implemented 04-build-executor.md incl. concurrency amendment: async builds in per-branch worktrees, builds.maxConcurrent, cancellation, live logs; fix .gitignore build/ rule that silently excluded the de.hoennig.gittally.build package (also recovers the step-01 domain files)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
2026-07-07 08:43:28 +02:00
parent ae3ae7aa04
commit b379bc0a6b
31 changed files with 1790 additions and 12 deletions
@@ -0,0 +1,27 @@
+package de.hoennig.gittally.build
+
+import java.security.MessageDigest
+import java.time.Instant
+
+/**
+ * Legacy-compatible artifact key naming: sanitized name plus a 12-char SHA-256 prefix,
+ * so keys are filesystem- and URL-safe but still unique for branch names that
+ * sanitize to the same string.
+ */
+object ArtifactKeys {
+    fun branchKey(branch: String): String = "${sanitize(branch)}-${sha256Prefix(branch)}"
+
+    fun buildKey(
+        branch: String,
+        startedAt: Instant,
+    ): String = "${branchKey(branch)}-${sanitize(startedAt.toString())}-${sha256Prefix("$branch\t$startedAt")}"
+
+    private fun sanitize(value: String): String = value.replace(Regex("[^A-Za-z0-9._-]"), "_")
+
+    private fun sha256Prefix(value: String): String =
+        MessageDigest
+            .getInstance("SHA-256")
+            .digest(value.toByteArray())
+            .joinToString("") { "%02x".format(it) }
+            .take(12)
+}
@@ -0,0 +1,29 @@
+package de.hoennig.gittally.build
+
+import org.slf4j.LoggerFactory
+import org.springframework.stereotype.Component
+import java.nio.file.Path
+
+/**
+ * Takes over the staging directory of a finished build.
+ * The real store (naming, retention, serving) arrives in step 05.
+ */
+interface ArtifactStore {
+    fun persist(
+        build: BuildResult,
+        stagingDir: Path,
+    )
+}
+
+/** Placeholder until step 05: logs and leaves the staging directory untouched. */
+@Component
+class NoOpArtifactStore : ArtifactStore {
+    private val log = LoggerFactory.getLogger(NoOpArtifactStore::class.java)
+
+    override fun persist(
+        build: BuildResult,
+        stagingDir: Path,
+    ) {
+        log.info("artifact store not implemented yet; leaving build output of {} in {}", build.artifactKey, stagingDir)
+    }
+}
@@ -0,0 +1,55 @@
+package de.hoennig.gittally.build
+
+import de.hoennig.gittally.git.GitService
+import org.slf4j.LoggerFactory
+import org.springframework.stereotype.Component
+import java.nio.file.Files
+import java.nio.file.Path
+
+/**
+ * Provides an isolated build workspace per branch so multiple branches can build
+ * concurrently without touching the primary checkout or each other.
+ */
+fun interface BranchWorkspaces {
+    /** A ready-to-build workspace for [branch] with [commit] checked out. */
+    fun prepare(
+        branch: String,
+        commit: String,
+        repoDir: Path,
+    ): Path
+}
+
+/**
+ * One reusable git worktree per branch under `.git/gittally/worktrees/<branchKey>`,
+ * checked out detached at the requested commit. Reuse keeps incremental build
+ * caches; the branch's `cleanCommand` decides how much of them survives.
