fix addurl concurrency issue

addurl: Support adding the same url to multiple files at the same time when
using -J with --batch --with-files.

Implementation was easier than expected, was able to reuse OnlyActionOn.

While it will download the url's content multiple times, that seems like
the best thing to do; see my comment for why.

Sponsored-by: Dartmouth College's DANDI project

CHANGELOG

@@ -27,6 +27,8 @@ git-annex (8.20211012) UNRELEASED; urgency=medium
     the hook to freeze the file in ways that prevent moving it, such as
     removing the Windows delete permission.
     Thanks, Reiko Asakura.
+  * addurl: Support adding the same url to multiple files at the same
+    time when using -J with --batch --with-files.
 
  -- Joey Hess <id@joeyh.name>  Mon, 11 Oct 2021 14:09:13 -0400
 

Command/AddUrl.hs

@@ -372,11 +372,21 @@ checkRaw o a
  - a filename. It's not displayed then for output consistency, 
  - but is added to the json when available. -}
 startingAddUrl :: SeekInput -> URLString -> AddUrlOptions -> CommandPerform -> CommandStart
-startingAddUrl si url o p = starting "addurl" (ActionItemOther (Just url)) si $ do
+startingAddUrl si url o p = starting "addurl" ai si $ do
 	case fileOption (downloadOptions o) of
 		Nothing -> noop
 		Just file -> maybeShowJSON $ JSONChunk [("file", file)]
 	p
+  where
+	-- Avoid failure when the same url is downloaded concurrently
+	-- to two different files, by using OnlyActionOn with a key
+	-- based on the url. Note that this may not be the actual key
+	-- that is used for the download; later size information may be
+	-- available and get added to it. That's ok, this is only
+	-- used to prevent two threads running concurrently when that would
+	-- likely fail.
+	ai = OnlyActionOn urlkey (ActionItemOther (Just url))
+	urlkey = Backend.URL.fromUrl url Nothing
 
 showDestinationFile :: FilePath -> Annex ()
 showDestinationFile file = do

doc/bugs/addurl_failure_has_empty_error-messages.mdwn

@@ -2,3 +2,5 @@ While running `git-annex addurl --batch --with-files --jobs 10 --json --json-err
 
 [[!meta author=jwodder]]
 [[!tag projects/dandi]]
+
+> [[fixed|done]] --[[Joey]]

doc/bugs/addurl_failure_has_empty_error-messages/comment_4_74e37e96d01bfb8b9521000d5faa7e53._comment

@@ -8,7 +8,7 @@ downloading, and the key transfer machinery prevents redundant downloads
 of the same Key at the same time.
 
 Arguably, the problem is not where the message gets put, but that
-it fails when adding an url to two different paths.
+it fails when adding an url to two different paths at the same time.
 
 I have, though, moved that message so it will appear in error-messages.
 """]]

doc/bugs/addurl_failure_has_empty_error-messages/comment_5_1b87390e69204b42878930cc1614437e._comment

@@ -0,0 +1,26 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 5"""
+ date="2021-10-27T18:56:23Z"
+ content="""
+The best solution I can find is for it to notice when another thread is
+downloading the same url, and wait until it finishes. Then proceed
+with downloading the url for a second time.
+
+It's not very satisfying to re-download. But once the url Key is downloaded,
+it does not keep that url Key populated, but hashes the content and moves
+the content to the final Key. It would be a real complication to
+communicate, across threads, what Key the content ended up at, and have the
+waiting thread use that. And addurl is already complicated well beyond a
+point I am comfortable with.
+
+Also, the content of an url can of course change over time. If I feed
+"$url foo" into git-annex addurl --batch -J10 and then some time
+later, I feed "$url bar", I might expect that file bar gets whatever
+content the url has now, not the content that the url had back when I added
+the same url to file foo. And if I cared about avoiding re-downloading,
+I could add the url to the first file, and then copy the annex link to the
+second file myself.
+
+Implemented this approach.
+"""]]