From c30c43f3690931218b88680b337d11a57a2fdc45 Mon Sep 17 00:00:00 2001
From: Luke Shumaker <lukeshu@lukeshu.com>
Date: Fri, 17 Mar 2023 01:46:40 -0400
Subject: tree-wide: Ensure that all existing type doc comments follow the
 expected format

This is a notable improvement in the docs for btrfsitem.
---
 lib/btrfs/btrfsitem/item_blockgroup.go      | 16 ++++++++++++++--
 lib/btrfs/btrfsitem/item_chunk.go           | 16 +++++++++++++---
 lib/btrfs/btrfsitem/item_dev.go             |  8 ++++++--
 lib/btrfs/btrfsitem/item_devextent.go       | 19 +++++++++++++++++--
 lib/btrfs/btrfsitem/item_dir.go             | 12 ++++++++----
 lib/btrfs/btrfsitem/item_extent.go          | 18 ++++++++++++++++--
 lib/btrfs/btrfsitem/item_extentcsum.go      |  8 ++++++--
 lib/btrfs/btrfsitem/item_extentdataref.go   |  8 ++++++--
 lib/btrfs/btrfsitem/item_fileextent.go      | 15 +++++++++++++--
 lib/btrfs/btrfsitem/item_freespacebitmap.go |  9 +++++++--
 lib/btrfs/btrfsitem/item_freespaceinfo.go   | 10 ++++++++--
 lib/btrfs/btrfsitem/item_inode.go           |  6 ++++--
 lib/btrfs/btrfsitem/item_inoderef.go        | 13 +++++++++----
 lib/btrfs/btrfsitem/item_metadata.go        | 15 +++++++++++++++
 lib/btrfs/btrfsitem/item_qgroupinfo.go      |  9 +++++++--
 lib/btrfs/btrfsitem/item_qgrouplimit.go     | 10 ++++++++--
 lib/btrfs/btrfsitem/item_qgroupstatus.go    | 10 ++++++++--
 lib/btrfs/btrfsitem/item_root.go            | 15 +++++++++++----
 lib/btrfs/btrfsitem/item_rootref.go         |  4 +++-
 lib/btrfs/btrfsitem/item_shareddataref.go   |  8 +++++---
 lib/btrfs/btrfsitem/item_uuid.go            | 11 ++++++++---
 21 files changed, 192 insertions(+), 48 deletions(-)

(limited to 'lib/btrfs')

diff --git a/lib/btrfs/btrfsitem/item_blockgroup.go b/lib/btrfs/btrfsitem/item_blockgroup.go
index ae0ca12..dc561bd 100644
--- a/lib/btrfs/btrfsitem/item_blockgroup.go
+++ b/lib/btrfs/btrfsitem/item_blockgroup.go
@@ -10,8 +10,20 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
 )
 
-// key.objectid = logical_addr
-// key.offset = size of chunk
+// A BlockGroup tracks allocation of the logical address space.
+//
+// Compare with:
+//   - DevExtents, which track allocation of the physical address space.
+//   - Chunks, which map logical addresses to physical addresses.
+//
+// The relationship between the three is
+//
+//	DevExtent---[many:one]---Chunk---[one:one]---BlockGroup
+//
+// Key:
+//
+//	key.objectid = logical_addr
+//	key.offset   = size of chunk
 type BlockGroup struct { // trivial BLOCK_GROUP_ITEM=192
 	Used          int64                    `bin:"off=0, siz=8"`
 	ChunkObjectID btrfsprim.ObjID          `bin:"off=8, siz=8"` // always FIRST_CHUNK_TREE_OBJECTID
diff --git a/lib/btrfs/btrfsitem/item_chunk.go b/lib/btrfs/btrfsitem/item_chunk.go
index 2280a0b..607df75 100644
--- a/lib/btrfs/btrfsitem/item_chunk.go
+++ b/lib/btrfs/btrfsitem/item_chunk.go
@@ -11,10 +11,20 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/containers"
 )
 