+ */
+@Component
+class GitWorktreeWorkspaces(
+    private val gitService: GitService,
+) : BranchWorkspaces {
+    private val log = LoggerFactory.getLogger(GitWorktreeWorkspaces::class.java)
+
+    override fun prepare(
+        branch: String,
+        commit: String,
+        repoDir: Path,
+    ): Path {
+        val workspace = repoDir.resolve(WORKTREES_DIR).resolve(ArtifactKeys.branchKey(branch))
+        if (Files.exists(workspace.resolve(".git"))) {
+            gitService.checkoutDetached(commit, workspace)
+        } else {
+            gitService.worktreePrune(repoDir)
+            if (Files.exists(workspace)) {
+                log.warn("removing broken workspace of branch {}: {}", branch, workspace)
+                workspace.toFile().deleteRecursively()
+            }
+            gitService.worktreeAdd(workspace, commit, repoDir)
+        }
+        return workspace
+    }
+
+    companion object {
+        const val WORKTREES_DIR = ".git/gittally/worktrees"
+    }
+}
@@ -0,0 +1,16 @@
+package de.hoennig.gittally.build
+
+import org.springframework.context.annotation.Bean
+import org.springframework.context.annotation.Configuration
+import java.nio.file.Paths
+
+@Configuration
+class BuildConfiguration {
+    /**
+     * Results file relative to the working directory, matching how `ConfigLoader`
+     * resolves the `.git/gittally/` override file. Nothing is touched until the
+     * first build runs, so the bean is safe outside a git repository.
+     */
+    @Bean
+    fun buildResultRepository(): BuildResultRepository = FileBuildResultRepository(Paths.get(".git/gittally/build-results.json"))
+}
@@ -0,0 +1,385 @@
+package de.hoennig.gittally.build
+
+import de.hoennig.gittally.config.BranchConfig
+import de.hoennig.gittally.config.ConfigLoader
+import de.hoennig.gittally.gitea.GiteaClient
+import org.slf4j.LoggerFactory
+import org.springframework.context.ApplicationEventPublisher
+import org.springframework.stereotype.Service
+import java.io.IOException
+import java.io.InputStream
+import java.io.OutputStream
+import java.nio.file.Files
+import java.nio.file.Path
+import java.nio.file.Paths
+import java.nio.file.StandardOpenOption
+import java.time.Duration
+import java.time.Instant
+import java.util.concurrent.ConcurrentHashMap
+import java.util.concurrent.ExecutorService
+import java.util.concurrent.Executors
+import java.util.concurrent.Semaphore
+import java.util.concurrent.atomic.AtomicBoolean
+import kotlin.concurrent.thread
+
+/**
+ * Runs builds asynchronously: up to `builds.maxConcurrent` branches at the same time
+ * (default 1), but never more than one build per branch. Each branch builds in its
+ * own git worktree via [BranchWorkspaces], never in the primary checkout.
+ * Every status transition is persisted via the [BuildResultRepository], published
+ * to Gitea (non-fatal), and emitted as a [BuildStatusChangedEvent].
+ */
+@Service
+class BuildExecutor(
+    private val repository: BuildResultRepository,
+    private val configLoader: ConfigLoader,
+    private val giteaClient: GiteaClient,
+    private val buildRunner: BuildRunner,
+    private val workspaces: BranchWorkspaces,
+    private val artifactStore: ArtifactStore,
+    private val eventPublisher: ApplicationEventPublisher,
+) {
+    private val log = LoggerFactory.getLogger(BuildExecutor::class.java)
+
+    /** One serial worker per branch enforces at most one build per branch. */
+    private val branchWorkers = ConcurrentHashMap<String, ExecutorService>()
+
+    /** All accepted, not yet finished builds by artifact key — queued and running. */
+    private val builds = ConcurrentHashMap<String, ActiveBuild>()
+
+    /** Global concurrency limit; sized from `builds.maxConcurrent` on first use. */
+    @Volatile
+    private var slots: Semaphore? = null
+
+    /** The builds currently executing, newest last (queued builds are PENDING in the repository). */
+    fun currentBuilds(): List<RunningBuild> = builds.values.filter { it.running }.map { it.runningBuild }
+
+    /**
+     * Persists a PENDING result and queues the build; returns immediately.
+     * A build of the same branch waits until the branch's previous build finished;
+     * builds of other branches run concurrently while slots are free.
