--explain for numcopies checks

And closed the todo as completed.

Sponsored-by: Dartmouth College's DANDI project

Annex/NumCopies.hs

@@ -197,8 +197,12 @@ numCopiesCheck file key vs = do
 
 numCopiesCheck' :: RawFilePath -> (Int -> Int -> v) -> [UUID] -> Annex v
 numCopiesCheck' file vs have = do
-	needed <- fst <$> getFileNumMinCopies file
-	return $ length have `vs` fromNumCopies needed
+	needed <- fromNumCopies . fst <$> getFileNumMinCopies file
+	let nhave = length have
+	explain (ActionItemTreeFile file) $ Just $ UnquotedString $
+		"has " ++ show nhave ++ " " ++ pluralCopies nhave ++ 
+		", and the configured annex.numcopies is " ++ show needed
+	return $ nhave `vs` needed
 
 data UnVerifiedCopy = UnVerifiedRemote Remote | UnVerifiedHere
 	deriving (Ord, Eq)
@@ -280,10 +284,10 @@ notEnoughCopies key neednum needmin have skip bad nolocmsg lockunsupported = do
 		then showLongNote $ UnquotedString $
 			"Could only verify the existence of " ++
 			show (length have) ++ " out of " ++ show (fromNumCopies neednum) ++ 
-			" necessary " ++ pluralcopies (fromNumCopies neednum)
+			" necessary " ++ pluralCopies (fromNumCopies neednum)
 		else do
 			showLongNote $ UnquotedString $ "Unable to lock down " ++ show (fromMinCopies needmin) ++ 
-				" " ++ pluralcopies (fromMinCopies needmin) ++ 
+				" " ++ pluralCopies (fromMinCopies needmin) ++ 
 				" of file necessary to safely drop it."
 			if null lockunsupported
 				then showLongNote "(This could have happened because of a concurrent drop, or because a remote has too old a version of git-annex-shell installed.)"
@@ -292,9 +296,10 @@ notEnoughCopies key neednum needmin have skip bad nolocmsg lockunsupported = do
 
 	Remote.showTriedRemotes bad
 	Remote.showLocations True key (map toUUID have++skip) nolocmsg
-  where
-	pluralcopies 1 = "copy"
-	pluralcopies _ = "copies"
+
+pluralCopies :: Int -> String
+pluralCopies 1 = "copy"
+pluralCopies _ = "copies"
 
 {- Finds locations of a key that can be used to get VerifiedCopies,
  - in order to allow dropping the key.

doc/todo/option_to_explain.mdwn

@@ -38,3 +38,5 @@ included expression, eg: "standard(group==backup, include==*)"
 [[!tag confirmed]] --[[Joey]]
 
 [[!tag projects/dandi]]
+
+> [[done]] --[[Joey]]

doc/todo/option_to_explain/comment_4_28919c6f816f11811b1e8c73b07403f7._comment

@@ -0,0 +1,32 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 4"""
+ date="2023-07-31T16:38:05Z"
+ content="""
+--explain now also shows when numcopies is taken into account when not
+dropping, eg `get --auto`.
+
+When dropping, git-annex already explains when numcopies is not satisfied
+of course. As to displaying the drop safety proof, I think it's too much in
+the weeds for a user to want it explained? Users probably don't want to
+worry about the distinction between RecentlyVerifiedCopy and LockedCopy,
+for example; those details are git-annex's job. It already gives a well
+calibrated explanation when it can't drop, so explaining when it can drop
+seems unncessary.
+
+I also looked at adding it for file content checking, but inAnnex is pretty core
+and is used in a lot of situations where --explain output would not be
+useful, eg after git-annex has downloaded a file, it will check inAnnex.
+While it would be possible to add something that explains that is only
+called when deciding whether to operate on a file, that would be an
+intrusive code change and would add complexity.
+
+Also, while it can be confusing to users sometimes why eg `git-annex get
+foo` doesn't output anything due to the file being present already, it
+seems like adding --explain output to that would generally not be useful to
+more experienced users. Explain needs to remain relatively concise, or it's
+just more debug output.
+
+So, closing this as completed, although of course more can be added
+to --explain later..
+"""]]