[Build] Rename library files if native sources of platform are unchanged

Rust rename the SWT native binary files for unchanged platforms to the
new version and only build those whose native sources really changed.

Avoiding a rebuild of effectively unchanged binaries has multiple
advantages, if only a sub-set of all supported platforms is build:
- reduced build times
- reduced occupation of precious native agents in Jenkins
- increased stability of the build because if a native build-agent
for a skipped platforms is unavailable the build isn't blocked.
- reduced future growth of the git large-file storage occupation as
renaming a binary file has basically zero-cost, while a modified binary
(even if just slightly) will add the entire file again.

Author: HannesWell

Jenkinsfile

@@ -42,8 +42,8 @@ def runOnNativeBuildAgent(String platform, Closure body) {
 	}
 }
 
-/** Returns the download URL of the JDK against whoose C headers (in the 'include/' folder) and native libraries the SWT natives are compiled.*/
-def getNativeJdkUrl(String os, String arch){ // To update the used JDK version update the URL template below
+/** Returns the download URL of the JDK against whose C headers (in the 'include/' folder) and native libraries the SWT natives are compiled.*/
+def getNativeJdkUrl(String os, String arch) { // To update the used JDK version update the URL template below
 	if('win32'.equals(os) && 'aarch64'.equals(arch)) {
 		// Temporary workaround until there are official Temurin GA releases for Windows on ARM that can be consumed through JustJ
 		dir("${WORKSPACE}/repackage-win32.aarch64-jdk") {
@@ -82,7 +82,7 @@ def getSWTVersions() { // must be called from the repository root
 	return props
 }
 
-boolean NATIVES_CHANGED = false
+def Set NATIVES_CHANGED = []
 
