Configuration files declare the GitTally they are written for

Until now a version that renames or drops a key did not fail — it silently
ignored what it no longer understood, and the effect surfaced as a build
doing the wrong thing. Both directions of that happened within two days:
a branch config using `??:00` on a GitTally that did not know it yet, and a
`builds.maxConcurrent` that had moved to another section.

    gitTally:
      version:
        since: "0.9.18"   # enforced
        below: "2.0"      # release marker; GitTally decides how strictly

There is deliberately no version of the file format (no `apiVersion`): no API
is involved — GitTally reads its own configuration — and only one generation
is ever supported. The declaration exists to make an incompatibility
nameable, never to run two parsers.

`since` is hard in both directions. Too new a requirement is refused, and so
is a file written before the version in which the configuration format last
broke (`ConfigVersions.FORMAT_BROKE_IN`, empty for now) — that check needs no
declared ceiling, because GitTally knows its own breaking changes.

`below` is the team's release marker and only warns: an unmaintained caution
value must never stop a CI. The routine it serves is the one known from IDE
plugins — new version, warning, try it, then raise the marker and commit.

The reach of a violation follows the layer: the machine and project configs
abort the start naming the file and the rollback, while an incompatible
branch config fails only that branch's builds. A branch cut before a
migration must not stop the server or hold up the branches that are fine.
A file that declares nothing keeps working, and the CLI prints one line
instead of a stack trace.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/main/kotlin/de/hoennig/gittally/config/ConfigLoader.kt

@@ -7,6 +7,8 @@ import com.fasterxml.jackson.dataformat.yaml.YAMLFactory
 import com.fasterxml.jackson.dataformat.yaml.YAMLGenerator
 import com.fasterxml.jackson.module.kotlin.registerKotlinModule
 import org.slf4j.LoggerFactory
+import org.springframework.beans.factory.ObjectProvider
+import org.springframework.boot.info.BuildProperties
 import org.springframework.stereotype.Service
 import java.io.File
 import java.nio.file.Path
@@ -14,7 +16,10 @@ import java.nio.file.Paths
 import java.util.concurrent.ConcurrentHashMap
 
 @Service
-class ConfigLoader {
+class ConfigLoader(
+    /** The running version, for the `gitTally.version` check; absent outside a built jar (IDE, tests). */
+    private val buildProperties: ObjectProvider<BuildProperties>? = null,
+) {
     private val log = LoggerFactory.getLogger(ConfigLoader::class.java)
 
     private val yaml =
@@ -26,6 +31,9 @@ class ConfigLoader {
     /** Keys already reported by [dropNonDefinitionBuilds]; the config is loaded on every poll cycle. */
     private val warnedBuildKeys = ConcurrentHashMap.newKeySet<String>()
 
+    /** Version warnings already reported; the config is loaded on every poll cycle, per branch. */
+    private val warnedVersions = ConcurrentHashMap.newKeySet<String>()
+
     fun load(workingDir: Path = Paths.get(".")): GitTallyConfig = toConfig(loadRaw(workingDir))
 
     /**
@@ -37,7 +45,7 @@ class ConfigLoader {
     fun loadForWorktree(
         workingDir: Path,
         worktreeDir: Path,
-    ): GitTallyConfig = toConfig(deepMerge(loadRaw(workingDir), stripPinned(loadFile(worktreeDir.resolve(".gittally.yml").toFile()))))
+    ): GitTallyConfig = withBranchLayer(workingDir, loadFile(worktreeDir.resolve(".gittally.yml").toFile()))
 
     /**
      * The primary/`.git` config with the committed `.gittally.yml` of one branch
@@ -49,8 +57,8 @@ class ConfigLoader {
      *
      * The [pinned][stripPinned] keys are the exception, and they are exactly the ones
      * that are not a description of this branch's build: secrets (`git`), the host- and
-     * repository-side sections (`server`, `gitea`, `executor`), the docker sandbox policy
-     * (`docker.enabled`/`docker.network`), and the trust gate
+     * repository-side sections (`server`, `gitea`, `executor`, `watcher`), the docker
+     * sandbox policy (`docker.enabled`/`docker.network`), and the trust gate
      * (`requirePullRequest`, which decides whether the branch is built at all).
      * They are stripped from the branch layer before merging, so a branch can neither
      * escape its container, nor bypass its own pull-request gate, nor raise the global
@@ -59,7 +67,17 @@ class ConfigLoader {
     fun loadWithBranchLayer(
         workingDir: Path,
         branchConfigYaml: String?,
-    ): GitTallyConfig = toConfig(deepMerge(loadRaw(workingDir), stripPinned(parseYaml(branchConfigYaml))))
+    ): GitTallyConfig = withBranchLayer(workingDir, parseYaml(branchConfigYaml))
+
+    private fun withBranchLayer(
+        workingDir: Path,
+        branchLayer: Map<String, Any?>,
+    ): GitTallyConfig {
+        // scoped to this branch: an incompatible branch config fails its own builds and
+        // must never stop the server or hold up the branches that are fine
+        checkVersion(branchLayer, "the committed .gittally.yml of this branch", BRANCH_HINT)
+        return toConfig(deepMerge(loadRaw(workingDir), stripPinned(branchLayer)))
+    }
 