-// Maps logical address to physical.
+// A Chunk maps logical addresses to physical addresses.
 //
-// key.objectid = BTRFS_FIRST_CHUNK_TREE_OBJECTID
-// key.offset = logical_addr
+// Compare with:
+//   - DevExtents, which track allocation of the physical address space.
+//   - BlockGroups, which track allocation of the logical address space.
+//
+// The relationship between the three is
+//
+//	DevExtent---[many:one]---Chunk---[one:one]---BlockGroup
+//
+// Key:
+//
+//	key.objectid = BTRFS_FIRST_CHUNK_TREE_OBJECTID
+//	key.offset   = logical_addr
 type Chunk struct { // complex CHUNK_ITEM=228
 	Head    ChunkHeader
 	Stripes []ChunkStripe
diff --git a/lib/btrfs/btrfsitem/item_dev.go b/lib/btrfs/btrfsitem/item_dev.go
index 188711e..7fd6833 100644
--- a/lib/btrfs/btrfsitem/item_dev.go
+++ b/lib/btrfs/btrfsitem/item_dev.go
@@ -10,8 +10,12 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
 )
 
-// key.objectid = BTRFS_DEV_ITEMS_OBJECTID
-// key.offset = device_id (starting at 1)
+// A Dev describes a physical volume that is part of the filesystem.
+//
+// Key:
+//
+//	key.objectid = BTRFS_DEV_ITEMS_OBJECTID
+//	key.offset   = device_id (starting at 1)
 type Dev struct { // trivial DEV_ITEM=216
 	DevID btrfsvol.DeviceID `bin:"off=0x0,    siz=0x8"`
 
diff --git a/lib/btrfs/btrfsitem/item_devextent.go b/lib/btrfs/btrfsitem/item_devextent.go
index cade165..6998277 100644
--- a/lib/btrfs/btrfsitem/item_devextent.go
+++ b/lib/btrfs/btrfsitem/item_devextent.go
@@ -10,8 +10,23 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
 )
 
-// key.objectid = device_id
-// key.offset = physical_addr
+// A DevExtent tracks allocation of the physical address space.
+//
+// Compare with:
+//   - BlockGroups, which track allocation of the logical address space.
+//   - Chunks, which map logical addresses to physical addresses.
+//
+// The relationship between the three is
+//
+//	DevExtent---[many:one]---Chunk---[one:one]---BlockGroup
+//
+// The device ID identifies which Dev item describes the physical
+// volume that the DevExtent is on.
+//
+// Key:
+//
+//	key.objectid = device_id
+//	key.offset   = physical_addr
 type DevExtent struct { // trivial DEV_EXTENT=204
 	ChunkTree     btrfsprim.ObjID      `bin:"off=0, siz=8"`  // always CHUNK_TREE_OBJECTID
 	ChunkObjectID btrfsprim.ObjID      `bin:"off=8, siz=8"`  // which chunk within .ChunkTree owns this extent, always FIRST_CHUNK_TREE_OBJECTID
diff --git a/lib/btrfs/btrfsitem/item_dir.go b/lib/btrfs/btrfsitem/item_dir.go
index c1b3c09..cd16d57 100644
--- a/lib/btrfs/btrfsitem/item_dir.go
+++ b/lib/btrfs/btrfsitem/item_dir.go
@@ -19,10 +19,14 @@ func NameHash(dat []byte) uint64 {
 	return uint64(^crc32.Update(1, crc32.MakeTable(crc32.Castagnoli), dat))
 }
 
-// key.objectid = inode of directory containing this entry
-// key.offset =
-//   - for DIR_ITEM and XATTR_ITEM = NameHash(name)
-//   - for DIR_INDEX               = index id in the directory (starting at 2, because "." and "..")
+// A DirEntry is an member of a directory.
+//
+// Key:
+//
+//	key.objectid = inode of directory containing this entry
+//	key.offset   = one of:
+//	  - for DIR_ITEM and XATTR_ITEM = NameHash(name)
+//	  - for DIR_INDEX               = index id in the directory (starting at 2, because of "." and "..")
 type DirEntry struct { // complex DIR_ITEM=84 DIR_INDEX=96 XATTR_ITEM=24
 	Location      btrfsprim.Key `bin:"off=0x0, siz=0x11"`
 	TransID       int64         `bin:"off=0x11, siz=8"`
diff --git a/lib/btrfs/btrfsitem/item_extent.go b/lib/btrfs/btrfsitem/item_extent.go
index 871ed90..edfef7c 100644
--- a/lib/btrfs/btrfsitem/item_extent.go
+++ b/lib/btrfs/btrfsitem/item_extent.go
@@ -13,8 +13,22 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/fmtutil"
 )
 
