Support user.useConfigOnly git config

When it's set and git cannot determine user.name or user.email, this will
result in git-annex init failing when committing to create the git-annex
branch. Other git-annex commands that commit can also fail.

Sponsored-by: Jack Hill on Patreon

Annex/Environment.hs

@@ -7,7 +7,11 @@
 
 {-# LANGUAGE OverloadedStrings #-}
 
-module Annex.Environment where
+module Annex.Environment (
+	checkEnvironment,
+	checkEnvironmentIO,
+	ensureCommit,
+) where
 
 import Annex.Common
 import qualified Annex
@@ -15,6 +19,8 @@ import Utility.UserInfo
 import qualified Git.Config
 import Utility.Env.Set
 
+import Control.Exception
+
 {- Checks that the system's environment allows git to function.
  - Git requires a GECOS username, or suitable git configuration, or
  - environment variables.
@@ -29,7 +35,8 @@ checkEnvironment :: Annex ()
 checkEnvironment = do
 	gitusername <- fromRepo $ Git.Config.getMaybe "user.name"
 	when (isNothing gitusername || gitusername == Just "") $
-		liftIO checkEnvironmentIO
+		unlessM userConfigOnly $
+			liftIO checkEnvironmentIO
 
 checkEnvironmentIO :: IO ()
 checkEnvironmentIO = whenM (isNothing <$> myUserGecos) $ do
@@ -46,8 +53,16 @@ checkEnvironmentIO = whenM (isNothing <$> myUserGecos) $ do
 ensureCommit :: Annex a -> Annex a
 ensureCommit a = either retry return =<< tryNonAsync a 
   where
-	retry _ = do
-		name <- liftIO $ either (const "unknown") id <$> myUserName
-		Annex.addGitConfigOverride ("user.name=" ++ name)
-		Annex.addGitConfigOverride ("user.email=" ++ name)
-		a
+	retry e = ifM userConfigOnly
+		( liftIO (throwIO e)
+		, do
+			name <- liftIO $ either (const "unknown") id <$> myUserName
+			Annex.addGitConfigOverride ("user.name=" ++ name)
+			Annex.addGitConfigOverride ("user.email=" ++ name)
+			a
+		)
+
+userConfigOnly :: Annex Bool
+userConfigOnly = do
+	v <- fromRepo $ Git.Config.getMaybe "user.useconfigonly"
+	return (fromMaybe False (Git.Config.isTrueFalse' =<< v))

CHANGELOG

@@ -7,6 +7,7 @@ git-annex (10.20230330) UNRELEASED; urgency=medium
     of --unlock-present and --hide-missing adjusted branches.
   * Avoid setting user.name and user.email in the git config
     when git is unable to detect them.
+  * Support user.useConfigOnly git config.
 
  -- Joey Hess <id@joeyh.name>  Fri, 31 Mar 2023 12:48:54 -0400