 pipeline {
 	options {
@@ -103,7 +103,9 @@ pipeline {
 		PR_VALIDATION_BUILD = "true"
 	}
 	parameters {
-		booleanParam(name: 'forceNativeBuilds', defaultValue: false, description: 'Forces to run the native builds of swt\'s binaries. Will push the built binaries to the master branch, unless \'skipCommit\' is set. Useful in debugging.')
+		booleanParam(name: 'forceNativeBuilds-cocoa', defaultValue: false, description: 'Enforce a re-build of SWT\'s native binaries for Mac OS X. Will push the built binaries to the master branch, unless \'skipCommit\' is set.')
+		booleanParam(name: 'forceNativeBuilds-gtk', defaultValue: false, description: 'Enforce a re-build of SWT\'s native binaries for Linux. Will push the built binaries to the master branch, unless \'skipCommit\' is set.')
+		booleanParam(name: 'forceNativeBuilds-win32', defaultValue: false, description: 'Enforce a re-build of SWT\'s native binaries for Windows. Will push the built binaries to the master branch, unless \'skipCommit\' is set.')
 		booleanParam(name: 'skipCommit', defaultValue: false, description: 'Stops committing to swt and swt binaries repo at the end. Useful in debugging.')
 	}
 	stages {
@@ -141,9 +143,10 @@ pipeline {
 						git config --global user.name 'Eclipse Releng Bot'
 					'''
 					script {
+						def allWS = ['cocoa', 'gtk', 'win32']
+						def libraryFilePattern = [ 'cocoa' : 'libswt-*.jnilib', 'gtk' : 'libswt-*.so', 'win32' : 'swt-*.dll' ]
 						def swtTag = getLatestGitTag()
 						echo "Current tag=${swtTag}."
-						boolean nativesChanged = false
 						dir('bundles/org.eclipse.swt') {
 							// Verify preprocessing is completed
 							sh '''
@@ -154,28 +157,42 @@ pipeline {
 							'''
 							def sourceFoldersProps = readProperties(file: 'nativeSourceFolders.properties')
 							def sources = sourceFoldersProps.collectEntries{ k, src -> [ k, src.split(',').collect{ f -> '\'' + f + '\''}.join(' ') ] }
-							def diff = sh(script: "git diff HEAD ${swtTag} ${sources.values().join(' ')}", returnStdout: true)
-							nativesChanged = !diff.strip().isEmpty()
-							echo "Natives changed since ${swtTag}: ${nativesChanged}"
+							for(ws in allWS) {
+								def diff = sh(script: "git diff HEAD ${swtTag} ${sources.src_common} ${sources['src_' + ws]}", returnStdout: true)
+								if (!diff.strip().isEmpty()) {
+									NATIVES_CHANGED += ws
+								}
+							}
+							echo "Natives changed since ${swtTag}: ${NATIVES_CHANGED.isEmpty() ? 'None': NATIVES_CHANGED}"
 						}
-						if (nativesChanged || params.forceNativeBuilds) {
-							NATIVES_CHANGED = true
+						NATIVES_CHANGED += allWS.findAll{ ws -> params[ 'forceNativeBuilds-' + ws] }
+						if (!NATIVES_CHANGED.isEmpty()) {
 							def versions = getSWTVersions()
 								sh """
-									# Delete native binaries to be replaced by subsequent binaries build
-									rm binaries/org.eclipse.swt.gtk.*/libswt-*.so
-									rm binaries/org.eclipse.swt.win32.*/swt-*.dll
-									rm binaries/org.eclipse.swt.cocoa.*/libswt-*.jnilib
-									
 									echo "Incrementing version from ${versions.swt_version} to ${versions.new_version}"
 									sed -i -e "s/REVISION = ${versions.rev}/REVISION = ${versions.new_rev}/g" \
 										'bundles/org.eclipse.swt/Eclipse SWT PI/common/org/eclipse/swt/internal/Library.java'
 									sed -i -e "s/rev=${versions.rev}/rev=${versions.new_rev}/g" \
 										'bundles/org.eclipse.swt/Eclipse SWT/common/library/make_common.mak'
 								"""
+							for(ws in allWS) {
+								if (NATIVES_CHANGED.contains(ws)) {
+								sh """
+									# Delete native binaries to be replaced by subsequent binaries build
+									rm binaries/org.eclipse.swt.${ws}.*/${libraryFilePattern[ws]}
+								"""
+								} else {
+									// Just rename existing native library files to new revision instead of rebuilding them
+									sh """
+										for f in binaries/org.eclipse.swt.${ws}.*/${libraryFilePattern[ws]}; do
+											mv "\$f" "\$(echo \$f | sed 's/-${ws}-${versions.swt_version}/-${ws}-${versions.new_version}/g')"
+										done
+									"""
+								}
+							}
 							// Collect SWT-native's sources
 							dir('bundles/org.eclipse.swt') {
-								for (ws in ['cocoa', 'gtk', 'win32']) {
+								for (ws in NATIVES_CHANGED) {
 									sh "java -Dws=${ws} build-scripts/CollectSources.java -nativeSources 'target/natives-build-temp/${ws}'"
 									dir("target/natives-build-temp/${ws}") {
 										stash(name:"swt.binaries.sources.${ws}")
@@ -207,8 +224,14 @@ pipeline {
 						'win32.win32.x86_64'
 					}
 				}
+				when {
+					expression {
+						def (ws, os, arch) = env.PLATFORM.split('\\.')
+						return NATIVES_CHANGED.any{ w -> ws.startsWith(w)} // handle also dedicated gtk4 build
+					}
+				}
 				stages {
-					stage('Build SWT-natives') {
+					stage('Initiate native build') {
 						options {
 							timeout(time: 120, unit: 'MINUTES') // Some build agents are rare and it might take awhile until they are available.
 						}
@@ -229,7 +252,7 @@ pipeline {
 										unstash "jdk.resources.${os}.${arch}"
 									}
 									withEnv(['MODEL=' + arch, "OUTPUT_DIR=${WORKSPACE}/libs", "SWT_JAVA_HOME=${WORKSPACE}/jdk.resources"]) {
-										if (isUnix()){
+										if (isUnix()) {
 											sh '''#!/bin/bash -x
 												mkdir libs
 												if [[ ${PLATFORM} == gtk.linux.* ]]; then