-// key.objectid = laddr of the extent
-// key.offset = length of the extent
+// Extent items map from regions in the logical address space to
+// regions in a file.
+//
+// Compare with:
+//
+//   - Metadata, which are like Extents but without .Info.
+//   - FileExtents, which map from regions in a file to regions in the
+//     logical address space.
+//
+// An Extent may contain (inline or not) several ExtentDataRef items
+// and/or ShareDataRef items.
+//
+// Key:
+//
+//	key.objectid = laddr of the extent
+//	key.offset   = length of the extent
 type Extent struct { // complex EXTENT_ITEM=168
 	Head ExtentHeader
 	Info TreeBlockInfo // only if .Head.Flags.Has(EXTENT_FLAG_TREE_BLOCK)
diff --git a/lib/btrfs/btrfsitem/item_extentcsum.go b/lib/btrfs/btrfsitem/item_extentcsum.go
index dfa166d..a963c16 100644
--- a/lib/btrfs/btrfsitem/item_extentcsum.go
+++ b/lib/btrfs/btrfsitem/item_extentcsum.go
@@ -11,8 +11,12 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
 )
 
-// key.objectid = BTRFS_EXTENT_CSUM_OBJECTID
-// key.offset = laddr of checksummed region
+// An ExtentCSum checksums regions of the logical address space.
+//
+// Key:
+//
+//	key.objectid = BTRFS_EXTENT_CSUM_OBJECTID
+//	key.offset   = laddr of checksummed region
 type ExtentCSum struct { // trivial EXTENT_CSUM=128
 	// Checksum of each sector starting at key.offset
 	btrfssum.SumRun[btrfsvol.LogicalAddr]
diff --git a/lib/btrfs/btrfsitem/item_extentdataref.go b/lib/btrfs/btrfsitem/item_extentdataref.go
index 6f2257b..bd7bb04 100644
--- a/lib/btrfs/btrfsitem/item_extentdataref.go
+++ b/lib/btrfs/btrfsitem/item_extentdataref.go
@@ -9,8 +9,12 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsprim"
 )
 
-// key.objectid = laddr of the extent being referenced
-// key.offset = crc32c([root,objectid,offset])
+// ExtentDataRef is part of an Extent.
+//
+// Key:
+//
+//	key.objectid = laddr of the extent being referenced
+//	key.offset   = crc32c([root,objectid,offset])
 type ExtentDataRef struct { // trivial EXTENT_DATA_REF=178
 	Root          btrfsprim.ObjID `bin:"off=0, siz=8"`  // subvolume tree ID that references this extent
 	ObjectID      btrfsprim.ObjID `bin:"off=8, siz=8"`  // inode number that references this extent within the .Root subvolume
diff --git a/lib/btrfs/btrfsitem/item_fileextent.go b/lib/btrfs/btrfsitem/item_fileextent.go
index 6b08897..df3d371 100644
--- a/lib/btrfs/btrfsitem/item_fileextent.go
+++ b/lib/btrfs/btrfsitem/item_fileextent.go
@@ -12,8 +12,19 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
 )
 
