Add tagRelease task to root

This task automates a series of checks and is used to prepare a release.

It checks if the release branches have been merged into, and creates
all the necessary tags. Main is tagged as requested, and release
branches get an appended "-xxx" which is their IJP version number.

build.gradle.kts

@@ -1,3 +1,5 @@
+import java.io.ByteArrayOutputStream
+
 plugins {
     alias(libs.plugins.composeDesktop) apply false
     `jewel-linting`
@@ -23,4 +25,113 @@ tasks {
     }
 
     register("check") { dependsOn(mergeSarifReports) }
+
+    register("tagRelease") {
+        dependsOn("check")
+
+        description = "Tags main branch and releases branches with provided tag name"
+        group = "release"
+
+        doFirst {
+            val tagName = (project.property("tagName") as String?)
+                ?.takeIf { it.isNotBlank() }
+                ?: throw GradleException("Please provide a tag name using -PtagName=[value]")
+
+            val stdOut = ByteArrayOutputStream()
+
+            // Check we're on the main branch
+            logger.info("Checking current branch is main...")
+            exec {
+                commandLine = listOf("git", "rev-parse", "--abbrev-ref", "HEAD")
+                standardOutput = stdOut
+            }.assertNormalExitValue()
+
+            val currentBranch = stdOut.use { it.toString() }.trim()
+            if (currentBranch != "main") {
+                throw GradleException("This task must only be run on the main branch")
+            }
+
+            // Check tag doesn't already exist
+            logger.info("Checking current branch is main...")
+            exec {
+                commandLine = listOf("git", "tag")
+                standardOutput = stdOut
+            }.assertNormalExitValue()
+
+            if (stdOut.toString().trim().lines().any { it == tagName }) {
+                throw GradleException("The tag $tagName already exists!")
+            }
+
+            // Get the current HEAD hash
+            logger.info("Getting HEAD hash...")
+            stdOut.reset()
+            exec {
+                commandLine = listOf("git", "rev-parse", "HEAD")
+                standardOutput = stdOut
+            }.assertNormalExitValue()
+
+            val currentHead = stdOut.use { it.toString() }.trim()
+
+            // Enumerate the release branches
+            logger.info("Enumerating release branches...")
+            stdOut.reset()
+            exec {
+                commandLine = listOf("git", "branch")
+                standardOutput = stdOut
+            }.assertNormalExitValue()
+
+            val releaseBranches = stdOut.use { it.toString() }
+                .lines()
+                .filter { it.trim().startsWith("releases/") }
+                .map { it.trim().removePrefix("releases/") }
+
+            logger.lifecycle("Release branches: ${releaseBranches.joinToString { "releases/$it" }}")
+
+            // Check all release branches have gotten the latest from main
+            logger.info("Validating release branches...")
+            for (branch in releaseBranches) {
+                stdOut.reset()
+                exec {
+                    commandLine = listOf("git", "merge-base", "main", "releases/$branch")
+                    standardOutput = stdOut
+                }.assertNormalExitValue()
+
+                val mergeBase = stdOut.use { it.toString() }.trim()
+                if (mergeBase != currentHead) {
+                    throw GradleException("Branch releases/$branch is not up-to-date with main!")
+                }
+            }
+
+            // Tag main branch
+            logger.lifecycle("Tagging head of main branch as $tagName...")
+            exec {
+                commandLine = listOf("git", "tag", tagName)
+            }.assertNormalExitValue()
+
+            // Tag release branches
+            for (branch in releaseBranches) {
+                val branchTagName = "$tagName-$branch"
+                logger.lifecycle("Tagging head of branch releases/$branch as $branchTagName...")
+                stdOut.reset()
+
+                logger.info("Getting branch head commit...")
+                exec {
+                    commandLine = listOf("git", "rev-parse", "releases/$branch")
+                    standardOutput = stdOut
+                }.assertNormalExitValue()
+
+                val branchHead = stdOut.use { it.toString() }.trim()
+                logger.info("HEAD of releases/$branch is $branchHead")
+
+                logger.info("Tagging commit ${branchHead.take(7)} as $branchTagName")
+                stdOut.reset()
+                exec {
+                    commandLine = listOf("git", "tag", branchTagName, branchHead)
+                    standardOutput = stdOut
+                }.assertNormalExitValue()
+            }
+
+            logger.info("All done!")
+        }
+    }
 }