+     */
+    fun startBuild(
+        branch: String,
+        commit: String,
+        workingDir: Path = Paths.get("."),
+    ): RunningBuild {
+        val startedAt = Instant.now()
+        val stagingDir = Files.createTempDirectory("gittally-build-")
+        val runningBuild =
+            RunningBuild(
+                branch = branch,
+                commit = commit,
+                artifactKey = ArtifactKeys.buildKey(branch, startedAt),
+                startedAt = startedAt,
+                stagingDir = stagingDir,
+                liveLogFile = stagingDir.resolve(LIVE_LOG_FILE),
+            )
+        val pending =
+            BuildResult(
+                branch = branch,
+                commit = commit,
+                status = BuildStatus.PENDING,
+                startedAt = startedAt,
+                duration = null,
+                artifactKey = runningBuild.artifactKey,
+            )
+        repository.append(pending)
+        eventPublisher.publishEvent(BuildStatusChangedEvent(pending))
+        val build = ActiveBuild(runningBuild, workingDir)
+        builds[runningBuild.artifactKey] = build
+        publishGiteaStatus(build, BuildStatus.PENDING, duration = null)
+        branchWorkers
+            .computeIfAbsent(branch) { serialWorker(it) }
+            .submit { execute(build) }
+        return runningBuild
+    }
+
+    /**
+     * Requests cancellation of the build with [artifactKey] and terminates its process
+     * tree (TERM, wait, KILL — like legacy `terminate_process_tree`). A queued build
+     * is recorded as CANCELLED once its worker picks it up.
+     * Returns false when no such build is queued or running.
+     */
+    fun cancel(artifactKey: String): Boolean {
+        val build = builds[artifactKey] ?: return false
+        build.cancelled.set(true)
+        build.process?.let { destroyProcessTree(it) }
+        return true
+    }
+
+    private fun execute(build: ActiveBuild) {
+        var slot: Semaphore? = null
+        var finalStatus = BuildStatus.FAILED
+        try {
+            slot = slotsFor(build.workingDir)
+            slot.acquire()
+            if (build.cancelled.get()) {
+                finalStatus = BuildStatus.CANCELLED
+                return
+            }
+            build.running = true
+            transition(build, BuildStatus.RUNNING, duration = null)
+            val workspace =
+                workspaces.prepare(
+                    branch = build.runningBuild.branch,
+                    commit = build.runningBuild.commit,
+                    repoDir = build.workingDir,
+                )
+            val exitCode = runBuildCommands(build, workspace)
+            finalStatus =
+                when {
+                    build.cancelled.get() -> BuildStatus.CANCELLED
+                    exitCode == 0 -> BuildStatus.SUCCESS
+                    else -> BuildStatus.FAILED
+                }
+        } catch (e: Exception) {
+            finalStatus = if (build.cancelled.get()) BuildStatus.CANCELLED else BuildStatus.FAILED
+            log.error("build of branch {} crashed", build.runningBuild.branch, e)
+            appendToLiveLog(build, "\nbuild crashed: ${e.message}\n")
+        } finally {
+            val duration = Duration.between(build.runningBuild.startedAt, Instant.now())
+            val result = transition(build, finalStatus, duration)
+            try {
+                artifactStore.persist(result, build.runningBuild.stagingDir)
+            } catch (e: Exception) {
+                log.warn("could not persist artifacts of {}: {}", result.artifactKey, e.message)
+            }
+            builds.remove(build.runningBuild.artifactKey)
+            slot?.release()
+        }
+    }
+
+    /**
+     * The semaphore is sized once from the first build's config;
+     * changing `builds.maxConcurrent` requires a restart.
