Add cli option sync-on-signal to trigger git-sync with a signal. (#664)

Operation:
git-sync --repo https://github.com/kubernetes/kubernetes --sync-on-signal SIGHUP
git-sync --repo https://github.com/kubernetes/kubernetes --sync-on-signal HUP
git-sync --repo https://github.com/kubernetes/kubernetes --sync-on-signal 1

Signals can be sent to docker containers with cmd:
docker kill -signal SIGHUP <Container ID>

cmd/git-sync/main.go

@@ -35,11 +35,13 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"syscall"
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/spf13/pflag"
+	"golang.org/x/sys/unix"
 	"k8s.io/git-sync/pkg/cmd"
 	"k8s.io/git-sync/pkg/hook"
 	"k8s.io/git-sync/pkg/logging"
@@ -77,6 +79,8 @@ var flSyncTimeout = pflag.Duration("sync-timeout", envDuration("GIT_SYNC_SYNC_TI
 	"the total time allowed for one complete sync, must be >= 10ms; --timeout overrides this")
 var flOneTime = pflag.Bool("one-time", envBool("GIT_SYNC_ONE_TIME", false),
 	"exit after the first sync")
+var flSyncOnSignal = pflag.String("sync-on-signal", envString("GIT_SYNC_SYNC_ON_SIGNAL", ""),
+	"sync on receipt of the specified signal (e.g. SIGHUP)")
 var flMaxFailures = pflag.Int("max-failures", envInt("GIT_SYNC_MAX_FAILURES", 0),
 	"the number of consecutive failures allowed before aborting (the first sync must succeed, -1 will retry forever")
 var flChmod = pflag.Int("change-permissions", envInt("GIT_SYNC_PERMISSIONS", 0),
@@ -397,6 +401,24 @@ func main() {
 		handleConfigError(log, true, "ERROR: --period must be at least 10ms")
 	}
 
+	var syncSig syscall.Signal
+	if *flSyncOnSignal != "" {
+		if num, err := strconv.ParseInt(*flSyncOnSignal, 0, 0); err == nil {
+			// sync-on-signal value is a number
+			syncSig = syscall.Signal(num)
+		} else {
+			// sync-on-signal value is a name
+			syncSig = unix.SignalNum(*flSyncOnSignal)
+			if syncSig == 0 {
+				// last resort - maybe they said "HUP", meaning "SIGHUP"
+				syncSig = unix.SignalNum("SIG" + *flSyncOnSignal)
+			}
+		}
+		if syncSig == 0 {
+			handleConfigError(log, true, "ERROR: --sync-on-signal must be a valid signal name or number")
+		}
+	}
+
 	if *flTimeout != 0 {
 		// Back-compat
 		*flSyncTimeout = time.Duration(*flTimeout) * time.Second
@@ -670,6 +692,13 @@ func main() {
 		go exechookRunner.Run(context.Background())
 	}
 
+	// Setup signal notify channel
+	sigChan := make(chan os.Signal, 1)
+	if syncSig != 0 {
+		log.V(2).Info("installing signal handler", "signal", unix.SignalName(syncSig))
+		signal.Notify(sigChan, syncSig)
+	}
+
 	// Craft a function that can be called to refresh credentials when needed.
 	refreshCreds := func(ctx context.Context) error {
 		// These should all be mutually-exclusive configs.
@@ -772,7 +801,16 @@ func main() {
 
 		log.V(1).Info("next sync", "waitTime", flPeriod.String())
 		cancel()
-		time.Sleep(*flPeriod)
+
+		// Sleep until the next sync. If syncSig is set then the sleep may
+		// be interrupted by that signal.
+		t := time.NewTimer(*flPeriod)
+		select {
+		case <-t.C:
+		case <-sigChan:
+			log.V(2).Info("caught signal", "signal", unix.SignalName(syncSig))
+			t.Stop()
+		}
 	}
 }
 
@@ -2139,6 +2177,13 @@ OPTIONS
             The git submodule behavior: one of "recursive", "shallow", or
             "off".  If not specified, this defaults to "recursive".
 
+    --sync-on-signal <string>, $GIT_SYNC_SYNC_ON_SIGNAL
+            Indicates that a sync attempt should occur upon receipt of the 
+            specified signal name (e.g. SIGHUP) or number (e.g. 1). If a sync
+            is already in progress, another sync will be triggered as soon as
+            the current one completes. If not specified, signals will not 
+            trigger syncs.
+
     --sync-timeout <duration>, $GIT_SYNC_SYNC_TIMEOUT
             The total time allowed for one complete sync.  This must be at least
             10ms.  This flag obsoletes --timeout, but if --timeout is specified,

test_e2e.sh

@@ -126,6 +126,14 @@ function docker_kill() {
     docker kill "$1" >/dev/null
 }
 
