feat: add ability to check for multiple default ssh key locations

Since Ubuntu 24.04 LTS and newer, the default location for ssh keys is
`~/.ssh/id_ed25519` instead of `~/.ssh/id_rsa`. Thus, when no ssh key
path is specified, this commit changes pycloudlib to check for an
existing ssh key at either  `~/.ssh/id_ed25519` or `~/.ssh/id_rsa`.

Fixes GH canonical#341

VERSION

@@ -1 +1 @@
-1!9.3.2
+1!9.4.0

pycloudlib/cloud.py

@@ -13,7 +13,11 @@
 import paramiko
 
 from pycloudlib.config import ConfigFile, parse_config
-from pycloudlib.errors import CleanupError, InvalidTagNameError
+from pycloudlib.errors import (
+    CleanupError,
+    InvalidTagNameError,
+    PycloudlibError,
+)
 from pycloudlib.instance import BaseInstance
 from pycloudlib.key import KeyPair
 from pycloudlib.util import (
@@ -62,13 +66,39 @@ def __init__(
         self._check_and_set_config(config_file, required_values)
 
         user = getpass.getuser()
+        # check if id_rsa or id_ed25519 keys exist in the user's .ssh directory
+        possible_default_keys = [
+            os.path.expanduser("~/.ssh/id_rsa.pub"),
+            os.path.expanduser("~/.ssh/id_ed25519.pub"),
+        ]
+        public_key_path: Optional[str] = os.path.expanduser(
+            self.config.get("public_key_path", "")
+        )
+        if not public_key_path:
+            for pubkey in possible_default_keys:
+                if os.path.exists(pubkey):
+                    self._log.debug(
+                        "No public key path provided, using: %s", pubkey
+                    )
+                    public_key_path = pubkey
+                    break
+            if not public_key_path:
+                raise PycloudlibError(
+                    "No public key path provided and no key found in default locations: "
+                    "'~/.ssh/id_rsa.pub' or '~/.ssh/id_ed25519.pub'"
+                )
+        if not os.path.exists(os.path.expanduser(public_key_path)):
+            raise PycloudlibError(
+                f"Provided public key path '{public_key_path}' does not exist"
+            )
+        if public_key_path not in possible_default_keys:
+            self._log.debug(
+                "Using provided public key path: '%s'", public_key_path
+            )
+        private_key_path = self.config.get("private_key_path", "")
         self.key_pair = KeyPair(
-            public_key_path=os.path.expandvars(
-                self.config.get("public_key_path", f"~{user}/.ssh/id_rsa.pub")
-            ),
-            private_key_path=os.path.expandvars(
-                self.config.get("private_key_path", "")
-            ),
+            public_key_path=public_key_path,
+            private_key_path=private_key_path,
             name=self.config.get("key_name", user),
         )