.objectid;
-			}
 			if (i.transid > rv) {
 				rv = i.transid;
 			}
 		}
 	}
-	m_transid_re.update(rv);
+
+	// transid must be greater than zero, or we did something very wrong
+	THROW_CHECK1(runtime_error, rv, rv > 0);
 	return rv;
 }
 
@@ -422,13 +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();
 		});
 
@@ -496,19 +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);
 	}
 }