+     */
+    private fun slotsFor(workingDir: Path): Semaphore {
+        slots?.let { return it }
+        synchronized(this) {
+            slots?.let { return it }
+            val maxConcurrent =
+                configLoader
+                    .load(workingDir)
+                    .builds.maxConcurrent
+                    .coerceAtLeast(1)
+            return Semaphore(maxConcurrent, true).also { slots = it }
+        }
+    }
+
+    private fun serialWorker(branch: String): ExecutorService =
+        Executors.newSingleThreadExecutor { runnable ->
+            Thread(runnable, "gittally-build-${ArtifactKeys.branchKey(branch)}").apply { isDaemon = true }
+        }
+
+    private fun runBuildCommands(
+        build: ActiveBuild,
+        workspace: Path,
+    ): Int {
+        val branchConfig = branchConfig(build.runningBuild.branch, build.workingDir)
+        val stagingDir = build.runningBuild.stagingDir
+        Files.newOutputStream(stagingDir.resolve(branchConfig.stdoutLog)).use { stdoutLog ->
+            Files.newOutputStream(stagingDir.resolve(branchConfig.stderrLog)).use { stderrLog ->
+                Files.newOutputStream(build.runningBuild.liveLogFile).use { liveLog ->
+                    writeLiveLogHeader(liveLog, build.runningBuild, branchConfig, workspace)
+                    if (branchConfig.cleanCommand.isNotBlank()) {
+                        val cleanExitCode = runCommand(build, branchConfig.cleanCommand, workspace, stdoutLog, stderrLog, liveLog)
+                        if (cleanExitCode != 0) {
+                            return cleanExitCode
+                        }
+                    }
+                    if (build.cancelled.get()) {
+                        return CANCELLED_EXIT_CODE
+                    }
+                    return runCommand(build, branchConfig.buildCommand, workspace, stdoutLog, stderrLog, liveLog)
+                }
+            }
+        }
+    }
+
+    private fun runCommand(
+        build: ActiveBuild,
+        command: String,
+        workspace: Path,
+        stdoutLog: OutputStream,
+        stderrLog: OutputStream,
+        liveLog: OutputStream,
+    ): Int {
+        val process = buildRunner.start(command, workspace, mapOf("branch" to build.runningBuild.branch))
+        build.process = process
+        if (build.cancelled.get()) {
+            destroyProcessTree(process)
+        }
+        val stdoutPump = pump(process.inputStream, stdoutLog, liveLog)
+        val stderrPump = pump(process.errorStream, stderrLog, liveLog)
+        try {
+            return process.waitFor()
+        } finally {
+            build.process = null
+            stdoutPump.join(PUMP_DRAIN_TIMEOUT_MILLIS)
+            stderrPump.join(PUMP_DRAIN_TIMEOUT_MILLIS)
+        }
+    }
+
+    /** Copies process output to both sinks as it arrives, flushing so the live log grows during the build. */
+    private fun pump(
+        input: InputStream,
+        vararg sinks: OutputStream,
+    ): Thread =
+        thread(isDaemon = true, name = "gittally-build-log") {
+            val buffer = ByteArray(8192)
+            try {
+                while (true) {
+                    val length = input.read(buffer)
+                    if (length < 0) {
+                        break
+                    }
+                    for (sink in sinks) {
+                        synchronized(sink) {
+                            sink.write(buffer, 0, length)
+                            sink.flush()
+                        }
+                    }
+                }
+            } catch (_: IOException) {
+                // the stream closes when the process dies; nothing left to copy
+            }
+        }
+
+    /** TERM to all descendants and the root, wait up to 2s, then KILL survivors. */
+    private fun destroyProcessTree(process: Process) {
+        val root = process.toHandle()
+        val tree = root.descendants().toList() + root
+        tree.forEach { it.destroy() }
+        val deadline = System.nanoTime() + Duration.ofSeconds(2).toNanos()
+        while (tree.any { it.isAlive } && System.nanoTime() < deadline) {
+            Thread.sleep(50)
+        }
+        tree.filter { it.isAlive }.forEach { it.destroyForcibly() }
+    }
+
+    private fun transition(
+        build: ActiveBuild,
+        status: BuildStatus,
+        duration: Duration?,
+    ): BuildResult {
+        val runningBuild = build.runningBuild
+        val updated =
+            repository.updateByArtifactKey(runningBuild.artifactKey) {
+                it.copy(status = status, duration = duration ?: it.duration)
+            } ?: BuildResult(
