fix: only refresh when there is a change

Reviewed-on: #3

Cargo.lock

@@ -654,9 +654,9 @@ dependencies = [
 
 [[package]]
 name = "libc"
-version = "0.2.171"
+version = "0.2.176"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c19937216e9d3aa9956d9bb8dfc0b0c8beb6058fc4f7a4dc4d850edf86a237d6"
+checksum = "58f929b4d672ea937a23a1ab494143d968337a5f47e56d0815df1e0890ddf174"
 
 [[package]]
 name = "libredox"

Dockerfile

@@ -0,0 +1,12 @@
+# Build Stage
+FROM rust:latest AS builder
+WORKDIR /app
+COPY . .
+RUN cargo build --release
+
+# Runtime Stage
+FROM debian:stable-slim
+WORKDIR /app
+COPY --from=builder /app/target/release/handler .
+COPY --from=builder /app/target/release/skubelb .
+CMD ["./handler"]

Makefile

@@ -0,0 +1,16 @@
+MAJOR_VERSION = 0
+MINOR_VERSION = 0
+PATH_VERSION = 1
+
+TAG = $(MAJOR_VERSION).$(MINOR_VERSION).$(PATH_VERSION)
+
+build:
+	docker build . -t skubelb-handler:$(TAG)
+	docker tag skubelb-handler:$(TAG) us-west4-docker.pkg.dev/nixernetes/images/skubelb-handler:$(TAG)
+
+kube:
+	cat kubernetes.yaml.tmpl | sed 's/TAG/$(TAG)/' > kubernetes.yaml
+
+deploy: build kube
+	docker push us-west4-docker.pkg.dev/nixernetes/images/skubelb-handler:$(TAG)
+	kubectl apply -f kubernetes.yaml

README.md

@@ -6,14 +6,173 @@ open a port on all nodes. When the request lands on any node,
 it is forwarded to the correct pod via the network mesh kubernetes
 is using. In theory, there is one a hop penalty.
 
-But lets be honest. You're running with a single LB, probably a GCE free
-tier N1 VM. That extra hop doesn't matter.
+But lets be honest. You're running with a single LB, probably on a
+GCE free tier N1 VM. That extra hop doesn't matter.
 
 ## Config
 
 Configure nginx to do what you want, test it. Use any Node IP for your testing.
-This tool accepts an argument (rewrite_string) which will be replaced using these rules:
+This will become the 'template_dir' in the argument to the LB.
 
-1. For each line that contains rewrite string:
-2. Copy the line once per node IP, replacing the string with host IPs
+Move that directory (i.e., `/etc/nginx`) to somewhere new,
+(i.e. `/etc/nginx-template/`).
 
