GEODE-6395: Use md5 as Spotless cache value. (apache#3186)

* This acts as a safeguard against developers forgetting to bump the input to 'bumpThisNumberIfACustomStepChanges'

gradle/spotless.gradle

@@ -15,6 +15,16 @@
  * limitations under the License.
  */
 
+// When a custom step changes, we need to bump the value passed to the method
+//   bumpThisNumberIfACustomStepChanges
+// This has been historicaly easy to forget, however, and can cause failures in some rare cases.
+// To safeguard against this, we instead use the (partial) md5 of this file as that method input.
+def thisFile = file("${scriptDir}/spotless.gradle")
+def thisFileMd5 = thisFile.text.md5() as String
+def thisFileMd5Piece = thisFileMd5.substring(0, 8)
+def thisFileIntegerHash = Integer.parseUnsignedInt(thisFileMd5Piece, 16)
+logger.debug("Using partial md5 (${thisFileIntegerHash}) of file ${thisFile} as spotless bump.")
+
 subprojects {
   apply plugin: "com.diffplug.gradle.spotless"
   spotless {
@@ -28,7 +38,7 @@ subprojects {
 
       // As the method name suggests, bump this number if any of the below "custom" rules change.
       // Spotless will not run on unchanged files unless this number changes.
-      bumpThisNumberIfACustomStepChanges(3)
+      bumpThisNumberIfACustomStepChanges(thisFileIntegerHash)
 
       removeUnusedImports()
 
@@ -115,7 +125,7 @@ subprojects {
 
       // As the method name suggests, bump this number if any of the below "custom" rules change.
       // Spotless will not run on unchanged files unless this number changes.
-      bumpThisNumberIfACustomStepChanges(0)
+      bumpThisNumberIfACustomStepChanges(thisFileIntegerHash)
 
       custom 'Use single-quote in project directives.', {
         it.replaceAll(/project\(":([^"]*)"\)/, 'project(\':$1\')')