dhcpcd: Fix startup process

The *dhcpcd.service* systemd unit file that ships with Buildroot is
broken.  It orders the unit after *network.target*, which makes no
sense.  It also configures the process to fork, but incorrectly
configures the PID file.  Finally, by relying on the fork to indicate
that the network is "online," it does not distinguish between "an
interface is up" and "the network is reachable."  As most daemons only
need the former, they only rely on *network.target*, and thus can fail
to start correctly.

To correct this brokenness, we provide our own unit file for
*dhcpcd.service*, based on the one included in the Fedora *dhcpcd* RPM
package.  For "online" signalling, we provide a pair of shell scripts:
one reads from a named pipe waiting for a message and the other sends a
message to the pipe when configuration is complete.

package/dhcpcd/dhcpcd-wait-online.service

@@ -0,0 +1,16 @@
+[Unit]
+Description=Wait for Network to be Configured
+DefaultDependencies=no
+Conflicts=shutdown.target
+After=dhcpcd.service
+Requires=dhcpcd.service
+Before=network-online.target shutdown.target
+
+[Service]
+Type=oneshot
+ExecStart=/usr/libexec/dhcpcd-wait-online
+TimeoutStartSec=1m
+RemainAfterExit=yes
+
+[Install]
+WantedBy=network-online.target

package/dhcpcd/dhcpcd-wait-online.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+# After it finishes configuring a network interface, dhcpcd will write
+# `online` to the `/run/dhcpcd/wait-online` pipe, if it exists.
+# Reading from the pipe will block until something is written to it,
+# allowing us to wait for the network to be configured, without
+# repeatedly polling interface information.
+#
+# If anything but `online` is read from the pipe, we assume that there
+# was an error configuring the network.
+
+cleanup() {
+    rm -f /run/dhcpcd/wait-online
+}
+
+mkdir -p /run/dhcpcd
+mkfifo /run/dhcpcd/wait-online || exit $?
+trap cleanup INT TERM QUIT EXIT
+status=$(cat /run/dhcpcd/wait-online)
+test "${status}" = online

package/dhcpcd/dhcpcd.mk

@@ -0,0 +1,14 @@
+define DHCPCD_INSTALL_INIT_SYSTEMD
+	$(INSTALL) -D -m u=rw,go=r \
+		$(BR2_EXTERNAL_metricspi_PATH)/package/dhcpcd/dhcpcd.service \
+		$(TARGET_DIR)/usr/lib/systemd/system/dhcpcd.service
+	$(INSTALL) -D -m u=rw,go=r \
+		$(BR2_EXTERNAL_metricspi_PATH)/package/dhcpcd/dhcpcd-wait-online.service \
+		$(TARGET_DIR)/usr/lib/systemd/system/dhcpcd-wait-online.service
+	$(INSTALL) -D -m u=rwx,go=rx \
+		$(BR2_EXTERNAL_metricspi_PATH)/package/dhcpcd/online.hook.sh \
+		$(TARGET_DIR)/usr/lib/dhcpcd/dhcpcd-hooks/99-online
+	$(INSTALL) -D -m u=rwx,go=rx \
+		$(BR2_EXTERNAL_metricspi_PATH)/package/dhcpcd/dhcpcd-wait-online.sh \
+		$(TARGET_DIR)/usr/libexec/dhcpcd-wait-online
+endef

package/dhcpcd/dhcpcd.service

@@ -0,0 +1,10 @@
+[Unit]
+Description=A minimalistic network configuration daemon with DHCPv4, rdisc and DHCPv6 support
+Wants=network.target
+Before=network.target
+
+[Service]
+ExecStart=/usr/sbin/dhcpcd -q --nobackground
+
+[Install]
+WantedBy=multi-user.target

package/dhcpcd/online.hook.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+
+# If the `/run/dhcpcd/wait-online` named pipe is present, it means
+# the *dhcpcd-wait-online.service* systemd unit is active.  This unit
+# helps order the activation of other units, especially during bootup.
+# Once the interface is configured and active, we write a message to
+# the pipe to indicate that the boot process can continue and services
+# that require the network to be online can be started.
+
+if $if_configured; then
+    if $if_up; then
+        if [ -p /run/dhcpcd/wait-online ]; then
+            echo online > /run/dhcpcd/wait-online
+        fi
+    fi
+fi

rootfs/overlay/usr/lib/systemd/system-preset/90-default.preset

@@ -1,6 +1,7 @@
 enable chrony-wait.service
 
 enable dhcpcd.service
+enable dhcpcd-wait-online.service
 
 enable dbus.socket
 enable dbus-broker.service

rootfs/overlay/usr/lib/systemd/system/dhcpcd.service.d/before-network-online.conf

@@ -1,2 +0,0 @@
-[Unit]
-Before=network-online.target

rootfs/overlay/usr/lib/systemd/system/dhcpcd.service.d/pidfile.conf

@@ -1,2 +0,0 @@
-[Service]
-PIDFile=/run/dhcpcd/pid