+                branch = runningBuild.branch,
+                commit = runningBuild.commit,
+                status = status,
+                startedAt = runningBuild.startedAt,
+                duration = duration,
+                artifactKey = runningBuild.artifactKey,
+            ).also { repository.append(it) }
+        eventPublisher.publishEvent(BuildStatusChangedEvent(updated))
+        publishGiteaStatus(build, status, duration)
+        return updated
+    }
+
+    private fun publishGiteaStatus(
+        build: ActiveBuild,
+        status: BuildStatus,
+        duration: Duration?,
+    ) {
+        try {
+            giteaClient.publishStatus(
+                sha = build.runningBuild.commit,
+                status = status,
+                description = description(status, duration),
+                targetUrl = null,
+                workingDir = build.workingDir,
+            )
+        } catch (e: Exception) {
+            log.warn("could not publish Gitea status {} for {}: {}", status, build.runningBuild.commit, e.message)
+        }
+    }
+
+    private fun description(
+        status: BuildStatus,
+        duration: Duration?,
+    ): String {
+        val after = duration?.let { " after ${formatDuration(it)}" } ?: ""
+        return when (status) {
+            BuildStatus.PENDING -> "build queued"
+            BuildStatus.RUNNING -> "build running"
+            BuildStatus.SUCCESS -> "build succeeded$after"
+            BuildStatus.FAILED -> "build failed$after"
+            BuildStatus.INTERRUPTED -> "build interrupted$after"
+            BuildStatus.CANCELLED -> "build cancelled$after"
+        }
+    }
+
+    private fun formatDuration(duration: Duration): String = "%02d:%02d".format(duration.toMinutes(), duration.toSecondsPart())
+
+    private fun writeLiveLogHeader(
+        liveLog: OutputStream,
+        runningBuild: RunningBuild,
+        branchConfig: BranchConfig,
+        workspace: Path,
+    ) {
+        val header =
+            buildString {
+                appendLine("building branch: ${runningBuild.branch}")
+                appendLine("commit: ${runningBuild.commit}")
+                appendLine("started: ${runningBuild.startedAt}")
+                appendLine("workspace: $workspace")
+                appendLine("build command: ${branchConfig.buildCommand}")
+                if (branchConfig.cleanCommand.isNotBlank()) {
+                    appendLine("clean command: ${branchConfig.cleanCommand}")
+                }
+                appendLine()
+            }
+        synchronized(liveLog) {
+            liveLog.write(header.toByteArray())
+            liveLog.flush()
+        }
+    }
+
+    private fun appendToLiveLog(
+        build: ActiveBuild,
+        message: String,
+    ) {
+        try {
+            Files.writeString(
+                build.runningBuild.liveLogFile,
+                message,
+                StandardOpenOption.CREATE,
+                StandardOpenOption.APPEND,
+            )
+        } catch (e: IOException) {
+            log.warn("could not append to live log of {}: {}", build.runningBuild.artifactKey, e.message)
+        }
+    }
+
+    private fun branchConfig(
+        branch: String,
+        workingDir: Path,
+    ): BranchConfig {
+        val branches = configLoader.load(workingDir).branches
+        return branches[branch] ?: branches["default"] ?: BranchConfig()
+    }
+
+    private class ActiveBuild(
+        val runningBuild: RunningBuild,
+        val workingDir: Path,
+    ) {
+        val cancelled = AtomicBoolean(false)
+
+        @Volatile
+        var running = false
+
+        @Volatile
+        var process: Process? = null
+    }
+
+    companion object {
+        /** Name of the combined live log inside the staging directory. */
+        const val LIVE_LOG_FILE = "build.log"
+        private const val CANCELLED_EXIT_CODE = 130
+        private const val PUMP_DRAIN_TIMEOUT_MILLIS = 10_000L
+    }
+}
@@ -0,0 +1,13 @@
+package de.hoennig.gittally.build
+
+import java.time.Duration
+import java.time.Instant
+
+data class BuildResult(
+    val branch: String,
+    val commit: String,
+    val status: BuildStatus,
+    val startedAt: Instant,
+    val duration: Duration? = null,
+    val artifactKey: String,
+)
@@ -0,0 +1,43 @@
+package de.hoennig.gittally.build
+
+interface BuildResultRepository {
+    fun append(result: BuildResult)
+
+    /** Applies [transform] to the newest entry of [branch]; returns null if the branch has no entries. */
+    fun updateLatest(
+        branch: String,
+        transform: (BuildResult) -> BuildResult,
+    ): BuildResult?
