adjust: If the adjusted branch already exists, avoid overwriting it, since it might contain changes that have not yet been propigated to the original branch.

Could not think of a foolproof way to detect if the old adjusted branch was
just behind the current branch. It's possible that the user amended the
adjusting commit at the head of the adjusted branch, for example.

I decided to bail in this situation, instead of just entering the old
branch, so that if git annex adjust succeeds the user is always in a
*current* adjusted branch, not some old and out of date one.

What could perhaps be done is enter the old branch and then update it. But
that seems too magical; the user may have rebased master or something or
may not want to propigate the changes from the old branch. Best to error
out.

Annex/AdjustedBranch.hs

@@ -160,21 +160,41 @@ originalBranch = fmap fromAdjustedBranch <$> inRepo Git.Branch.current
  - adjusted version of a branch, changes the adjustment of the original
  - branch).
  -
- - Can fail, if no branch is checked out, or perhaps if staged changes
- - conflict with the adjusted branch.
+ - Can fail, if no branch is checked out, or if the adjusted branch already
+ - exists, or perhaps if staged changes conflict with the adjusted branch.
  -}
-enterAdjustedBranch :: Adjustment -> Annex ()
+enterAdjustedBranch :: Adjustment -> Annex Bool
 enterAdjustedBranch adj = go =<< originalBranch
   where
 	go (Just origbranch) = do
+		let adjbranch = adjBranch $ originalToAdjusted origbranch adj
+		ifM (inRepo $ Git.Ref.exists adjbranch)
+			( do
+				mapM_ (warning . unwords)
+					[ [ "adjusted branch"
+					  , Git.Ref.describe adjbranch
+					  , "already exists."
+					  ]
+					, [ "Aborting because that branch may have changes that have not yet reached"
+					  , Git.Ref.describe origbranch
+					  ]
+					, [ "You can check out the adjusted branch manually to enter it,"
+					  , "or delete the adjusted branch and re-run this command."
+					  ]
+					]
+				return False
+			, do
 				AdjBranch b <- preventCommits $ const $ 
 					adjustBranch adj origbranch
 				showOutput -- checkout can have output in large repos
-		inRepo $ Git.Command.run
+				inRepo $ Git.Command.runBool
 					[ Param "checkout"
 					, Param $ fromRef $ Git.Ref.base b
 					]
-	go Nothing = error "not on any branch!"
+			)
+	go Nothing = do
+		warning "not on any branch!"
+		return False
 
 adjustToCrippledFileSystem :: Annex ()
 adjustToCrippledFileSystem = do
@@ -186,7 +206,8 @@ adjustToCrippledFileSystem = do
 			, Param "-m"
 			, Param "commit before entering adjusted unlocked branch"
 			]
-	enterAdjustedBranch UnlockAdjustment
+	unlessM (enterAdjustedBranch UnlockAdjustment) $
+		warning "Failed to enter adjusted branch!"
 
 setBasisBranch :: BasisBranch -> Ref -> Annex ()
 setBasisBranch (BasisBranch basis) new = 

Command/Adjust.hs

@@ -35,5 +35,4 @@ start :: Adjustment -> CommandStart
 start adj = do
 	checkVersionSupported
 	showStart "adjust" ""
-	enterAdjustedBranch adj
-	next $ next $ return True
+	next $ next $ enterAdjustedBranch adj

debian/changelog

@@ -3,6 +3,9 @@ git-annex (6.20160512) UNRELEASED; urgency=medium
   * Change git annex info remote encryption description to use wording
     closer to what's used in initremote.
   * webapp: Avoid confusing display of dead remotes.
+  * adjust: If the adjusted branch already exists, avoid overwriting it,
+    since it might contain changes that have not yet been propigated to the
+    original branch.
 
  -- Joey Hess <id@joeyh.name>  Wed, 11 May 2016 16:08:38 -0400