[PATCH v2 3/5] git-mergetool--lib.sh: add error message if 'setup_user_tool' fails

["Philippe Blain via GitGitGadget" <gitgitgadget@gmail.com>]

From: Philippe Blain <levraiphilippeblain@gmail.com>

In git-mergetool--lib.sh::setup_tool, we check if the given tool is a
known builtin tool, a known variant, or a user-defined tool by calling
setup_user_tool, and we return with the exit code from setup_user_tool
if it was called. setup_user_tool checks if {diff,merge}tool.$tool.cmd
is set and quietly returns with an error if not.

This leads to the following invocation quietly failing:

	git mergetool --tool=unknown

which is not very user-friendly. Adjust setup_tool to output an error
message before returning if setup_user_tool returned with an error.

Note that we do not check the result of the second call to
setup_user_tool in setup_tool, as this call is only meant to allow users
to redefine 'cmd' for a builtin tool; it is not an error if they have
not done so.

Note that this behaviour of quietly failing is a regression dating back
to de8dafbada (mergetool: break setup_tool out into separate
initialization function, 2021-02-09), as before this commit an unknown
mergetool would be diagnosed in get_merge_tool_path when called from
run_merge_tool.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
---
 git-mergetool--lib.sh | 9 +++++++--
 t/t7610-mergetool.sh  | 8 ++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/git-mergetool--lib.sh b/git-mergetool--lib.sh
index 269a60ea44c..d7e410d9481 100644
--- a/git-mergetool--lib.sh
+++ b/git-mergetool--lib.sh
@@ -159,7 +159,7 @@ check_unchanged () {
 }
 
 valid_tool () {
-	setup_tool "$1" && return 0
+	setup_tool "$1" 2>/dev/null && return 0
 	cmd=$(get_merge_tool_cmd "$1")
 	test -n "$cmd"
 }
@@ -250,7 +250,12 @@ setup_tool () {
 		. "$MERGE_TOOLS_DIR/${tool%[0-9]}"
 	else
 		setup_user_tool
-		return $?
+		rc=$?
+		if test $rc -ne 0
+		then
+			echo >&2 "error: ${TOOL_MODE}tool.$tool.cmd not set for tool '$tool'"
+		fi
+		return $rc
 	fi
 
 	# Now let the user override the default command for the tool.  If
diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
index 22b3a85b3e9..c077aba7ced 100755
--- a/t/t7610-mergetool.sh
+++ b/t/t7610-mergetool.sh
@@ -898,4 +898,12 @@ test_expect_success 'mergetool with guiDefault' '
 	git commit -m "branch1 resolved with mergetool"
 '
 
+test_expect_success 'mergetool with non-existent tool' '
+	test_when_finished "git reset --hard" &&
+	git checkout -b test$test_count branch1 &&
+	test_must_fail git merge main &&
+	yes "" | test_must_fail git mergetool --tool=absent >out 2>&1 &&
+	test_grep "mergetool.absent.cmd not set for tool" out
+'
+
 test_done
-- 
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