Automatically set version from git

- Uses last tag on master branch (this is set on 'git flow release finish')
- branch name snapshot otherwise, e.g. 'feature-nio-SNAPSHOT', 'develop-SNAPSHOT'

buildSrc/src/main/groovy/ch/dissem/gradle/GitFlowVersion.groovy

@@ -0,0 +1,53 @@
+package ch.dissem.gradle
+
+import org.gradle.api.Plugin
+import org.gradle.api.Project
+
+/**
+ * Sets the version as follows:
+ * <ul>
+ *     <li>If the branch is 'master', the version is set to the latest tag (which is expected to be set by Git flow)</li>
+ *     <li>Otherwise, the version is set to the branch name, with '-SNAPSHOT' appended</li>
+ * </ul>
+ */
+class GitFlowVersion implements Plugin<Project> {
+    def getBranch(Project project) {
+        def stdout = new ByteArrayOutputStream()
+        project.exec {
+            commandLine 'git', 'rev-parse', '--abbrev-ref', 'HEAD'
+            standardOutput = stdout
+        }
+        return stdout.toString().trim()
+    }
+
+    def getTag(Project project) {
+        def stdout = new ByteArrayOutputStream()
+        project.exec {
+            commandLine 'git', 'describe', '--abbrev=0'
+            standardOutput = stdout
+        }
+        return stdout.toString().trim()
+    }
+
+    def isRelease(Project project) {
+        return "master" == getBranch(project);
+    }
+
+    def getVersion(Project project) {
+        if (project.ext.isRelease) {
+            return getTag(project)
+        } else {
+            return getBranch(project).replaceAll("/", "-") + "-SNAPSHOT"
+        }
+    }
+
+    @Override
+    void apply(Project project) {
+        project.ext.isRelease = isRelease(project)
+        project.version = getVersion(project)
+
+        project.task('version') << {
+            println "Version deduced from git: '${project.version}'"
+        }
+    }
+}

buildSrc/src/main/resources/META-INF/gradle-plugins/gitflow-version.properties

@@ -0,0 +1 @@
+implementation-class=ch.dissem.gradle.GitFlowVersion