t/lib-httpd: try harder to find a port for apache

When running multiple builds concurrently, tests which run daemons, like
apache httpd, sometimes conflict with each other, leading to spurious
failures:

    ++ /usr/sbin/httpd -d '/tmp/git-t.ck9I/trash directory.t9118-git-svn-funky-branch-names/httpd' \
       -f /builddir/build/BUILD/git-2.37.2/t/lib-httpd/apache.conf -DDAV -DSVN -c 'Listen 127.0.0.1:9118' \
       -k start
    (98)Address already in use: AH00072: make_sock: could not bind to address 127.0.0.1:9118
    no listening sockets available, shutting down
    AH00015: Unable to open logs
    ++ test 1 -ne 0

Try a bit harder to find an open port to use to avoid these intermittent
failures.  If we fail to start httpd, increment the port number and try
again.  By default, we make 3 attempts.  This may be overridden by
setting GIT_TEST_START_HTTPD_TRIES to a different value.

Helped-by: Ondřej Pohořelský <opohorel@redhat.com>
Signed-off-by: Todd Zullinger <tmz@pobox.com>

Author: tmzullinger

t/lib-httpd.sh

@@ -175,19 +175,26 @@ prepare_httpd() {
 }
 
 start_httpd() {
-	prepare_httpd >&3 2>&4
-
 	test_atexit stop_httpd
 
-	"$LIB_HTTPD_PATH" -d "$HTTPD_ROOT_PATH" \
-		-f "$TEST_PATH/apache.conf" $HTTPD_PARA \
-		-c "Listen 127.0.0.1:$LIB_HTTPD_PORT" -k start \
-		>&3 2>&4
-	if test $? -ne 0
-	then
-		cat "$HTTPD_ROOT_PATH"/error.log >&4 2>/dev/null
-		test_skip_or_die GIT_TEST_HTTPD "web server setup failed"
-	fi
+	i=0
+	while test $i -lt ${GIT_TEST_START_HTTPD_TRIES:-3}
+	do
+		i=$(($i + 1))
+		prepare_httpd >&3 2>&4
+		say >&3 "Starting httpd on port $LIB_HTTPD_PORT"
+		"$LIB_HTTPD_PATH" -d "$HTTPD_ROOT_PATH" \
+			-f "$TEST_PATH/apache.conf" $HTTPD_PARA \
+			-c "Listen 127.0.0.1:$LIB_HTTPD_PORT" -k start \
+			>&3 2>&4
+		test $? -eq 0 && return
+		LIB_HTTPD_PORT=$(($LIB_HTTPD_PORT + 1))
+		export LIB_HTTPD_PORT
+		# clean up modules symlink, prepare_httpd will re-create it
+		rm -f "$HTTPD_ROOT_PATH/modules"
+	done
+	cat "$HTTPD_ROOT_PATH"/error.log >&4 2>/dev/null
+	test_skip_or_die GIT_TEST_HTTPD "web server setup failed"
 }
 
 stop_httpd() {