From 45ac8c6c2277b72560b7f56c5b4033d8b8f11cea Mon Sep 17 00:00:00 2001
From: Songmin Li <lisongmin@protonmail.com>
Date: Tue, 17 Dec 2024 20:34:26 +0800
Subject: [PATCH] Support docker-compose like default network behavior by
 default_net_behavior_compat switch

Signed-off-by: Songmin Li <lisongmin@protonmail.com>
---
 docs/Extensions.md                            | 20 +++++++++++++++++++
 .../default_net_behavior_compat.feature       |  2 ++
 podman_compose.py                             | 20 ++++++++++++++-----
 3 files changed, 37 insertions(+), 5 deletions(-)
 create mode 100644 newsfragments/default_net_behavior_compat.feature

diff --git a/docs/Extensions.md b/docs/Extensions.md
index 300e071a..c5575d1a 100644
--- a/docs/Extensions.md
+++ b/docs/Extensions.md
@@ -110,6 +110,26 @@ x-podman:
 By default `default_net_name_compat` is `false`. This will change to `true` at some point and the
 setting will be removed.
 
+## Compatibility of default network behavior between docker-compose and podman-compose
+
+When there is no network defined (neither network-mode nor networks) in service,
+The behavior of default network in docker-compose and podman-compose are different.
+
+| Top-level networks             | podman-compose             | docker-compose |
+| ------------------------------ | -------------------------- | -------------- |
+| No networks                    | default                    | default        |
+| One network named net0         | net0                       | default        |
+| Two networks named net0, net1  | podman(`--network=bridge`) | default        |
+| Contains network named default | default                    | default        |
+
+To enable compatibility between docker-compose and podman-compose, specify
+`default_net_behavior_compat: true` under global `x-podman` key:
+
+```yaml
+x-podman:
+    default_net_behavior_compat: true
+```
+
 ## Custom pods management
 
 Podman-compose can have containers in pods. This can be controlled by extension key x-podman in_pod.
diff --git a/newsfragments/default_net_behavior_compat.feature b/newsfragments/default_net_behavior_compat.feature
new file mode 100644
index 00000000..133e9967
--- /dev/null
+++ b/newsfragments/default_net_behavior_compat.feature
@@ -0,0 +1,2 @@
+Support docker-compose like default network behavior when no network defined in service via `default_net_behavior_compat` feature flag.
+
diff --git a/podman_compose.py b/podman_compose.py
index 81dcba39..09086317 100755
--- a/podman_compose.py
+++ b/podman_compose.py
@@ -1998,13 +1998,23 @@ def _parse_compose_file(self):
         nets = compose.get("networks", {})
         if not nets:
             nets["default"] = None
+
         self.networks = nets
-        if len(self.networks) == 1:
-            self.default_net = list(nets.keys())[0]
-        elif "default" in nets:
-            self.default_net = "default"
+        if compose.get("x-podman", {}).get("default_net_behavior_compat", False):
+            # If there is no network_mode and networks in service,
+            # docker-compose will create default network named '<project_name>_default'
+            # and add the service to the default network.
+            # So we always set `default_net = 'default'` for compatibility
+            if "default" not in self.networks:
+                self.networks["default"] = None
         else:
-            self.default_net = None
+            if len(self.networks) == 1:
+                self.default_net = list(nets.keys())[0]
+            elif "default" in nets:
+                self.default_net = "default"
+            else:
+                self.default_net = None
+
         allnets = set()
         for name, srv in services.items():
             srv_nets = srv.get("networks", self.default_net)