Allow 5 seconds for child to handle SIGINT before destroying it

Previously, when RunProcess handled a SIGINT it would immediately
attempt to destroy the process that it had run. This created a race
condition between the SIGINT being handled by the child process
and RunProcess destroying the child. The exact behavior of destroy
is implementation dependent and it may result in forcible termination
of the process where shutdown hooks are not called. This is what
happens on Windows. The exit code in such a case is 1 which prevents
anything from waiting for the process to complete from detecting
that it ended as a result of a SIGINT, leaving it with no choice but
to report an error. This is what happens with mvn spring-boot:run
with a forked process on Windows and results in the build failing.

This commit updates RunProcess to allow the child process to handle
the SIGINT itself, waiting for up to five seconds for that to happen
before the process is then destroyed. Given this time, the child
process exits with 130 which RunMojo already handles correctly as
indicating that the process died due to SIGINT and the build completes
with success as a result.

Fixes gh-18936

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/RunProcess.java

@@ -113,13 +113,33 @@ public Process getRunningProcess() {
 	 * @return {@code true} if stopped
 	 */
 	public boolean handleSigInt() {
-		// if the process has just ended, probably due to this SIGINT, consider handled.
-		if (hasJustEnded()) {
+		if (allowChildToHandleSigInt()) {
 			return true;
 		}
 		return doKill();
 	}
 
+	private boolean allowChildToHandleSigInt() {
+		Process process = this.process;
+		if (process == null) {
+			return true;
+		}
+		long end = System.currentTimeMillis() + 5000;
+		while (System.currentTimeMillis() < end) {
+			if (!process.isAlive()) {
+				return true;
+			}
+			try {
+				Thread.sleep(500);
+			}
+			catch (InterruptedException ex) {
+				Thread.currentThread().interrupt();
+				return false;
+			}
+		}
+		return false;
+	}
+
 	/**
 	 * Kill this process.
 	 */