+function docker_signal() {
+    if [[ -z "$1" || -z "$2" ]]; then
+        echo "usage: $0 <id> <signal>"
+        return 1
+    fi
+    docker kill "--signal=$2" "$1" >/dev/null
+}
+
 # E2E_TAG is the tag used for docker builds.  This is needed because docker
 # tags are system-global, but one might have multiple repos checked out.
 E2E_TAG=$(git rev-parse --show-toplevel | sed 's|/|_|g')
@@ -1111,6 +1119,108 @@ function e2e::sync_slow_git_long_timeout() {
     assert_metric_eq "${METRIC_GOOD_SYNC_COUNT}" 2
 }
 
+##############################################
+# Test sync-on-signal with SIGHUP
+##############################################
+function e2e::sync_on_signal_sighup() {
+     # First sync
+    echo "$FUNCNAME 1" > "$REPO"/file
+    git -C "$REPO" commit -qam "$FUNCNAME 1"
+
+    GIT_SYNC \
+        --period=100s \
+        --sync-on-signal="SIGHUP" \
+        --repo="file://$REPO" \
+        --branch="$MAIN_BRANCH" \
+        --rev=HEAD \
+        --root="$ROOT" \
+        --link="link" \
+        >> "$1" 2>&1 &
+    wait_for_sync 3
+    assert_link_exists "$ROOT"/link
+    assert_file_exists "$ROOT"/link/file
+    assert_file_eq "$ROOT"/link/file "$FUNCNAME 1"
+
+    # Move HEAD forward
+    echo "$FUNCNAME 2" > "$REPO"/file
+    git -C "$REPO" commit -qam "$FUNCNAME 2"
+    # Send signal (note --period is 100s, signal should trigger sync)
+    CTR=$(docker ps --filter label="git-sync-e2e=$RUNID" --format="{{.ID}}")
+    docker_signal "$CTR" SIGHUP
+    wait_for_sync 3
+    assert_link_exists "$ROOT"/link
+    assert_file_exists "$ROOT"/link/file
+    assert_file_eq "$ROOT"/link/file "$FUNCNAME 2"
+}
+
+##############################################
+# Test sync-on-signal with HUP
+##############################################
+function e2e::sync_on_signal_hup() {
+     # First sync
+    echo "$FUNCNAME 1" > "$REPO"/file
+    git -C "$REPO" commit -qam "$FUNCNAME 1"
+
+    GIT_SYNC \
+        --period=100s \
+        --sync-on-signal="HUP" \
+        --repo="file://$REPO" \
+        --branch="$MAIN_BRANCH" \
+        --rev=HEAD \
+        --root="$ROOT" \
+        --link="link" \
+        >> "$1" 2>&1 &
+    wait_for_sync 3
+    assert_link_exists "$ROOT"/link
+    assert_file_exists "$ROOT"/link/file
+    assert_file_eq "$ROOT"/link/file "$FUNCNAME 1"
+
+    # Move HEAD forward
+    echo "$FUNCNAME 2" > "$REPO"/file
+    git -C "$REPO" commit -qam "$FUNCNAME 2"
+    # Send signal (note --period is 100s, signal should trigger sync)
+    CTR=$(docker ps --filter label="git-sync-e2e=$RUNID" --format="{{.ID}}")
+    docker_signal "$CTR" SIGHUP
+    wait_for_sync 3
+    assert_link_exists "$ROOT"/link
+    assert_file_exists "$ROOT"/link/file
+    assert_file_eq "$ROOT"/link/file "$FUNCNAME 2"
+}
+
+##############################################
+# Test sync-on-signal with 1 (SIGHUP)
+##############################################
+function e2e::sync_on_signal_1() {
+     # First sync
+    echo "$FUNCNAME 1" > "$REPO"/file
+    git -C "$REPO" commit -qam "$FUNCNAME 1"
+
+    GIT_SYNC \
+        --period=100s \
+        --sync-on-signal=1 \
+        --repo="file://$REPO" \
+        --branch="$MAIN_BRANCH" \
+        --rev=HEAD \
+        --root="$ROOT" \
+        --link="link" \
+        >> "$1" 2>&1 &
+    wait_for_sync 3
+    assert_link_exists "$ROOT"/link
+    assert_file_exists "$ROOT"/link/file
+    assert_file_eq "$ROOT"/link/file "$FUNCNAME 1"
+
+    # Move HEAD forward
+    echo "$FUNCNAME 2" > "$REPO"/file
+    git -C "$REPO" commit -qam "$FUNCNAME 2"
+    # Send signal (note --period is 100s, signal should trigger sync)
+    CTR=$(docker ps --filter label="git-sync-e2e=$RUNID" --format="{{.ID}}")
+    docker_signal "$CTR" SIGHUP
+    wait_for_sync 3
+    assert_link_exists "$ROOT"/link
+    assert_file_exists "$ROOT"/link/file
+    assert_file_eq "$ROOT"/link/file "$FUNCNAME 2"
+}
+
 ##############################################
 # Test depth syncing
 ##############################################