-// key.objectid = inode
-// key.offset = offset within file
+// FileExtent items map from regions in a file to regions in the
+// logical address space.
+//
+// Compare with:
+//
+//   - Extents, which map from regions in the logical address space to
+//     regions in a file.
+//   - Metadata, which are like Extents but without .Info.
+//
+// Key:
+//
+//	key.objectid = inode
+//	key.offset   = offset within file
 type FileExtent struct { // complex EXTENT_DATA=108
 	Generation btrfsprim.Generation `bin:"off=0x0, siz=0x8"` // transaction ID that created this extent
 	RAMBytes   int64                `bin:"off=0x8, siz=0x8"` // upper bound of what compressed data will decompress to
diff --git a/lib/btrfs/btrfsitem/item_freespacebitmap.go b/lib/btrfs/btrfsitem/item_freespacebitmap.go
index ebc00e4..d70a2b5 100644
--- a/lib/btrfs/btrfsitem/item_freespacebitmap.go
+++ b/lib/btrfs/btrfsitem/item_freespacebitmap.go
@@ -4,8 +4,13 @@
 
 package btrfsitem
 
-// key.objectid = object ID of the FreeSpaceInfo (logical_addr)
-// key.offset = offset of the FreeSpaceInfo (size)
+// FreeSpaceBitmap is used in conjunction with FreeSpaceInfo for
+// highly-fragmented blockgroups.
+//
+// Key:
+//
+//	key.objectid = object ID of the FreeSpaceInfo (logical_addr)
+//	key.offset   = offset of the FreeSpaceInfo (size)
 type FreeSpaceBitmap struct { // complex FREE_SPACE_BITMAP=200
 	Bitmap []byte
 }
diff --git a/lib/btrfs/btrfsitem/item_freespaceinfo.go b/lib/btrfs/btrfsitem/item_freespaceinfo.go
index 9b886af..7782d27 100644
--- a/lib/btrfs/btrfsitem/item_freespaceinfo.go
+++ b/lib/btrfs/btrfsitem/item_freespaceinfo.go
@@ -9,8 +9,14 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/fmtutil"
 )
 
-// key.objectid = object ID of the BlockGroup (logical_addr)
-// key.offset = offset of the BlockGroup (size)
+// FreeSpaceInfo is the main way (v2) that free space is tracked in a
+// BlockGroup.  For highly-fragmented blockgorups, it may be augmented
+// by a FreeSpaceBitmap.
+//
+// Key:
+//
+//	key.objectid = object ID of the BlockGroup (logical_addr)
+//	key.offset   = offset of the BlockGroup (size)
 type FreeSpaceInfo struct { // trivial FREE_SPACE_INFO=198
 	ExtentCount   int32          `bin:"off=0, siz=4"`
 	Flags         FreeSpaceFlags `bin:"off=4, siz=4"`
diff --git a/lib/btrfs/btrfsitem/item_inode.go b/lib/btrfs/btrfsitem/item_inode.go
index 6951c76..0b6a1c2 100644
--- a/lib/btrfs/btrfsitem/item_inode.go
+++ b/lib/btrfs/btrfsitem/item_inode.go
@@ -10,8 +10,10 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/fmtutil"
 )
 
-// key.objectid = inode number
-// key.offset = 0
+// Inode is a file/dir/whatever in the filesystem.
+//
+//	key.objectid = inode number
+//	key.offset   = 0
 type Inode struct { // trivial INODE_ITEM=1
 	Generation    btrfsprim.Generation `bin:"off=0x00, siz=0x08"`
 	TransID       int64                `bin:"off=0x08, siz=0x08"`
diff --git a/lib/btrfs/btrfsitem/item_inoderef.go b/lib/btrfs/btrfsitem/item_inoderef.go
index 074b26d..f38295d 100644
--- a/lib/btrfs/btrfsitem/item_inoderef.go
+++ b/lib/btrfs/btrfsitem/item_inoderef.go
@@ -12,11 +12,16 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/containers"
 )
 
-// key.objectid = inode number of the file
-// key.offset = inode number of the parent directory
+// An InodeRefs item is a set of back-references that point to a given
+// Inode.
 //
-// Might have multiple entries if the same file has multiple hardlinks
-// in the same directory.
+// Key:
+//
+//	key.objectid = inode number of the file
+//	key.offset   = inode number of the parent directory
+//
+// There might be multiple back-references in a single InodeRef item
+// if the same file has multiple hardlinks in the same directory.
 type InodeRefs struct { // complex INODE_REF=12
 	Refs []InodeRef
 }
diff --git a/lib/btrfs/btrfsitem/item_metadata.go b/lib/btrfs/btrfsitem/item_metadata.go
index db2315e..10ae994 100644
--- a/lib/btrfs/btrfsitem/item_metadata.go
+++ b/lib/btrfs/btrfsitem/item_metadata.go
@@ -8,7 +8,22 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/binstruct"
 )
 
