t5536: new test of refspec conflicts when fetching

Add some tests that "git fetch" handles refspec conflicts (i.e., when
the same local reference should be updated from two different remote
references) correctly.

There is a small bug when updating references opportunistically,
namely that an explicit user wish like

    git fetch origin \
        refs/heads/branch1:refs/remotes/origin/branch2 \
        refs/heads/branch2:refs/remotes/origin/branch1

should override a configured refspec like

    +refs/heads/*:refs/remotes/origin/*

The current code incorrectly treats this as a fatal error.

In a few commits we will improve the error messages for refspec
conflicts in general and also turn this buggy fatal error into a
warning.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5536-fetch-conflicts.sh

@@ -0,0 +1,96 @@
+#!/bin/sh
+
+test_description='fetch handles conflicting refspecs correctly'
+
+. ./test-lib.sh
+
+D=$(pwd)
+
+setup_repository () {
+	git init "$1" && (
+		cd "$1" &&
+		git config remote.origin.url "$D" &&
+		shift &&
+		for refspec in "$@"
+		do
+			git config --add remote.origin.fetch "$refspec"
+		done
+	)
+}
+
+verify_stderr () {
+	cat >expected &&
+	# We're not interested in the error
+	# "fatal: The remote end hung up unexpectedly":
+	grep -v "hung up" <error >actual &&
+	test_cmp expected actual
+}
+
+test_expect_success 'setup' '
+	git commit --allow-empty -m "Initial" &&
+	git branch branch1 &&
+	git tag tag1 &&
+	git commit --allow-empty -m "First" &&
+	git branch branch2 &&
+	git tag tag2
+'
+
+test_expect_success 'fetch with no conflict' '
+	setup_repository ok "+refs/heads/*:refs/remotes/origin/*" && (
+		cd ok &&
+		git fetch origin
+	)
+'
+
+test_expect_success 'fetch conflict: config vs. config' '
+	setup_repository ccc \
+		"+refs/heads/branch1:refs/remotes/origin/branch1" \
+		"+refs/heads/branch2:refs/remotes/origin/branch1" && (
+		cd ccc &&
+		test_must_fail git fetch origin 2>error &&
+		verify_stderr <<-\EOF
+		fatal: refs/remotes/origin/branch1 tracks both refs/heads/branch1 and refs/heads/branch2
+		EOF
+	)
+'
+
+test_expect_success 'fetch duplicate: config vs. config' '
+	setup_repository dcc \
+		"+refs/heads/*:refs/remotes/origin/*" \
+		"+refs/heads/branch1:refs/remotes/origin/branch1" && (
+		cd dcc &&
+		git fetch origin
+	)
+'
+
+test_expect_success 'fetch conflict: arg overrides config' '
+	setup_repository aoc \
+		"+refs/heads/*:refs/remotes/origin/*" && (
+		cd aoc &&
+		git fetch origin refs/heads/branch2:refs/remotes/origin/branch1
+	)
+'
+
+test_expect_success 'fetch conflict: arg vs. arg' '
+	setup_repository caa && (
+		cd caa &&
+		test_must_fail git fetch origin \
+			refs/heads/*:refs/remotes/origin/* \
+			refs/heads/branch2:refs/remotes/origin/branch1 2>error &&
+		verify_stderr <<-\EOF
+		fatal: refs/remotes/origin/branch1 tracks both refs/heads/branch1 and refs/heads/branch2
+		EOF
+	)
+'
+
+test_expect_failure 'fetch conflict: criss-cross args' '
+	setup_repository xaa \
+		"+refs/heads/*:refs/remotes/origin/*" && (
+		cd xaa &&
+		git fetch origin \
+			refs/heads/branch1:refs/remotes/origin/branch2 \
+			refs/heads/branch2:refs/remotes/origin/branch1
+	)
+'
+
+test_done