+
+    /** Applies [transform] to the entry with [artifactKey]; returns null if no entry matches. */
+    fun updateByArtifactKey(
+        artifactKey: String,
+        transform: (BuildResult) -> BuildResult,
+    ): BuildResult?
+
+    fun latestFor(branch: String): BuildResult?
+
+    /** The newest entry of each branch, newest first. */
+    fun latestPerBranch(): List<BuildResult>
+
+    /** All entries, newest first. */
+    fun history(): List<BuildResult>
+
+    /** Removes all entries with the given artifact key; returns true if anything was removed. */
+    fun delete(artifactKey: String): Boolean
+
+    /**
+     * Startup recovery: RUNNING entries and PENDING entries superseded by a newer entry
+     * of the same branch become INTERRUPTED. Returns the changed entries.
+     */
+    fun markStaleRunningAsInterrupted(): List<BuildResult>
+
+    /**
+     * Keeps the newest [retentionPerBranch] entries per branch and drops entries of branches
+     * not contained in [originBranches]. Returns the removed entries.
+     */
+    fun prune(
+        originBranches: Collection<String>,
+        retentionPerBranch: Int,
+    ): List<BuildResult>
+}
@@ -0,0 +1,31 @@
+package de.hoennig.gittally.build
+
+import org.springframework.stereotype.Component
+import java.nio.file.Path
+
+/**
+ * Starts a single build or clean command and hands the [Process] back to the caller,
+ * which owns log streaming and process-tree termination.
+ * Native shell execution for now; a Docker runner can plug in later (step 11).
+ */
+interface BuildRunner {
+    fun start(
+        command: String,
+        workingDir: Path,
+        environment: Map<String, String>,
+    ): Process
+}
+
+@Component
+class ProcessBuildRunner : BuildRunner {
+    override fun start(
+        command: String,
+        workingDir: Path,
+        environment: Map<String, String>,
+    ): Process {
+        val processBuilder = ProcessBuilder("bash", "-c", command)
+        processBuilder.directory(workingDir.toFile())
+        processBuilder.environment().putAll(environment)
+        return processBuilder.start()
+    }
+}
@@ -0,0 +1,17 @@
+package de.hoennig.gittally.build
+
+enum class BuildStatus {
+    PENDING,
+    RUNNING,
+    SUCCESS,
+    FAILED,
+    INTERRUPTED,
+    CANCELLED,
+    ;
+
+    val isTerminal: Boolean
+        get() = this != PENDING && this != RUNNING
+
+    val isRestartable: Boolean
+        get() = this == PENDING || this == RUNNING || this == INTERRUPTED
+}
@@ -0,0 +1,185 @@
+package de.hoennig.gittally.build
+
+import com.fasterxml.jackson.databind.DeserializationFeature
+import com.fasterxml.jackson.databind.ObjectMapper
+import com.fasterxml.jackson.databind.SerializationFeature
+import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule
+import com.fasterxml.jackson.module.kotlin.readValue
+import com.fasterxml.jackson.module.kotlin.registerKotlinModule
+import org.slf4j.LoggerFactory
+import java.nio.file.Files
+import java.nio.file.Path
+import java.nio.file.StandardCopyOption
+
+/**
+ * Stores build results as a JSON file, e.g. `.git/gittally/build-results.json`.
+ * Writes are atomic (temp file + atomic move) so readers never see partial content.
