From 93da42d4512838c1a49a57fdf6a773fe823ba935 Mon Sep 17 00:00:00 2001
From: Jorge Aparicio <jorge.aparicio@ferrous-systems.com>
Date: Wed, 22 May 2024 19:55:41 +0200
Subject: [PATCH] use smaller Docker networks

---
 packages/dns-test/src/container/network.rs | 111 ++++++++++++++++++++-
 1 file changed, 109 insertions(+), 2 deletions(-)

diff --git a/packages/dns-test/src/container/network.rs b/packages/dns-test/src/container/network.rs
index ebe9c6d..3315950 100644
--- a/packages/dns-test/src/container/network.rs
+++ b/packages/dns-test/src/container/network.rs
@@ -1,8 +1,9 @@
 use std::{
+    net::Ipv4Addr,
     process::{self, Command, Stdio},
     sync::{
         atomic::{self, AtomicUsize},
-        Arc,
+        Arc, Mutex,
     },
 };
 
@@ -50,17 +51,23 @@ impl Drop for NetworkInner {
 
 impl NetworkInner {
     pub fn new(pid: u32, network_name: &str) -> Result<Self> {
+        static LOCK: Mutex<()> = Mutex::new(());
+
+        let guard = LOCK.lock()?;
         let count = network_count();
         let network_name = format!("{network_name}-{pid}-{count}");
 
+        let in_use = in_use()?;
+        let subnet = format!("{}/24", choose_network(&in_use));
         let mut command = Command::new("docker");
         command
             .args(["network", "create"])
-            .args(["--internal", "--attachable"])
+            .args(["--internal", "--attachable", "--subnet", &subnet])
             .arg(&network_name);
 
         // create network
         let output = command.output()?;
+        drop(guard);
         let stdout = String::from_utf8_lossy(&output.stdout);
         let stderr = String::from_utf8_lossy(&output.stderr);
 
@@ -78,6 +85,96 @@ impl NetworkInner {
     }
 }
 
+fn choose_network(in_use: &[(Ipv4Addr, u32)]) -> Ipv4Addr {
+    for c in 0..=255 {
+        let candidate = Ipv4Addr::new(192, 168, c, 0);
+
+        if !overlaps_with_any(candidate, in_use) {
+            return candidate;
+        }
+    }
+
+    for b in 16..=31 {
+        for c in 0..=255 {
+            let candidate = Ipv4Addr::new(172, b, c, 0);
+
+            if !overlaps_with_any(candidate, in_use) {
+                return candidate;
+            }
+        }
+    }
+
+    for b in 0..=255 {
+        for c in 0..=255 {
+            let candidate = Ipv4Addr::new(10, b, c, 0);
+
+            if !overlaps_with_any(candidate, in_use) {
+                return candidate;
+            }
+        }
+    }
+
+    // should wait until a docker network is released
+    unimplemented!()
+}
+
+fn overlaps_with(lhs: Ipv4Addr, rhs: Ipv4Addr, rhs_netmask_bits: u32) -> bool {
+    // LHS has netmask /24
+    if rhs_netmask_bits == 24 {
+        let [a1, b1, c1, _] = lhs.octets();
+        let [a2, b2, c2, _] = rhs.octets();
+
+        (a1, b1, c1) == (a2, b2, c2)
+    } else if rhs_netmask_bits == 16 {
+        let [a1, b1, _, _] = lhs.octets();
+        let [a2, b2, _, _] = rhs.octets();
+
+        (a1, b1) == (a2, b2)
+    } else if rhs_netmask_bits == 8 {
+        let [a1, _, _, _] = lhs.octets();
+        let [a2, _, _, _] = rhs.octets();
+
+        a1 == a2
+    } else {
+        unreachable!()
+    }
+}
+
+fn overlaps_with_any(lhs: Ipv4Addr, in_use: &[(Ipv4Addr, u32)]) -> bool {
+    for (rhs, rhs_netmask_bits) in in_use {
+        if overlaps_with(lhs, *rhs, *rhs_netmask_bits) {
+            return true;
+        }
+    }
+
+    false
+}
+
+fn in_use() -> Result<Vec<(Ipv4Addr, u32)>> {
+    let ifconfig = String::from_utf8(Command::new("ifconfig").output()?.stdout)?;
+    let mut in_use = vec![];
+    for line in ifconfig.lines() {
+        if let Some((_before, after)) = line.split_once("inet ") {
+            let mut parts = after.split_whitespace();
+            if let (Some(ip_addr), Some(_), Some(netmask)) =
+                (parts.next(), parts.next(), parts.next())
+            {
+                if let Ok(ip_addr) = ip_addr.parse() {
+                    let netmask_bits = netmask
+                        .split('.')
+                        .filter_map(|part| part.parse::<u8>().ok())
+                        .map(|mask| mask.count_ones())
+                        .sum::<u32>();
+
+                    in_use.push((ip_addr, netmask_bits))
+                }
+            }
+        }
+    }
+
+    Ok(in_use)
+}
+
 /// Collects all important configs.
 pub struct NetworkConfig {
     /// The CIDR subnet mask, e.g. "172.21.0.0/16"
@@ -153,4 +250,14 @@ mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn stress() {
+        let mut networks = vec![];
+        for index in 0..256 {
+            let network = Network::new().unwrap_or_else(|e| panic!("{}: {e}", index));
+            eprintln!("{}", network.0.config.subnet);
+            networks.push(network);
+        }
+    }
 }