+Make a workspace directory for this tool; it will write configs to this folder
+before updating the symlink you created above. It needs to be persistent so on
+server reboot the service starts ok (i.e., `mkdir /var/skubelb/`).
+
+Create a symlink in the workspace which will point to the 'active' configuration;
+this will be updated by the tool (i.e., `ln -s /etc/nginx-template /var/skubelb/nginx`).
+Make a symlink from the old config directory to that symlink (i.e., 
+`ln -s /var/skubelb/nginx /etc/nginx`). Two layers are symlinks are used so we can
+have a non-root user run the rool when we setup the service.
+
+Make sure the user running the tool has read access to the template folder, read-write
+access to the workspace folder and config symlink.
+
+Run the server with a command like:
+
+```sh
+skubelb --needle some_node_ip \
+    --workspace_dir /var/skubelb \
+    --config_symlink /var/skubelb/nginx \
+    --template_dir /etc/nginx-template
+    --listen 0.0.0.0:8888
+```
+
+Replacing `some_node_ip` with the node IP you used during the initial setup.
+
+Next, configure the Kubernetes nodes to POST `http://loadbalancer:8888/register/${NODE_IP}` when
+they started, and DELETE `http://loadbalancer:8888/register/${NODE_IP}` when they shutdown. The easiest
+way to do this is with a daemonset; see the example below, or in daemon_set.yaml.
+
+#### Running as a system service
+
+Setup a user to run the service; make that user
+with `useradd -M skubelb`, Prevent logins with `usermod -L skubelb`.
+
+Make a workspace dir, `mkdir /var/skubelb/`, and give access to the
+daemon user, `chown skubelb:skubelb /var/skubelb/`.
+
+Add the systemd config to `/etc/systemd/system/skubelb.service`:
+
+```toml
+[Unit]
+Description=Simple Kubernetes Load Balancer
+After=network.target
+StartLimitIntervalSec=0
+
+[Service]
+Type=simple
+Restart=always
+RestartSec=1
+User=skubelb
+ExecStart=/usr/local/bin/skubelb --needle some_node_ip \
+    --workspace-dir /var/skubelb \
+    --config-symlink /var/skubelb/nginx \
+    --template-dir /etc/nginx-template \
+    --listen 0.0.0.0:8888 \
+    --reload-cmd '/usr/bin/sudo systemctl reload nginx'
+
+[Install]
+WantedBy=multi-user.target
+```
+
+Make sure you update `--needle some_node_ip` with something
+like `--needle 123.44.55.123`. The IP of node you tested with.
+
+### Sample Kubernets configuration
+
+Deploy this [daemon set](https://kubernetes.io/docs/concepts/workloads/controllers/daemonset/)
+to your cluster, replacing `lb_address` with the address of your load balancer.
+
+```yaml
+apiVersion: apps/v1
+kind: DaemonSet
+metadata:
+  name: skubelb
+  namespace: skubelb
+  labels:
+    k8s-app: skubelb
+spec:
+  selector:
+    matchLabels:
+      name: skubelb
+  template:
+    metadata:
+      labels:
+        name: skubelb
+    spec:
+      tolerations:
+      # these tolerations are to have the daemonset runnable on control plane nodes
+      # remove them if your control plane nodes should not run pods
+      - key: node-role.kubernetes.io/control-plane
+        operator: Exists
+        effect: NoSchedule
+      - key: node-role.kubernetes.io/master
+        operator: Exists
+        effect: NoSchedule
+      containers:
+      - name: skubelb
+        image: alpine/curl:latest
+        env:
+        - name: NODE_IP
+          valueFrom:
+            fieldRef:
+              fieldPath: status.hostIP
+        command: ['sh', '-c', 'echo "Wait for heat death of universe" && sleep 999999d']
+        lifecycle:
+          preStop:
+            exec:
+              command:
+              - sh
+              - "-c"
+              - "curl -X POST 10.128.0.2:8888/register/${NODE_IP}"
+          postStart:
+            exec:
+              command:
+              - sh
+              - "-c"
+              - "curl -X POST 10.128.0.2:8888/register/${NODE_IP}"
+        resources:
+          limits:
+            memory: 200Mi
+          requests:
+            cpu: 10m
+            memory: 100Mi
+      terminationGracePeriodSeconds: 30
+```
+
+NOTE: you should need to make an entry in the firewall to allow this request through. It is very important that the firewall entry has a source filter; it should only be allowed from the Kubernetes cluster. Nginx will forward traffic to any host that registers, and this could easily become a MitM vulnerability.
+
+## Other tips
+
+### Use 'upstream' in nginx
+
+Do this:
+
+```
+upstream hosts {
+        server 10.182.0.36:30004;
+        server 10.182.0.39:30004;
+}
+server {
+        server_name git.tipsy.codes tipsy.codes;
+
+        location / {
+                proxy_pass http://hosts;
+        }
+}
+```
+
+Rather than just writing out the IP in the proxy_pass.
+
+### visudo to only allow the nginx reload command
+
+Use `sudo visudo` to update the sudoers file and add this line:
+
+```
+skubelb ALL=(root) NOPASSWD: /usr/bin/systemctl reload nginx
+```
+
+This will prevent the user from running commands other than reload.

daemon_set.yaml

@@ -0,0 +1,54 @@
+apiVersion: apps/v1
+kind: DaemonSet
+metadata:
+  name: skubelb
+  namespace: skubelb
+  labels:
+    k8s-app: skubelb
+spec:
+  selector:
+    matchLabels:
+      name: skubelb
+  template:
+    metadata:
+      labels:
+        name: skubelb
+    spec:
+      tolerations:
+      # these tolerations are to have the daemonset runnable on control plane nodes
+      # remove them if your control plane nodes should not run pods
+      - key: node-role.kubernetes.io/control-plane
+        operator: Exists
+        effect: NoSchedule
+      - key: node-role.kubernetes.io/master
+        operator: Exists
+        effect: NoSchedule
+      containers:
+      - name: skubelb
+        image: alpine/curl:latest
+        env:
+        - name: NODE_IP
+          valueFrom:
+            fieldRef:
+              fieldPath: status.hostIP
+        command: ['sh', '-c', 'echo "Wait for heat death of universe" && sleep 999999d']
+        lifecycle:
+          preStop:
+            exec:
+              command:
+              - sh
+              - "-c"
+              - "curl -X POST 10.128.0.2:8888/register/${NODE_IP}"
+          postStart:
+            exec:
+              command:
+              - sh
+              - "-c"
+              - "curl -X POST 10.128.0.2:8888/register/${NODE_IP}"
+        resources:
+          limits:
+            memory: 200Mi
+          requests:
+            cpu: 10m
+            memory: 100Mi
+      terminationGracePeriodSeconds: 30