     private fun toConfig(raw: Map<String, Any?>): GitTallyConfig {
         val config =
@@ -144,9 +162,47 @@ class ConfigLoader {
     fun loadRaw(workingDir: Path = Paths.get(".")): Map<String, Any?> {
         val repoInstall = loadFile(workingDir.resolve(".git/gittally/.gittally.yml").toFile())
         val project = loadFile(workingDir.resolve(".gittally.yml").toFile())
+        // per file, so the message names the file to fix — the merged map has no provenance
+        checkVersion(project, ".gittally.yml", ROLLBACK_HINT)
+        checkVersion(repoInstall, ".git/gittally/.gittally.yml", ROLLBACK_HINT)
         return deepMerge(project, repoInstall)
     }
 
+    /**
+     * Enforces the `gitTally.version` declaration of one configuration file.
+     * An incompatible file throws — reading it would mean honoring keys that mean
+     * something else now, which is worse than not building. A file that merely exceeds
+     * its own `below` marker is a warning, logged once: an unmaintained marker must
+     * never stop a CI.
+     */
+    private fun checkVersion(
+        raw: Map<String, Any?>,
+        source: String,
+        hint: String,
+    ) {
+        if (raw.isEmpty()) {
+            return
+        }
+        val running = buildProperties?.getIfAvailable()?.version
+        when (val verdict = ConfigVersions.verdict(requirementOf(raw), running)) {
+            is VersionVerdict.Compatible -> Unit
+            is VersionVerdict.Warn ->
+                if (warnedVersions.add("$source: ${verdict.message}")) {
+                    log.warn("{} {}", source, verdict.message)
+                }
+            is VersionVerdict.Incompatible -> throw ConfigVersionException("$source ${verdict.message}. $hint")
+        }
+    }
+
+    @Suppress("UNCHECKED_CAST")
+    private fun requirementOf(raw: Map<String, Any?>): VersionRequirement {
+        val version = (raw["gitTally"] as? Map<String, Any?>)?.get("version") as? Map<String, Any?> ?: return VersionRequirement()
+        return VersionRequirement(
+            since = version["since"]?.toString()?.trim().orEmpty(),
+            below = version["below"]?.toString()?.trim().orEmpty(),
+        )
+    }
+
     fun toYaml(value: Any): String = yaml.writeValueAsString(value)
 
     private fun loadFile(file: File): Map<String, Any?> {
@@ -216,5 +272,11 @@ class ConfigLoader {
 
         /** Per-branch `docker` keys a branch must never override: the sandbox policy. */
         private val PINNED_DOCKER_KEYS = setOf("enabled", "network")
+
+        private const val ROLLBACK_HINT =
+            "Migrate the file, or roll back to the GitTally version it was written for."
+
+        private const val BRANCH_HINT =
+            "Migrate the file on this branch; the other branches keep building."
     }
 }

