From 3390d7065e857e280e6415d6bfec55ff106e0f1f Mon Sep 17 00:00:00 2001
From: clawbot <35+clawbot@noreply.example.org>
Date: Thu, 1 Oct 2026 21:15:02 +0200
Subject: [PATCH] docker: set up the data directory in an entrypoint (closes
 #166)

The runtime image no longer sets USER. Its new entrypoint,
deploy/docker-entrypoint.sh, runs as root: it creates the data
directory if needed, gives it and everything in it to the dnswatcher
user (uid 10001) with mode 700 on the directory, then runs dnswatcher
as that user with su-exec. A bind-mounted host directory, whether
empty and root-owned or holding a state file left by another uid, no
longer has to be chowned first, and the README's upaas section now says
only which path to mount. The startup check that the data directory is
writable stays.

Model: opus-5-5
---
 Dockerfile                  | 18 ++++++++----------
 README.md                   | 12 +-----------
 TODO.md                     |  2 ++
 deploy/docker-entrypoint.sh | 17 +++++++++++++++++
 4 files changed, 28 insertions(+), 21 deletions(-)
 create mode 100755 deploy/docker-entrypoint.sh

diff --git a/Dockerfile b/Dockerfile
index b4f7273..711588c 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -41,18 +41,15 @@ RUN make build
 # alpine 3.21, 2026-02-28
 FROM alpine@sha256:c3f8e73fdb79deaebaa2037150150191b9dcbfba68b4a46d70103204c53f4709
 
-RUN apk add --no-cache ca-certificates tzdata
+RUN apk add --no-cache ca-certificates tzdata su-exec
 
 COPY --from=builder /src/bin/dnswatcher /usr/local/bin/dnswatcher
+COPY deploy/docker-entrypoint.sh /usr/local/bin/docker-entrypoint.sh
 
-# Run as an unprivileged user that owns the data directory. A fresh named
-# volume inherits this ownership; a bind-mounted host directory must be
-# owned by uid 10001 (see "Running under upaas" in README.md), or startup
-# fails.
+# dnswatcher runs as this unprivileged user. The entrypoint creates the
+# data directory and gives it to this user on every start.
 RUN addgroup -S -g 10001 dnswatcher \
-    && adduser -S -G dnswatcher -u 10001 dnswatcher \
-    && mkdir -p /var/lib/dnswatcher \
-    && chown dnswatcher:dnswatcher /var/lib/dnswatcher
+    && adduser -S -G dnswatcher -u 10001 dnswatcher
 
 ENV DNSWATCHER_DATA_DIR=/var/lib/dnswatcher
 
@@ -62,7 +59,8 @@ ENV DNSWATCHER_DATA_DIR=/var/lib/dnswatcher
 # data directory, or the binary's directory, the working directory.
 WORKDIR /
 
-USER dnswatcher
+# No USER: the entrypoint must start as root to set up the data
+# directory; it then runs dnswatcher as the dnswatcher user.
 
 EXPOSE 8080
 
@@ -72,4 +70,4 @@ EXPOSE 8080
 HEALTHCHECK --interval=10s --timeout=5s --start-period=10s --retries=3 \
     CMD wget -q -O /dev/null "http://127.0.0.1:${PORT:-8080}/.well-known/healthcheck" || exit 1
 
-ENTRYPOINT ["/usr/local/bin/dnswatcher"]
+ENTRYPOINT ["/usr/local/bin/docker-entrypoint.sh"]
diff --git a/README.md b/README.md
index 1e6294b..f971096 100644
--- a/README.md
+++ b/README.md
@@ -533,17 +533,7 @@ repository's `Dockerfile` and runs it. The app needs:
 - **Branch:** `prod`. `prod` is cut from `main`, and merging a `main` to
   `prod` pull request is a deploy.
 - **Volume:** one host directory mounted at `/var/lib/dnswatcher`, where
-  the state file lives. upaas bind-mounts the host path it is given and
-  does not create it. The container runs as uid 10001 and does not start
-  unless it can write there. Create the directory before the first
-  deploy:
-
-  ```sh
-  mkdir -p /path/to/data
-  chown 10001:10001 /path/to/data
-  chmod 700 /path/to/data
-  ```
-
+  the state file lives.
 - **Network and port:** the dashboard is unauthenticated and shows every
   watched name and recent alert, and upaas publishes every mapped port on
   all interfaces of the host
diff --git a/TODO.md b/TODO.md
index 23a1f8f..7462510 100644
--- a/TODO.md
+++ b/TODO.md
@@ -25,6 +25,8 @@ Rationale, Design, TODO, License, Author) if any are still missing.
   constructors: two moved to `export_test.go`, one is deleted (closes #111).
 - 2026-10-01: notify shutdown tests use one timing constant per meaning, name
   the bound they check, and require the drain's debug line (closes #116).
+- 2026-09-29: the entrypoint chowns the data directory to `dnswatcher` and runs
+  dnswatcher as that user, so a host bind mount needs no chown (closes #166).
 - 2026-09-29: the live-DNS test package is renamed `internal/livednstest` and
   added to the `test-support` `deny` list in `.golangci.yml`, so `make lint`
   fails when program code imports it (closes #164).
diff --git a/deploy/docker-entrypoint.sh b/deploy/docker-entrypoint.sh
new file mode 100755
index 0000000..62b95cd
--- /dev/null
+++ b/deploy/docker-entrypoint.sh
@@ -0,0 +1,17 @@
+#!/bin/sh
+# deploy/docker-entrypoint.sh: the Docker image's ENTRYPOINT. It runs as
+# root only to give the data directory to the dnswatcher user: a host
+# directory bind-mounted there keeps its host owner, often root, and may
+# hold a state file left by another uid, which dnswatcher could neither
+# read nor replace. dnswatcher itself always runs as the dnswatcher user.
+set -eu
+
+main() {
+    dir="${DNSWATCHER_DATA_DIR:-/var/lib/dnswatcher}"
+    mkdir -p "$dir"
+    chown -R dnswatcher:dnswatcher "$dir"
+    chmod 700 "$dir"
+    exec su-exec dnswatcher /usr/local/bin/dnswatcher "$@"
+}
+
+main "$@"