kubernetes.yaml

@@ -0,0 +1,41 @@
+apiVersion: apps/v1
+kind: DaemonSet
+metadata:
+  name: skubelb
+  namespace: skubelb
+  labels:
+    k8s-app: skubelb
+spec:
+  selector:
+    matchLabels:
+      name: skubelb
+  template:
+    metadata:
+      labels:
+        name: skubelb
+    spec:
+      tolerations:
+      # these tolerations are to have the daemonset runnable on control plane nodes
+      # remove them if your control plane nodes should not run pods
+      - key: node-role.kubernetes.io/control-plane
+        operator: Exists
+        effect: NoSchedule
+      - key: node-role.kubernetes.io/master
+        operator: Exists
+        effect: NoSchedule
+      containers:
+      - name: skubelb
+        image: us-west4-docker.pkg.dev/nixernetes/images/skubelb-handler:0.0.1
+        env:
+        - name: NODE_IP
+          valueFrom:
+            fieldRef:
+              fieldPath: status.hostIP
+        command: ["sh", "-c", "./handler -s 10.128.0.2:8888 -l ${NODE_IP}"]
+        resources:
+          limits:
+            memory: 200Mi
+          requests:
+            cpu: 10m
+            memory: 100Mi
+      terminationGracePeriodSeconds: 30

kubernetes.yaml.tmpl

@@ -0,0 +1,41 @@
+apiVersion: apps/v1
+kind: DaemonSet
+metadata:
+  name: skubelb
+  namespace: skubelb
+  labels:
+    k8s-app: skubelb
+spec:
+  selector:
+    matchLabels:
+      name: skubelb
+  template:
+    metadata:
+      labels:
+        name: skubelb
+    spec:
+      tolerations:
+      # these tolerations are to have the daemonset runnable on control plane nodes
+      # remove them if your control plane nodes should not run pods
+      - key: node-role.kubernetes.io/control-plane
+        operator: Exists
+        effect: NoSchedule
+      - key: node-role.kubernetes.io/master
+        operator: Exists
+        effect: NoSchedule
+      containers:
+      - name: skubelb
+        image: us-west4-docker.pkg.dev/nixernetes/images/skubelb-handler:TAG
+        env:
+        - name: NODE_IP
+          valueFrom:
+            fieldRef:
+              fieldPath: status.hostIP
+        command: ["sh", "-c", "./handler -s 10.128.0.2:8888 -l ${NODE_IP}"]
+        resources:
+          limits:
+            memory: 200Mi
+          requests:
+            cpu: 10m
+            memory: 100Mi
+      terminationGracePeriodSeconds: 30

src/main.rs

@@ -1,14 +1,19 @@
+use std::process::Command;
+use std::sync::Arc;
 use std::sync::Mutex;
+use std::thread;
+use std::thread::sleep;
+use std::time::Duration;
 
 use clap::Parser;
 
 use skubelb::Rewriter;
 use skubelb::Server;
 
+use anyhow::{Result, anyhow};
 use env_logger::Env;
 use log::{info, warn};
-use anyhow::Result;
-use rouille::{router, Request, Response};
+use rouille::{Request, Response, router};
 
 /// Implements a HTTP server which allows clients to 'register'
 /// themselves. Their IP address will be used to replace a
@@ -28,7 +33,7 @@ struct Args {
     /// and instead N lines (one per replacement) is added to
     /// the output.
     #[arg(short, long)]
-    rewrite_string: String,
+    needle: String,
 
     /// The folder which contains the templates that
     /// will be be searched for the needle.
@@ -45,8 +50,13 @@ struct Args {
     #[arg(short, long)]
     workspace_dir: String,
 
+    /// Address to listen for http requests on.
     #[arg(short, long, default_value_t = String::from("0.0.0.0:8080"))]
     listen: String,
+
+    /// Command to reload nginx.
+    #[arg(short, long, default_value_t = String::from("sudo nginx -s reload"))]
+    reload_cmd: String,
 }
 
 fn main() {
@@ -54,31 +64,92 @@ fn main() {
     env_logger::Builder::from_env(Env::default().default_filter_or("info")).init();
     let args = Args::parse();
 