src/main/kotlin/de/hoennig/gittally/config/ConfigVersion.kt

@@ -0,0 +1,123 @@
+package de.hoennig.gittally.config
+
+/**
+ * The GitTally version a configuration file declares itself for, the `gitTally.version`
+ * section:
+ *
+ * ```yaml
+ * gitTally:
+ *   version:
+ *     since: "0.9.16"   # always hard: an older GitTally refuses this file
+ *     below: "2.0"      # GitTally decides how hard, see ConfigVersions.verdict
+ * ```
+ *
+ * There is deliberately no version of the file format itself (no `apiVersion`): no API is
+ * involved — GitTally reads its own configuration — and only one configuration generation
+ * is ever supported. The declared version exists to make an incompatibility nameable,
+ * never to run two parsers.
+ */
+data class VersionRequirement(
+    /** Oldest GitTally that understands this file; empty means the file does not say. */
+    val since: String = "",
+    /** First GitTally this file was not released for; empty means no ceiling. */
+    val below: String = "",
+)
+
+data class GitTallyMeta(
+    val version: VersionRequirement = VersionRequirement(),
+)
+
+/** What a [VersionRequirement] means for the GitTally that reads the file. */
+sealed interface VersionVerdict {
+    /** The running version is covered by the declaration. */
+    data object Compatible : VersionVerdict
+
+    /** Usable, but the file was not released for this version. */
+    data class Warn(
+        val message: String,
+    ) : VersionVerdict
+
+    /** Not usable: the file predates a change that GitTally cannot bridge. */
+    data class Incompatible(
+        val message: String,
+    ) : VersionVerdict
+}
+
+/** A configuration file this GitTally must not read; carries the file's name in its message. */
+class ConfigVersionException(
+    message: String,
+) : RuntimeException(message)
+
+object ConfigVersions {
+    /**
+     * The version in which the configuration format last changed incompatibly — a file
+     * written before it cannot be read by this GitTally. Empty while no such change has
+     * happened; set it to the release that introduces one, together with the migration
+     * note the message points at.
+     */
+    const val FORMAT_BROKE_IN = ""
+
+    /** Human-readable description of that change, shown in the error message. */
+    const val FORMAT_BROKE_DESCRIPTION = ""
+
+    /**
+     * Decides what [requirement] means for [running].
+     *
+     * `since` is always hard — a file that needs a newer GitTally cannot be honored, and
+     * silently ignoring its unknown keys is exactly the failure mode this section exists
+     * to prevent.
+     *
+     * `below` alone only warns: it is the team's release marker, and an unmaintained
+     * marker must never stop a CI. Whether the running version really broke the file is
+     * GitTally's own knowledge ([FORMAT_BROKE_IN]) — a file written before that change
+     * and read after it is incompatible regardless of what it declares as its ceiling.
+     */
+    fun verdict(
+        requirement: VersionRequirement,
+        running: String?,
+        brokeIn: String = FORMAT_BROKE_IN,
+        brokeDescription: String = FORMAT_BROKE_DESCRIPTION,
+    ): VersionVerdict {
+        val version = parse(running) ?: return VersionVerdict.Compatible
+        val since = parse(requirement.since)
+        if (since != null && version < since) {
+            return VersionVerdict.Incompatible(
+                "needs GitTally ${requirement.since} or newer (gitTally.version.since), this is $running",
+            )
+        }
+        val broke = parse(brokeIn)
+        if (since != null && broke != null && since < broke && version >= broke) {
+            return VersionVerdict.Incompatible(
+                "is written for GitTally ${requirement.since} (gitTally.version.since), " +
+                    "but the configuration format changed incompatibly in $brokeIn" +
+                    brokeDescription.takeIf { it.isNotBlank() }?.let { ": $it" }.orEmpty(),
+            )
+        }
+        val below = parse(requirement.below)
+        if (below != null && version >= below) {
+            return VersionVerdict.Warn(
+                "was released for GitTally below ${requirement.below} (gitTally.version.below), this is $running",
+            )
+        }
+        return VersionVerdict.Compatible
+    }
+
+    /**
+     * `1.2.3` and shorter prefixes like `2.0`, compared numerically part by part with
+     * missing parts as 0 — `below: "2.0"` is the point 2.0.0, which is why the ceiling is
+     * exclusive: an inclusive one could not tell `1` (the release) from `1.x` (the series).
+     * A pre-release suffix (`1.0.0-rc1`) is ignored, and anything unparseable yields null,
+     * so a typo can never make a file look incompatible.
+     */
+    fun parse(version: String?): List<Int>? {
+        val text = version?.trim()?.substringBefore('-').orEmpty()
+        if (text.isEmpty()) {
+            return null
+        }
+        val parts = text.split('.').map { it.toIntOrNull() ?: return null }
+        return (parts + listOf(0, 0, 0)).take(3)
+    }
+
+    private operator fun List<Int>.compareTo(other: List<Int>): Int =
+        indices.firstNotNullOfOrNull { i -> (this[i] - other[i]).takeIf { it != 0 } } ?: 0
+}

src/main/kotlin/de/hoennig/gittally/config/GitTallyConfig.kt

@@ -3,6 +3,8 @@ package de.hoennig.gittally.config
 import com.fasterxml.jackson.annotation.JsonProperty
 
 data class GitTallyConfig(
+    /** What this file declares about the GitTally that reads it; see [VersionRequirement]. */
+    val gitTally: GitTallyMeta = GitTallyMeta(),
     val server: ServerConfig = ServerConfig(),
     val git: GitConfig = GitConfig(),
     val gitea: GiteaConfig = GiteaConfig(),