feat: Add pool sharing for domains

Allow multiple domains to share the same backend pool using share_with parameter.
This saves pool slots when domains point to the same servers.

- Add share_with parameter to haproxy_add_domain
- Add helper functions for shared domain management
- Protect shared pools from being cleared on domain removal
- Update documentation with pool sharing examples

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

haproxy_mcp/tools/domains.py

@@ -28,6 +28,9 @@ from ..file_ops import (
     add_server_to_config,
     remove_server_from_config,
     remove_domain_from_config,
+    add_shared_domain_to_config,
+    get_domains_sharing_pool,
+    is_shared_domain,
 )
 from ..utils import parse_servers_state, disable_server_slot
 
@@ -204,13 +207,18 @@ def register_domain_tools(mcp):
     def haproxy_add_domain(
         domain: Annotated[str, Field(description="Domain name to add (e.g., api.example.com, example.com)")],
         ip: Annotated[str, Field(default="", description="Optional: Initial server IP. If provided, adds server to slot 1")],
-        http_port: Annotated[int, Field(default=80, description="HTTP port for backend server (default: 80)")]
+        http_port: Annotated[int, Field(default=80, description="HTTP port for backend server (default: 80)")],
+        share_with: Annotated[str, Field(default="", description="Optional: Existing domain to share pool with. New domain uses same backend servers.")]
     ) -> str:
         """Add a new domain to HAProxy (no reload required).
 
         Creates domain→pool mapping. Use haproxy_add_server to add more servers later.
 
+        Pool sharing: Use share_with to reuse an existing domain's pool. This saves pool
+        slots when multiple domains point to the same backend servers.
+
         Example: haproxy_add_domain("api.example.com", ip="10.0.0.1", http_port=8080)
+        Example: haproxy_add_domain("www.example.com", share_with="example.com")
         """
         # Validate inputs
         if domain.startswith("."):
@@ -221,6 +229,10 @@ def register_domain_tools(mcp):
             return "Error: Invalid IP address format"
         if not validate_port_int(http_port):
             return "Error: Port must be between 1 and 65535"
+        if share_with and not validate_domain(share_with):
+            return "Error: Invalid share_with domain format"
+        if share_with and ip:
+            return "Error: Cannot specify both ip and share_with (shared domains use existing servers)"
 
         # Use file locking for the entire pool allocation operation
         lock_path = f"{MAP_FILE}.lock"
@@ -244,10 +256,19 @@ def register_domain_tools(mcp):
                     if not entry_domain.startswith("."):
                         registered_domains.add(entry_domain)
 
-                # Find available pool
-                pool = _find_available_pool(entries, used_pools)
-                if not pool:
-                    return f"Error: All {POOL_COUNT} pool backends are in use"
+                # Handle share_with: reuse existing domain's pool
+                if share_with:
+                    share_backend = get_domain_backend(share_with)
+                    if not share_backend:
+                        return f"Error: Domain {share_with} not found"
+                    if not share_backend.startswith("pool_"):
+                        return f"Error: Cannot share with legacy backend {share_backend}"
+                    pool = share_backend
+                else:
+                    # Find available pool
+                    pool = _find_available_pool(entries, used_pools)
+                    if not pool:
+                        return f"Error: All {POOL_COUNT} pool backends are in use"
 
                 # Check if this is a subdomain of an existing domain
                 is_subdomain, parent_domain = _check_subdomain(domain, registered_domains)
@@ -269,8 +290,13 @@ def register_domain_tools(mcp):
                         _rollback_domain_addition(domain, entries)
                         return f"Error: Failed to update HAProxy map: {e}"
 
-                    # If IP provided, add server to slot 1
-                    if ip:
+                    # Handle server configuration based on mode
+                    if share_with:
+                        # Save shared domain reference
+                        add_shared_domain_to_config(domain, share_with)
+                        result = f"Domain {domain} added, sharing pool {pool} with {share_with}"
+                    elif ip:
+                        # Add server to slot 1
                         add_server_to_config(domain, 1, ip, http_port)
                         try:
                             server = f"{pool}_1"
@@ -318,6 +344,13 @@ def register_domain_tools(mcp):
             return f"Error: Cannot remove legacy domain {domain} (uses static backend {backend})"
 
         try:
+            # Check if this domain is sharing another domain's pool
+            domain_is_shared = is_shared_domain(domain)
+
+            # Check if other domains are sharing this pool
+            domains_using_pool = get_domains_sharing_pool(backend)
+            other_domains = [d for d in domains_using_pool if d != domain]
+
             # Save to disk first (atomic write for persistence)
             entries = get_map_contents()
             new_entries = [(d, b) for d, b in entries if d != domain and d != f".{domain}"]
@@ -334,6 +367,14 @@ def register_domain_tools(mcp):
             except HaproxyError as e:
                 logger.warning("Failed to remove wildcard entry for %s: %s", domain, e)
 
+            # Only clear servers if no other domains are using this pool
+            if other_domains:
+                return f"Domain {domain} removed from {backend} (pool still used by: {', '.join(other_domains)})"
+
+            # If this domain was sharing another domain's pool, don't clear servers
+            if domain_is_shared:
+                return f"Domain {domain} removed from shared pool {backend}"
+
             # Disable all servers in the pool (reset to 0.0.0.0:0)
             for slot in range(1, MAX_SLOTS + 1):
                 server = f"{backend}_{slot}"