-    let rewriter = Rewriter::new(args.rewrite_string);
-    let server_impl = Mutex::new(Server::new(rewriter, args.workspace_dir, args.template_dir, args.config_symlink));
+    let rewriter = Rewriter::new(args.needle);
+    let server_impl = Arc::new(Mutex::new(Server::new(
+        rewriter,
+        args.workspace_dir,
+        args.template_dir,
+        args.config_symlink,
+    )));
+    let reload_command = args.reload_cmd.leak();
+    let reload_command: Vec<&str> = reload_command.split_ascii_whitespace().collect();
 
-    rouille::start_server(args.listen, move |request| {
-        info!("Processing request: {:?}", request);
-        match handle(request, &server_impl) {
-            Ok(resp) => resp,
+    // Start cleanup thread
+    {
+        let server_impl = server_impl.clone();
+        thread::spawn(move || {
+            loop {
+                sleep(Duration::from_secs(30));
+                match cleanup_worker(&server_impl) {
+                    Ok(_) => (),
                     Err(e) => {
-                warn!("{:?}", e);
-                Response{status_code: 500, ..Response::empty_400()}
+                        warn!("Error cleaning up handlers {}", e);
+                    }
                 }
             }
         });
     }
 
-fn handle(request: &Request, server_impl: &Mutex<Server>) -> Result<Response> {
+    rouille::start_server(args.listen, move |request| {
+        info!("Processing request: {:?}", request);
+        match handle(request, &server_impl) {
+            Ok((resp, reload)) => {
+                if reload && reload_command.len() > 0 {
+                    let output = Command::new(reload_command[0])
+                        .args(&reload_command[1..])
+                        .output();
+                    match output {
+                        Ok(o) => {
+                            info!("Ran {:?}; exit code: {}", reload_command, o.status);
+                            info!(
+                                "Ran {:?}; stdout: {}",
+                                reload_command,
+                                String::from_utf8_lossy(&o.stdout)
+                            );
+                            info!(
+                                "Ran {:?}; stderr: {}",
+                                reload_command,
+                                String::from_utf8_lossy(&o.stderr)
+                            );
+                        }
+                        Err(e) => {
+                            warn!("Failed to run {:?}: {:?}", reload_command, e);
+                        }
+                    };
+                }
+                resp
+            }
+            Err(e) => {
+                warn!("{:?}", e);
+                Response {
+                    status_code: 500,
+                    ..Response::empty_400()
+                }
+            }
+        }
+    });
+}
+
+fn handle(request: &Request, server_impl: &Mutex<Server>) -> Result<(Response, bool)> {
     router!(request,
-        (POST) (/register) => {
-            server_impl.lock().unwrap().register(request)?;
-            Ok(Response{status_code: 200, ..Response::empty_204()})
+        (POST) (/register/{ip: String}) => {
+            let mut server_impl = server_impl.lock().map_err(|_| anyhow!("failed to acquire lock"))?;
+            server_impl.register(request, &ip)?;
+            let must_reload = server_impl.cleanup()?;
+            Ok((Response{status_code: 200, ..Response::empty_204()}, must_reload))
         },
-        (DELETE) (/register) => {
-            server_impl.lock().unwrap().unregister(request)?;
-            Ok(Response{status_code: 200, ..Response::empty_204()})
+        (DELETE) (/register/{ip: String}) => {
+            let mut server_impl = server_impl.lock().map_err(|_| anyhow!("failed to acquire lock"))?;
+            server_impl.unregister(request, &ip)?;
+            let must_reload = server_impl.cleanup()?;
+            Ok((Response{status_code: 200, ..Response::empty_204()}, must_reload))
         },
-        _ => Ok(Response::empty_404()),
+        _ => Ok((Response::empty_404(), false)),
     )
 }
+
+fn cleanup_worker(server_impl: &Mutex<Server>) -> Result<()> {
+    let mut server_impl = server_impl.lock().map_err(|_| anyhow!("failed to acquire lock"))?;
+    server_impl.cleanup()?;
+    Ok(())
+}

src/rewriter.rs

@@ -1,6 +1,7 @@
 use anyhow::Result;
-use std::collections::HashSet;
+use std::collections::{HashMap, HashSet};
 use std::path::Path;
+use std::time::Instant;
 use std::{
     fs::{self, File},
     io::{BufReader, prelude::*},
@@ -9,6 +10,8 @@ use std::{
 pub struct Rewriter {
     source: String,
     replacements: HashSet<String>,
+    // When each replacement should be cleaned up
+    replacement_cleanup: HashMap<String, Instant>,
 }
 
 impl Rewriter {
@@ -16,15 +19,18 @@ impl Rewriter {
         Self {
             source,
             replacements: HashSet::new(),
+            replacement_cleanup: HashMap::default(),
         }
     }
 
