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

["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_user_tool to output an
error message before returning if {diff,merge}tool.$tool.cmd is not set.

Adjust the second call to setup_user_tool in setup_tool to silence this
new error, 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 (which
is why we do not check the return status of this call).

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 | 10 +++++++---
 t/t7610-mergetool.sh  |  8 ++++++++
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/git-mergetool--lib.sh b/git-mergetool--lib.sh
index 269a60ea44c..f4786afc63f 100644
--- a/git-mergetool--lib.sh
+++ b/git-mergetool--lib.sh
@@ -159,14 +159,18 @@ 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"
 }
 
 setup_user_tool () {
 	merge_tool_cmd=$(get_merge_tool_cmd "$tool")
-	test -n "$merge_tool_cmd" || return 1
+	if test -z "$merge_tool_cmd"
+	then
+		echo >&2 "error: ${TOOL_MODE}tool.$tool.cmd not set for tool '$tool'"
+		return 1
+	fi
 
 	diff_cmd () {
 		( eval $merge_tool_cmd )
@@ -255,7 +259,7 @@ setup_tool () {
 
 	# Now let the user override the default command for the tool.  If
 	# they have not done so then this will return 1 which we ignore.
-	setup_user_tool
+	setup_user_tool 2>/dev/null
 
 	if ! list_tool_variants | grep -q "^$tool$"
 	then
diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
index 22b3a85b3e9..82a88107850 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 -i "not set for tool" out
+'
+
 test_done
-- 
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