From: Zygo Blaxell <bees@furryterror.org>
Date: Sun, 24 May 2026 19:09:18 +0000 (-0400)
Subject: fs: make FILE_EXTENT_SAME failures self-describing
X-Git-Url: http://git.hungrycats.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=refs%2Fheads%2Fnext;p=bees

fs: make FILE_EXTENT_SAME failures self-describing

The old error dumped the raw btrfs_ioctl_same_args struct after the
source path.  With long backup-snapshot pathnames the line was
truncated before the destination offset, so the log rarely contained
enough to understand or reproduce the failure.

Emit the parameters of dedupe_file_range directly: the src file and
offset, the length, and every dst file and offset.  That is the full
set of inputs needed to replay the ioctl by hand.

Assisted-by: Claude-Code:claude-opus-4-7
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
---

diff --git a/lib/fs.cc b/lib/fs.cc
index ee7d9b68..45c0d841 100644
--- a/lib/fs.cc
+++ b/lib/fs.cc
@@ -145,7 +145,24 @@ namespace crucible {
 		}
 		int rv = ioctl(m_fd, BTRFS_IOC_FILE_EXTENT_SAME, ioctl_ptr);
 		if (rv) {
-			THROW_ERRNO("After FILE_EXTENT_SAME (fd = " << m_fd << " '" << name_fd(m_fd) << "') : " << ioctl_ptr);
+			// Build a self-contained description of the failed
+			// request: the src file/offset, the length, and every
+			// dst file/offset.  That is exactly the set of
+			// parameters needed to replay the dedupe_file_range
+			// ioctl by hand.  (name_fd() preserves errno across its
+			// readlink(), so THROW_ERRNO still reports the ioctl's.)
+			ostringstream oss;
+			oss << "FILE_EXTENT_SAME failed:"
+				<< " src fd " << m_fd << " '" << name_fd(m_fd) << "'"
+				<< " offset " << to_hex(m_logical_offset)
+				<< " length " << to_hex(m_length);
+			for (size_t i = 0; i < m_info.size(); ++i) {
+				const auto &info = m_info[i];
+				oss << " -> dst[" << i << "] fd " << info.fd
+					<< " '" << name_fd(info.fd) << "'"
+					<< " offset " << to_hex(info.logical_offset);
+			}
+			THROW_ERRNO(oss.str());
 		}
 		count = 0;
 		for (auto i = m_info.cbegin(); i != m_info.cend(); ++i) {