-    pub fn add_replacement(&mut self, replacement: String) {
-        self.replacements.insert(replacement);
+    pub fn add_replacement(&mut self, replacement: String, cleanup: Instant) -> bool {
+        self.replacement_cleanup.insert(replacement.clone(), cleanup);
+        self.replacements.insert(replacement)
     }
 
-    pub fn remove_replacement(&mut self, replacement: &str) {
-        self.replacements.remove(replacement);
+    pub fn remove_replacement(&mut self, replacement: &str) -> bool {
+        self.replacement_cleanup.remove(replacement);
+        self.replacements.remove(replacement)
     }
 
     pub fn rewrite_folder(&self, src: &str, dst: &str) -> Result<()> {
@@ -58,32 +64,75 @@ impl Rewriter {
                 // Open 2 files; one to read and translate, and one to write.
                 let source_file = File::open(&src_path)?;
                 let mut dest_file = File::create(&dst_path)?;
-                let reader = BufReader::new(source_file);
+                let mut buff = Vec::with_capacity(2048);
+                let mut reader = BufReader::new(source_file);
 
-                for line in reader.lines() {
-                    let line = line?;
+                while let Ok(count) = reader.read_until(b'\n', &mut buff) {
+                    if count == 0 {
+                        break;
+                    }
                     // If the line is not subject to replacement, copy it and
                     // carry on.
-                    if !line.contains(&self.source) {
-                        writeln!(dest_file, "{}", line)?;
+                    let line = &buff[0..count];
+                    let m = contains(&self.source.as_bytes(), line);
+                    if m.is_none() {
+                        dest_file.write(&line)?;
+                        buff.clear();
                         continue;
                     }
+                    let m = m.unwrap();
+                    let start = &line[0..m.0];
+                    let end = &line[m.1..];
 
                     // Else, repeat the line multiple times, replacing the string
                     // in question
                     for replacement in &replacements {
-                        let new_line = line.replace(&self.source, &replacement);
-                        writeln!(dest_file, "{}", new_line)?;
+                        dest_file.write(start)?;
+                        dest_file.write(replacement.as_bytes())?;
+                        dest_file.write(end)?;
                     }
+                    buff.clear();
                 }
             }
         }
         Ok(())
     }
+
+    pub fn cleanup(&mut self) -> bool {
+        let now = Instant::now();
+        let mut to_remove = vec![];
+        for (name, when) in self.replacement_cleanup.iter() {
+            if when < &now {
+                to_remove.push(name.clone());
+            }
+        }
+        let will_cleanup = to_remove.len() > 0;
+        for name in to_remove {
+            self.remove_replacement(&name);
+        }
+        will_cleanup
+    }
+}
+
+fn contains(needle: &[u8], haystack: &[u8]) -> Option<(usize, usize)> {
+    let mut i = 0;
+    while i + needle.len() <= haystack.len() {
+        let mut j = 0;
+        while i+j < haystack.len() && j < needle.len() && haystack[i+j] == needle[j] {
+            j += 1;
+        }
+        if j == needle.len() {
+            return Some((i, i+j));
+        }
+        i += 1;
+    }
+    None
 }
 
 #[cfg(test)]
 mod tests {
+    use std::time::{Duration, Instant};
+
     use include_directory::{Dir, include_directory};
     use tempdir::TempDir;
 
@@ -97,10 +146,11 @@ mod tests {
         let src = testdata.path().join("testsrc");
         let dst = TempDir::new("").unwrap();
 
