Retry hdiutil commands when generating DMG

hdiutil can fail transiently with "Resource busy" while macOS scans the
new image, which is common on CI runners. Retry each hdiutil command up
to 3 times before failing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: fvarrui

.minispec/features/fix-hdiutil-resource-busy.md

@@ -0,0 +1,18 @@
+# DMG generation fails intermittently with "Resource busy"
+
+## Problem
+
+DMG generation sometimes fails with `hdiutil: create failed - Resource busy`. The rest of the build succeeds, but no DMG is produced. Seen in 1 of 7 macOS smoke test jobs (run 37165552012, `macos-14 / ARM64`). Common on CI runners, where the system scans new disk images.
+
+## Cause
+
+`hdiutil` can fail transiently while macOS (Spotlight, XProtect) holds the image or volume. `GenerateDmg` ran each `hdiutil` command once and gave up on the first error.
+
+## Solution
+
+`GenerateDmg` runs every `hdiutil` command (`create`, `attach`, `detach`, `convert`) through a private `hdiutil(...)` helper. It retries up to 3 times, waiting 5 s and then 10 s, and logs a warning for each retry. After the last attempt the original error is thrown as before.
+
+## Verification
+
+- The failure is intermittent, so it can't be reproduced on demand.
+- The macOS smoke tests must stay green. If the error shows up again, the build log must show the `hdiutil ... failed (attempt n of 3), retrying` warning, followed by a DMG.

src/main/java/io/github/fvarrui/javapackager/packagers/GenerateDmg.java

@@ -22,6 +22,8 @@
  */
 public class GenerateDmg extends ArtifactGenerator<MacPackager> {
 
+	private static final int HDIUTIL_ATTEMPTS = 3;
+
 	public GenerateDmg() {
 		super("DMG image");
 	}
@@ -90,16 +92,16 @@ protected File doApply(MacPackager packager) throws Exception {
 		boolean isAarch64 = osArchitecture.equalsIgnoreCase("aarch64");
 		String fileSystem = isAarch64 ? "APFS" : "HFS+";
 		Logger.warn(osArchitecture + " architecture detected. Using " + fileSystem + " filesystem");
-		execute("hdiutil", "create", "-srcfolder", appFolder, "-volname", volumeName, "-ov", "-fs", fileSystem, "-format", "UDRW", tempDmgFile);
+		hdiutil("create", "-srcfolder", appFolder, "-volname", volumeName, "-ov", "-fs", fileSystem, "-format", "UDRW", tempDmgFile);
 
 		if (mountFolder.exists()) {
 			Logger.info("Unmounting volume: " + mountFolder);
-			execute("hdiutil", "detach", mountFolder);
+			hdiutil("detach", mountFolder);
 		}
 
 		// mounts image
 		Logger.info("Mounting image: " + tempDmgFile.getAbsolutePath());
-		String result = execute("hdiutil", "attach", "-readwrite", "-noverify", "-noautoopen", tempDmgFile);
+		String result = hdiutil("attach", "-readwrite", "-noverify", "-noautoopen", tempDmgFile);
 		Optional<String> optDeviceName = Arrays.stream(result.split("\n"))
 								.filter(s -> s.contains(mountFolder.getAbsolutePath()))
 								.map(StringUtils::normalizeSpace)
@@ -146,11 +148,11 @@ protected File doApply(MacPackager packager) throws Exception {
 
 		// unmounts
 		Logger.info("Unmounting volume: " + mountFolder);
-		execute("hdiutil", "detach", mountFolder);
+		hdiutil("detach", mountFolder);
 
 		// compress image
 		Logger.info("Compressing disk image...");
-		execute("hdiutil", "convert", tempDmgFile, "-ov", "-format", "UDZO", "-imagekey", "zlib-level=9", "-o", dmgFile);
+		hdiutil("convert", tempDmgFile, "-ov", "-format", "UDZO", "-imagekey", "zlib-level=9", "-o", dmgFile);
 		tempDmgFile.delete();
 
 		// checks if dmg file was created
@@ -160,5 +162,20 @@ protected File doApply(MacPackager packager) throws Exception {
 
 		return dmgFile;
 	}
+
+	/**
+	 * Runs hdiutil, retrying if it fails (e.g. "Resource busy" while the image is being scanned)
+	 */
+	private String hdiutil(Object... arguments) throws Exception {
+		for (int attempt = 1; ; attempt++) {
+			try {
+				return execute("hdiutil", arguments);
+			} catch (Exception e) {
+				if (attempt == HDIUTIL_ATTEMPTS) throw e;
+				Logger.warn("hdiutil " + arguments[0] + " failed (attempt " + attempt + " of " + HDIUTIL_ATTEMPTS + "), retrying: " + e.getMessage());
+				ThreadUtils.sleep(attempt * 5000L);
+			}
+		}
+	}
 
 }