user-manual: git-fsck, dangling objects

Initial import of fsck and dangling objects discussion, mostly lifted from
an email from Linus.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>

Documentation/user-manual.txt

@@ -1373,12 +1373,37 @@ Ensuring reliability
 Checking the repository for corruption
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-TODO:
-	git-fsck
-	"dangling objects" explanation
-		Brief explanation here,
-		include forward reference to longer explanation from
-			Linus, to be added to later chapter
+The gitlink:git-fsck-objects[1] command runs a number of self-consistency
+checks on the repository, and reports on any problems.  This may take some
+time.  The most common warning by far is about "dangling" objects:
+
+-------------------------------------------------
+$ git fsck-objects
+dangling commit 7281251ddd2a61e38657c827739c57015671a6b3
+dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63
+dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5
+dangling blob 218761f9d90712d37a9c5e36f406f92202db07eb
+dangling commit bf093535a34a4d35731aa2bd90fe6b176302f14f
+dangling commit 8e4bec7f2ddaa268bef999853c25755452100f8e
+dangling tree d50bb86186bf27b681d25af89d3b5b68382e4085
+dangling tree b24c2473f1fd3d91352a624795be026d64c8841f
+...
+-------------------------------------------------
+
+Dangling objects are objects that are harmless, but also unnecessary; you can
+remove them at any time with gitlink:git-prune[1] or the --prune option to
+gitlink:git-gc[1]:
+
+-------------------------------------------------
+$ git gc --prune
+-------------------------------------------------
+
+This may be time-consuming.  Unlike most other git operations (including git-gc
+when run without any options), it is not safe to prune while other git
+operations are in progress in the same repository.
+
+For more about dangling merges, see <<dangling-merges>>.
+
 
 Recovering lost changes
 ~~~~~~~~~~~~~~~~~~~~~~~
@@ -2693,6 +2718,93 @@ objects will work exactly as they did before.
 The gitlink:git-gc[1] command performs packing, pruning, and more for
 you, so is normally the only high-level command you need.
 
+[[dangling-objects]]
+Dangling objects
+^^^^^^^^^^^^^^^^
+
+The gitlink:git-fsck-objects[1] command will sometimes complain about dangling
+objects.  They are not a problem.
+
+The most common cause of dangling objects is that you've rebased a branch, or
+you have pulled from somebody else who rebased a branch--see
+<<cleaning-up-history>>.  In that case, the old head of the original branch
+still exists, as does obviously everything it pointed to. The branch pointer
+itself just doesn't, since you replaced it with another one.
+
+There are also other situations too that cause dangling objects. For example, a
+"dangling blob" may arise because you did a "git add" of a file, but then,
+before you actually committed it and made it part of the bigger picture, you
+changed something else in that file and committed that *updated* thing - the
+old state that you added originally ends up not being pointed to by any
+commit or tree, so it's now a dangling blob object.
+
+Similarly, when the "recursive" merge strategy runs, and finds that there 
+are criss-cross merges and thus more than one merge base (which is fairly 
+unusual, but it does happen), it will generate one temporary midway tree 
+(or possibly even more, if you had lots of criss-crossing merges and 
+more than two merge bases) as a temporary internal merge base, and again, 
+those are real objects, but the end result will not end up pointing to 
+them, so they end up "dangling" in your repository.
+
+Generally, dangling objects aren't anything to worry about. They can even 
+be very useful: if you screw something up, the dangling objects can be how 
+you recover your old tree (say, you did a rebase, and realized that you 
+really didn't want to - you can look at what dangling objects you have, 
+and decide to reset your head to some old dangling state).
+
+For commits, the most useful thing to do with dangling objects tends to be 
+to do a simple
+
+------------------------------------------------
+$ gitk <dangling-commit-sha-goes-here> --not --all
+------------------------------------------------
+
+which means exactly what it sounds like: it says that you want to see the 
+commit history that is described by the dangling commit(s), but you do NOT 
+want to see the history that is described by all your branches and tags 
+(which are the things you normally reach). That basically shows you in a 
+nice way what the dangling commit was (and notice that it might not be 
+just one commit: we only report the "tip of the line" as being dangling, 
+but there might be a whole deep and complex commit history that has gotten 
+dropped - rebasing will do that).
+
+For blobs and trees, you can't do the same, but you can examine them. You 
+can just do
+
+------------------------------------------------
+$ git show <dangling-blob/tree-sha-goes-here>
+------------------------------------------------
+
+to show what the contents of the blob were (or, for a tree, basically what 
+the "ls" for that directory was), and that may give you some idea of what 
+the operation was that left that dangling object.
+
+Usually, dangling blobs and trees aren't very interesting. They're almost 
+always the result of either being a half-way mergebase (the blob will 
+often even have the conflict markers from a merge in it, if you have had 
+conflicting merges that you fixed up by hand), or simply because you 
+interrupted a "git fetch" with ^C or something like that, leaving _some_ 
+of the new objects in the object database, but just dangling and useless.
+
+Anyway, once you are sure that you're not interested in any dangling 
+state, you can just prune all unreachable objects:
+
+------------------------------------------------
+$ git prune
+------------------------------------------------
+
+and they'll be gone. But you should only run "git prune" on a quiescent 
+repository - it's kind of like doing a filesystem fsck recovery: you don't 
+want to do that while the filesystem is mounted.
+
+(The same is true of "git-fsck-objects" itself, btw - but since 
+git-fsck-objects never actually *changes* the repository, it just reports 
+on what it found, git-fsck-objects itself is never "dangerous" to run. 
+Running it while somebody is actually changing the repository can cause 
+confusing and scary messages, but it won't actually do anything bad. In 
+contrast, running "git prune" while somebody is actively changing the 
+repository is a *BAD* idea).
+
 Glossary of git terms
 =====================