examples/deploy-qemu-vm: show progress when waiting

This improves the user experience on VM startup, which can take a few
seconds.

examples/deploy-qemu-vm.sh

@@ -38,14 +38,13 @@ sshPort=60734
 runVM $tmpDir/vm $vmNumCPUs $vmMemoryMiB $sshPort
 
 vmWaitForSSH
-echo "Waiting until services are ready..."
-c '
-attempts=300
-while ! systemctl is-active clightning &> /dev/null; do
-    ((attempts-- == 0)) && { echo "timeout"; exit 1; }
-    sleep 0.2
-done
-'
+printf "Waiting until services are ready"
+c "
+$(cat qemu-vm/wait-until.sh)
+waitUntil 'systemctl is-active clightning &> /dev/null' 100
+"
+echo
+
 echo
 echo "Bitcoind service:"
 c systemctl status bitcoind

examples/qemu-vm/run-vm.sh

@@ -1,5 +1,7 @@
 qemuDir=$(cd "${BASH_SOURCE[0]%/*}" && pwd)
 
+source "$qemuDir/wait-until.sh"
+
 tmpDir=/tmp/nix-bitcoin-qemu-vm
 mkdir -p $tmpDir
 
@@ -31,7 +33,7 @@ runVM() {
 vmWaitForSSH() {
     echo
     printf "Waiting for SSH connection..."
-    while ! c : 2>/dev/null; do :; done
+    waitUntil "c : 2>/dev/null" 500
     echo
 }
 

examples/qemu-vm/wait-until.sh

@@ -0,0 +1,21 @@
+# Wait until $condition is true, retrying every $intervalMs milliseconds.
+# Print a '.' character every second as a progress indicator.
+waitUntil() {
+    condition=$1
+    intervalMs=$2
+
+    lastDotTime=$(getTimeMs)
+    while ! { t0=$(getTimeMs); eval "$condition"; }; do
+        now=$(getTimeMs)
+        if ((now - lastDotTime >= 1000)); then
+            printf .
+            lastDotTime=$now
+        fi
+        toSleep=$((t0 + intervalMs - now))
+        if ((toSleep > 0)); then
+            sleep $((toSleep / 1000)).$((toSleep % 1000));
+        fi
+    done
+}
+
+getTimeMs() { date +%s%3N; }