context: wait for btrfs send to finish, then try dedupe again

Dedupe is not possible on a subvol where a btrfs send is running:

    BTRFS warning (device dm-22): cannot deduplicate to root 259417 while send operations are using it (1 in progress)

btrfs informs a process with EAGAIN that a dedupe could not be performed
due to a running send operation.

It would be possible to save the crawler state at the affected point,
fork a new crawler that avoids the subvol under send, and resume the
crawler state after a successful dedupe is detected; however, this only
helps the intersection of the set of users who have unrelated subvols
that don't share extents, and the set of users who cannot simply delay
dedupe until send is finished.  The simplest approach is to simply stop
and wait until the send goes away.

The simplest approach is taken here.  When a dedupe fails with EAGAIN,
affected Tasks will poll, approximately once per transaction, until the
dedupe succeeds or fails with a different error.

bees dedupe performance corresponds with the availability of subvols that
can accept dedupe requests.  While the dedupe is paused, no new Tasks can
be performed by the worker thread.  If subvols are small and isolated
from the bulk of the filesystem data, the result will be a small but
partial loss of dedupe performance during the send as some worker threads
get stuck on the sending subvol.  If subvols heavily share extents with
duplicate data in other subvols, worker threads will all become blocked,
and the entire bees process will pause until at least some of the running
sends terminate.

During the polling for btrfs send, the dedupe Task will hold its dst
file open.  This open FD won't interfere with snapshot or file delete
because send subvols are always read-only (it is not possible to delete
a file on a RO subvol, open or otherwise) and send itself holds the
affected subvol open, preventing its deletion.  Once the send terminates,
the dedupe will terminate soon after, and the normal FD release can occur.

This pausing during btrfs send is unrelated to the
`--workaround-btrfs-send` option, although `--workaround-btrfs-send` will
cause the pausing to trigger less often.  It applies to all scan modes.

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

src/bees-context.cc

@@ -243,25 +243,36 @@ BeesContext::dedup(const BeesRangePair &brp_in)
 	BEESNOTE("waiting to dedup " << brp);
 	const auto lock = MultiLocker::get_lock("dedupe");
 
-	Timer dedup_timer;
-
 	BEESLOGINFO("dedup: src " << pretty(brp.first.size())  << " [" << to_hex(brp.first.begin())  << ".." << to_hex(brp.first.end())  << "] {" << first_addr  << "} " << name_fd(brp.first.fd()) << "\n"
 		 << "       dst " << pretty(brp.second.size()) << " [" << to_hex(brp.second.begin()) << ".." << to_hex(brp.second.end()) << "] {" << second_addr << "} " << name_fd(brp.second.fd()));
 	BEESNOTE("dedup: src " << pretty(brp.first.size())  << " [" << to_hex(brp.first.begin())  << ".." << to_hex(brp.first.end())  << "] {" << first_addr  << "} " << name_fd(brp.first.fd()) << "\n"
 		 << "       dst " << pretty(brp.second.size()) << " [" << to_hex(brp.second.begin()) << ".." << to_hex(brp.second.end()) << "] {" << second_addr << "} " << name_fd(brp.second.fd()));
 
-	const bool rv = btrfs_extent_same(brp.first.fd(), brp.first.begin(), brp.first.size(), brp.second.fd(), brp.second.begin());
-	BEESCOUNTADD(dedup_ms, dedup_timer.age() * 1000);
+	while (true) {
+		try {
+			Timer dedup_timer;
+			const bool rv = btrfs_extent_same(brp.first.fd(), brp.first.begin(), brp.first.size(), brp.second.fd(), brp.second.begin());
+			BEESCOUNTADD(dedup_ms, dedup_timer.age() * 1000);
 
-	if (rv) {
-		BEESCOUNT(dedup_hit);
-		BEESCOUNTADD(dedup_bytes, brp.first.size());
-	} else {
-		BEESCOUNT(dedup_miss);
-		BEESLOGWARN("NO Dedup! " << brp);
+			if (rv) {
+				BEESCOUNT(dedup_hit);
+				BEESCOUNTADD(dedup_bytes, brp.first.size());
+			} else {
+				BEESCOUNT(dedup_miss);
+				BEESLOGWARN("NO Dedup! " << brp);
+			}
+
+			return rv;
+		} catch (const std::system_error &e) {
+			if (e.code().value() == EAGAIN) {
+				BEESNOTE("dedup waiting for btrfs send on " << brp.second);
+				BEESLOGDEBUG("dedup waiting for btrfs send on " << brp.second);
+				roots()->wait_for_transid(1);
+			} else {
+				throw;
+			}
+		}
 	}
-
-	return rv;
 }
 
 BeesRangePair

src/bees-roots.cc

@@ -1479,6 +1479,15 @@ BeesRoots::clear_caches()
 	m_ctx->resolve_cache_clear();
 }
 
+void
+BeesRoots::wait_for_transid(const uint64_t count)
+{
+	const auto now_transid = transid_max_nocache();
+	const auto target_transid = now_transid + count;
+	BEESLOGDEBUG("Waiting for transid " << target_transid << ", current transid is " << now_transid);
+	m_transid_re.wait_until(target_transid);
+}
+
 void
 BeesRoots::crawl_thread()
 {

src/bees.h

@@ -552,6 +552,8 @@ class BeesRoots : public enable_shared_from_this<BeesRoots> {
 	BeesThread				m_writeback_thread;
 	bool					m_workaround_btrfs_send = false;
 
+	RateEstimator				m_transid_re;
+
 	shared_ptr<BeesScanMode>		m_scanner;
 
 	mutex					m_tmpfiles_mutex;
@@ -613,6 +615,8 @@ public:
 
 	uint64_t transid_min();
 	uint64_t transid_max();
+
+	void wait_for_transid(const uint64_t count);
 };
 
 struct BeesHash {