+ */
+class FileBuildResultRepository(
+    private val file: Path,
+) : BuildResultRepository {
+    private val log = LoggerFactory.getLogger(FileBuildResultRepository::class.java)
+    private val lock = Any()
+
+    private val json =
+        ObjectMapper()
+            .registerKotlinModule()
+            .registerModule(JavaTimeModule())
+            .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
+            .configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false)
+            .configure(SerializationFeature.WRITE_DURATIONS_AS_TIMESTAMPS, false)
+            .configure(SerializationFeature.INDENT_OUTPUT, true)
+
+    override fun append(result: BuildResult) {
+        synchronized(lock) {
+            save(load() + result)
+        }
+    }
+
+    override fun updateLatest(
+        branch: String,
+        transform: (BuildResult) -> BuildResult,
+    ): BuildResult? {
+        synchronized(lock) {
+            val results = load()
+            val index = indexOfLatest(results, branch) ?: return null
+            val updated = transform(results[index])
+            save(results.toMutableList().also { it[index] = updated })
+            return updated
+        }
+    }
+
+    override fun updateByArtifactKey(
+        artifactKey: String,
+        transform: (BuildResult) -> BuildResult,
+    ): BuildResult? {
+        synchronized(lock) {
+            val results = load()
+            val index = results.indexOfLast { it.artifactKey == artifactKey }
+            if (index < 0) {
+                return null
+            }
+            val updated = transform(results[index])
+            save(results.toMutableList().also { it[index] = updated })
+            return updated
+        }
+    }
+
+    override fun latestFor(branch: String): BuildResult? {
+        val results = load()
+        return indexOfLatest(results, branch)?.let { results[it] }
+    }
+
+    override fun latestPerBranch(): List<BuildResult> =
+        load()
+            .groupBy { it.branch }
+            .values
+            .map { entries -> entries.reduce(::laterOf) }
+            .sortedByDescending { it.startedAt }
+
+    override fun history(): List<BuildResult> = load().sortedByDescending { it.startedAt }
+
+    override fun delete(artifactKey: String): Boolean {
+        synchronized(lock) {
+            val results = load()
+            val remaining = results.filterNot { it.artifactKey == artifactKey }
+            if (remaining.size == results.size) {
+                return false
+            }
+            save(remaining)
+            return true
+        }
+    }
+
+    override fun markStaleRunningAsInterrupted(): List<BuildResult> {
+        synchronized(lock) {
+            val results = load()
+            val changed = mutableListOf<BuildResult>()
+            val updated =
+                results.map { result ->
+                    val superseded =
+                        results.any { it.branch == result.branch && it.startedAt.isAfter(result.startedAt) }
+                    if (result.status == BuildStatus.RUNNING ||
+                        (result.status == BuildStatus.PENDING && superseded)
+                    ) {
+                        result.copy(status = BuildStatus.INTERRUPTED).also { changed += it }
+                    } else {
+                        result
+                    }
+                }
+            if (changed.isNotEmpty()) {
+                save(updated)
+            }
+            return changed
+        }
+    }
+
+    override fun prune(
+        originBranches: Collection<String>,
+        retentionPerBranch: Int,
+    ): List<BuildResult> {
+        synchronized(lock) {
+            val results = load()
+            val originBranchSet = originBranches.toSet()
+            val kept =
+                results
+                    .filter { it.branch in originBranchSet }
+                    .groupBy { it.branch }
+                    .values
+                    .flatMap { entries ->
+                        entries
+                            .sortedByDescending { it.startedAt }
+                            .take(retentionPerBranch.coerceAtLeast(0))
+                    }.toSet()
+            val removed = results.filterNot { it in kept }
+            if (removed.isNotEmpty()) {
+                save(results.filter { it in kept })
+            }
+            return removed
+        }
+    }
+
+    /** The index of the newest entry of [branch]; on equal timestamps the later appended entry wins. */