+// Metadata items map from regions in the logical address space to
+// regions in a file.
+//
 // Metadata is like Extent, but doesn't have .Info.
+//
+// Compare with:
+//
+//   - Extents, which are the same as Metadata, but have an extra
+//     .Info member.
+//   - FileExtents, which map from regions in a file to regions in the
+//     logical address space.
+//
+// Key:
+//
+//	key.objectid = laddr of the extent
+//	key.offset   = length of the extent
 type Metadata struct { // complex METADATA_ITEM=169
 	Head ExtentHeader
 	Refs []ExtentInlineRef
diff --git a/lib/btrfs/btrfsitem/item_qgroupinfo.go b/lib/btrfs/btrfsitem/item_qgroupinfo.go
index 6699030..2bf38f6 100644
--- a/lib/btrfs/btrfsitem/item_qgroupinfo.go
+++ b/lib/btrfs/btrfsitem/item_qgroupinfo.go
@@ -9,8 +9,13 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsprim"
 )
 
-// key.objectid = 0
-// key.offset = ID of the qgroup
+// QGroupInfo tracks the amount of space used by a given qgroup in the
+// containing subvolume.
+//
+// Key:
+//
+//	key.objectid = 0
+//	key.offset   = ID of the qgroup
 type QGroupInfo struct { // trivial QGROUP_INFO=242
 	Generation                btrfsprim.Generation `bin:"off=0, siz=8"`
 	ReferencedBytes           uint64               `bin:"off=8, siz=8"`
diff --git a/lib/btrfs/btrfsitem/item_qgrouplimit.go b/lib/btrfs/btrfsitem/item_qgrouplimit.go
index 47f7eca..e303b04 100644
--- a/lib/btrfs/btrfsitem/item_qgrouplimit.go
+++ b/lib/btrfs/btrfsitem/item_qgrouplimit.go
@@ -34,8 +34,14 @@ func (f QGroupLimitFlags) String() string {
 	return fmtutil.BitfieldString(f, qgroupLimitFlagNames, fmtutil.HexNone)
 }
 
-// key.objectid = 0
-// key.offset = ID of the qgroup
+// QGroupLimit configures the maximum permissible amount of space that
+// a given qgroup can consume (tracked in a QGroupInfo item) on the
+// containing subvolume.
+//
+// Key:
+//
+//	key.objectid = 0
+//	key.offset   = ID of the qgroup
 type QGroupLimit struct { // trivial QGROUP_LIMIT=244
 	Flags         QGroupLimitFlags `bin:"off=0, siz=8"`
 	MaxReferenced uint64           `bin:"off=8, siz=8"`
diff --git a/lib/btrfs/btrfsitem/item_qgroupstatus.go b/lib/btrfs/btrfsitem/item_qgroupstatus.go
index 346c913..b140371 100644
--- a/lib/btrfs/btrfsitem/item_qgroupstatus.go
+++ b/lib/btrfs/btrfsitem/item_qgroupstatus.go
@@ -32,8 +32,14 @@ func (f QGroupStatusFlags) String() string {
 
 const QGroupStatusVersion uint64 = 1
 
-// key.objectid = 0
-// key.offset = 0
+// A QGroupStatus holds the qgroup state of a subvolume (I think that
+// implies that the QGroupStatus goes in the subvolume tree that it is
+// describing?).
+//
+// Key:
+//
+//	key.objectid = 0
+//	key.offset   = 0
 type QGroupStatus struct { // trivial QGROUP_STATUS=240
 	Version        uint64               `bin:"off=0, siz=8"`
 	Generation     btrfsprim.Generation `bin:"off=8, siz=8"`
diff --git a/lib/btrfs/btrfsitem/item_root.go b/lib/btrfs/btrfsitem/item_root.go
index d39bd70..4ffad9a 100644
--- a/lib/btrfs/btrfsitem/item_root.go
+++ b/lib/btrfs/btrfsitem/item_root.go
@@ -11,10 +11,17 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/fmtutil"
 )
 