+        let now = Instant::now();
         let mut rewriter = Rewriter::new("to_be_replaced".into());
-        rewriter.add_replacement("abc".into());
-        rewriter.add_replacement("def".into());
-        rewriter.add_replacement("zyx".into());
+        rewriter.add_replacement("abc".into(), now.checked_add(Duration::new(60*60*24, 0)).unwrap());
+        rewriter.add_replacement("def".into(), now);
+        rewriter.add_replacement("zyx".into(), now);
         rewriter.remove_replacement("zyx");
         rewriter
             .rewrite_folder(
@@ -113,5 +163,20 @@ mod tests {
         assert!(
             dir_diff::is_different(testdata.path().join("testdst"), dst.path()).unwrap() == false
         );
+
+        // Trigger the cleanup, which should GC abc
+        let dst = TempDir::new("").unwrap();
+        rewriter.cleanup();
+        rewriter
+            .rewrite_folder(
+                src.as_os_str().to_str().unwrap(),
+                dst.path().as_os_str().to_str().unwrap(),
+            )
+            .unwrap();
+
+        // Validate that everything matches
+        assert!(
+            dir_diff::is_different(testdata.path().join("testdst_after_gc"), dst.path()).unwrap() == false
+        );
     }
 }

src/server.rs

@@ -1,9 +1,13 @@
-use std::{fs, path::Path};
+use std::{
+    fs,
+    path::Path,
+    time::{Duration, Instant},
+};
 
+use anyhow::{Context, Result, anyhow};
 use chrono::Utc;
 use log::info;
 use rouille::Request;
-use anyhow::{Context, Result};
 
 use crate::Rewriter;
 
@@ -18,7 +22,12 @@ pub struct Server {
 }
 
 impl Server {
-    pub fn new(rewriter: Rewriter, workspace_dir: String, template_dir: String, config_dir: String) -> Self {
+    pub fn new(
+        rewriter: Rewriter,
+        workspace_dir: String,
+        template_dir: String,
+        config_dir: String,
+    ) -> Self {
         Self {
             rewriter,
             workspace_dir,
@@ -27,19 +36,30 @@ impl Server {
         }
     }
 
-    pub fn register(&mut self, request: &Request) -> Result<()> {
-        let ip = request.remote_addr().ip().to_string();
-        info!("Registering {} as a handler", ip);
-        self.rewriter.add_replacement(ip);
+    pub fn cleanup(&mut self) -> Result<bool> {
+        let cleaned_up = self.rewriter.cleanup();
+        if cleaned_up {
             self.generate_config()?;
+        }
+        Ok(cleaned_up)
+    }
+
+    pub fn register(&mut self, _request: &Request, ip: &str) -> Result<()> {
+        info!("Registering {} as a handler", ip);
+        let cleanup_time = Instant::now()
+            .checked_add(Duration::from_secs(60))
+            .ok_or(anyhow!("failed to convert time"))?;
+        if self.rewriter.add_replacement(ip.to_string(), cleanup_time) {
+            self.generate_config()?;
+        }
         Ok(())
     }
 
-    pub fn unregister(&mut self, request: &Request) -> Result<()> {
-        let ip = request.remote_addr().ip().to_string();
+    pub fn unregister(&mut self, _request: &Request, ip: &str) -> Result<()> {
         info!("Deregistering {} as a handler", ip);
-        self.rewriter.remove_replacement(&ip);
+        if self.rewriter.remove_replacement(ip) {
             self.generate_config()?;
+        }
         Ok(())
     }
 
@@ -51,10 +71,13 @@ impl Server {
         let path = Path::new(&self.workspace_dir).join(&now.format("%Y/%m/%d/%s").to_string());
         let path = path.as_os_str().to_str().unwrap();
         fs::create_dir_all(path).with_context(|| "creating directory")?;
-        self.rewriter.rewrite_folder(&self.template_dir, path).with_context(|| "generating configs")?;
+        self.rewriter
+            .rewrite_folder(&self.template_dir, path)
+            .with_context(|| "generating configs")?;
         // Finally, symlink it to the output folder; only support Linux for now
-        fs::remove_file(&self.config_dir).with_context(|| "removing old symlink")?;
-        std::os::unix::fs::symlink(path, &self.config_dir).with_context(|| "updating symlink")?;
+        let symlink = Path::new(&self.workspace_dir).join("symlink.tmp");
+        std::os::unix::fs::symlink(path, &symlink).with_context(|| "creating symlink")?;
+        fs::rename(symlink, &self.config_dir).with_context(|| "renaming symlink")?;
         Ok(())
     }
 }

src/testdata/testdst_after_gc/hello.txt

@@ -0,0 +1,4 @@
+This is a line
+This is abc line
+
+This is another line

src/testdata/testdst_after_gc/recursive/world.txt

@@ -0,0 +1,3 @@
+This is a abc line.
+
+In a nested directory.