+    private fun indexOfLatest(
+        results: List<BuildResult>,
+        branch: String,
+    ): Int? {
+        var latest: Int? = null
+        results.forEachIndexed { index, result ->
+            if (result.branch == branch &&
+                (latest == null || !result.startedAt.isBefore(results[latest].startedAt))
+            ) {
+                latest = index
+            }
+        }
+        return latest
+    }
+
+    private fun laterOf(
+        first: BuildResult,
+        second: BuildResult,
+    ): BuildResult = if (second.startedAt.isBefore(first.startedAt)) first else second
+
+    private fun load(): List<BuildResult> {
+        if (!Files.exists(file)) {
+            return emptyList()
+        }
+        return try {
+            json.readValue<List<BuildResult>>(file.toFile())
+        } catch (e: Exception) {
+            log.warn("ignoring unreadable build results file {}: {}", file, e.message)
+            emptyList()
+        }
+    }
+
+    private fun save(results: List<BuildResult>) {
+        Files.createDirectories(file.parent)
+        val tempFile = Files.createTempFile(file.parent, file.fileName.toString(), ".tmp")
+        try {
+            json.writeValue(tempFile.toFile(), results)
+            Files.move(tempFile, file, StandardCopyOption.ATOMIC_MOVE, StandardCopyOption.REPLACE_EXISTING)
+        } finally {
+            Files.deleteIfExists(tempFile)
+        }
+    }
+}
@@ -0,0 +1,21 @@
+package de.hoennig.gittally.build
+
+import java.nio.file.Path
+import java.time.Instant
+
+/** Handle to a build accepted by the [BuildExecutor]; log paths become valid once the build runs. */
+data class RunningBuild(
+    val branch: String,
+    val commit: String,
+    val artifactKey: String,
+    val startedAt: Instant,
+    /** Working directory for build output; handed to the [ArtifactStore] when the build ends. */
+    val stagingDir: Path,
+    /** Combined stdout+stderr log, written live while the build runs. */
+    val liveLogFile: Path,
+)
+
+/** Published via Spring's `ApplicationEventPublisher` on every persisted status transition. */
+data class BuildStatusChangedEvent(
+    val result: BuildResult,
+)
@@ -112,6 +112,11 @@ class InitCommand(
              repo: ${detected.repo}                      # repository name
              statusContext: GitTally            # label shown on Gitea commit status checks (default: GitTally)

+            # Build execution.
+            builds:
+              # how many branches may build at the same time (at most one build per branch regardless)
+              maxConcurrent: 1
+
            # Build artifact retention.
            artifacts:
              # number of builds to keep per branch
@@ -4,6 +4,7 @@ data class GitTallyConfig(
    val server: ServerConfig = ServerConfig(),
    val git: GitConfig = GitConfig(),
    val gitea: GiteaConfig = GiteaConfig(),
+    val builds: BuildsConfig = BuildsConfig(),
    val artifacts: ArtifactsConfig = ArtifactsConfig(),
    val watcher: WatcherConfig = WatcherConfig(),
    val branches: Map<String, BranchConfig> = mapOf("default" to BranchConfig()),
@@ -25,6 +26,11 @@ data class GiteaConfig(
    val statusContext: String = "GitTally",
 )

+data class BuildsConfig(
+    /** How many branches may build at the same time; at most one build per branch regardless. */
+    val maxConcurrent: Int = 1,
+)
+
 data class ArtifactsConfig(
    val retentionPerBranch: Int = 3,
 )
@@ -157,6 +157,28 @@ class GitService(
            .stdout
            .trim()

+    /** Creates a worktree at [path] with [commit] checked out as a detached HEAD; [path] must not exist yet. */
+    fun worktreeAdd(
+        path: Path,
+        commit: String,
+        workingDir: Path = Paths.get("."),
+    ) {
+        runner.runOrThrow(listOf("git", "worktree", "add", "--detach", path.toString(), commit), workingDir)
+    }
+
+    /** Removes registrations of worktrees whose directories no longer exist. */
+    fun worktreePrune(workingDir: Path = Paths.get(".")) {
+        runner.runOrThrow(listOf("git", "worktree", "prune"), workingDir)
+    }
+
+    /** Checks out [commit] as a detached HEAD, discarding local modifications to tracked files. */
+    fun checkoutDetached(
+        commit: String,
+        workingDir: Path = Paths.get("."),
+    ) {
+        runner.runOrThrow(listOf("git", "checkout", "--force", "--detach", commit), workingDir)
+    }
+
    private fun refExists(
        ref: String,
        workingDir: Path,