-// key.objectid = tree ID
-// key.offset = either
-//   - 0 if objectid is one of the BTRFS_*_TREE_OBJECTID defines or a non-snapshot volume; or
-//   - transaction_id of when this snapshot was created
+// A Root goes in the ROOT_TREE and defines one of the other trees in
+// the filesystem.  All trees have a Root item describing them, except
+// for the ROOT_TREE, CHUNK_TREE, TREE_LOG, and BLOCK_GROUP_TREE,
+// which are defined directly in the superblock.
+//
+// Key:
+//
+//	key.objectid = tree ID
+//	key.offset   = one of:
+//	   - 0 if objectid is one of the BTRFS_*_TREE_OBJECTID defines or a non-snapshot volume; or
+//	   - transaction_id of when this snapshot was created
 type Root struct { // trivial ROOT_ITEM=132
 	Inode         Inode                `bin:"off=0x000, siz=0xa0"` // ???
 	Generation    btrfsprim.Generation `bin:"off=0x0a0, siz=0x08"`
diff --git a/lib/btrfs/btrfsitem/item_rootref.go b/lib/btrfs/btrfsitem/item_rootref.go
index 4179890..e40b278 100644
--- a/lib/btrfs/btrfsitem/item_rootref.go
+++ b/lib/btrfs/btrfsitem/item_rootref.go
@@ -12,10 +12,12 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsprim"
 )
 
-// RootRefs link subvolumes parent→child for normal subvolumes and
+// A RootRef links subvolumes parent→child for normal subvolumes and
 // base→snapshot for snapshot subvolumes.  BACKREF items go the other
 // direction; child→parent and snapshot→base.
 //
+// Key:
+//
 //	               ROOT_REF                   | ROOT_BACKREF
 //	key.objectid = ID of the parent subvolume | ID of the child subvolume
 //	key.offset   = ID of the child subvolume  | ID of the parent subvolume
diff --git a/lib/btrfs/btrfsitem/item_shareddataref.go b/lib/btrfs/btrfsitem/item_shareddataref.go
index 6143a5c..5d88522 100644
--- a/lib/btrfs/btrfsitem/item_shareddataref.go
+++ b/lib/btrfs/btrfsitem/item_shareddataref.go
@@ -8,10 +8,12 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/binstruct"
 )
 
-// key.objectid = laddr of the extent being referenced
+// SharedDataRef is part of an Extent.
 //
-// key.offset = laddr of the leaf node containing the FileExtent
-// (EXTENT_DATA_KEY) for this reference.
+// Key:
+//
+//	key.objectid = laddr of the extent being referenced
+//	key.offset   = laddr of the leaf node containing the FileExtent (EXTENT_DATA_KEY) for this reference.
 type SharedDataRef struct { // trivial SHARED_DATA_REF=184
 	Count         int32 `bin:"off=0, siz=4"` // reference count
 	binstruct.End `bin:"off=4"`
diff --git a/lib/btrfs/btrfsitem/item_uuid.go b/lib/btrfs/btrfsitem/item_uuid.go
index fca409d..a5ace33 100644
--- a/lib/btrfs/btrfsitem/item_uuid.go
+++ b/lib/btrfs/btrfsitem/item_uuid.go
@@ -11,11 +11,16 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsprim"
 )
 
-// The Key for this item is a UUID, and the item is a subvolume IDs
+// A UUIDMap item goes in the UUID_TREE and maps from a UUID to a
+// btrfsprim.ObjID.
+//
+// The Key for this item is a UUID, and the item is a subvolume ID
 // that UUID maps to.
 //
-// key.objectid = first half of UUID
-// key.offset = second half of UUID
+// Key:
+//
+//	key.objectid = first half of UUID
+//	key.offset   = second half of UUID
 type UUIDMap struct { // trivial UUID_SUBVOL=251 UUID_RECEIVED_SUBVOL=252
 	ObjID         btrfsprim.ObjID `bin:"off=0, siz=8"`
 	binstruct.End `bin:"off=8"`
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