Avoid recreating unchanged global aliases, and use hard links where possible.

Fixes python#184

Author: zooba

src/manage/commands.py

@@ -351,6 +351,10 @@ class BaseCommand:
     show_help = False
 
     def __init__(self, args, root=None):
+        # Storage for command-specific data per-command execution.
+        # All data should use a unique key.
+        self.scratch = {}
+
         cmd_args = {
             k: v for k, v in
             [*CLI_SCHEMA.items(), *CLI_SCHEMA.get(self.CMD, {}).items()]

src/manage/install_command.py

@@ -225,11 +225,12 @@ def _if_exists(launcher, plat):
 
 def _write_alias(cmd, install, alias, target):
     p = (cmd.global_dir / alias["name"])
+    target = Path(target)
     ensure_tree(p)
-    unlink(p)
     launcher = cmd.launcher_exe
     if alias.get("windowed"):
         launcher = cmd.launcherw_exe or launcher
+
     plat = install["tag"].rpartition("-")[-1]
     if plat:
         LOGGER.debug("Checking for launcher for platform -%s", plat)
@@ -247,8 +248,61 @@ def _write_alias(cmd, install, alias, target):
         else:
             LOGGER.debug("Skipping %s alias because the launcher template was not found.", alias["name"])
         return
-    p.write_bytes(launcher.read_bytes())
-    p.with_name(p.name + ".__target__").write_text(str(target), encoding="utf-8")
+
+    launcher_remap = cmd.scratch.setdefault("install_command._write_alias.launcher_remap", {})
+    LOGGER.debug("remap %s", launcher_remap)
+    try:
+        launcher = launcher_remap[str(launcher)]
+    except KeyError:
+        pass
+
+    try:
+        launcher_bytes = launcher.read_bytes()
+    except OSError:
+        warnings_shown = cmd.scratch.setdefault("install_command._write_alias.warnings_shown", set())
+        if str(launcher) not in warnings_shown:
+            LOGGER.warn("Failed to read launcher template at %s.", launcher)
+            warnings_shown.add(str(launcher))
+        LOGGER.debug(exc_info=True)
+        return
+
+    existing_bytes = b''
+    try:
+        with open(p, 'rb') as f:
+            existing_bytes = f.read(len(launcher_bytes) + 1)
+    except FileNotFoundError:
+        pass
+    except OSError:
+        LOGGER.debug("Failed to read existing alias launcher.")
+
+    if existing_bytes != launcher_bytes:
+        # First try and create a hard link
+        unlink(p)
+        try:
+            os.link(launcher, p)
+            LOGGER.debug("Created %s as hard link to %s", p.name, launcher.name)
+        except OSError as ex:
+            if ex.winerror != 17:
+                # Report errors other than cross-drive links
+                LOGGER.debug("Failed to create hard link for command.", exc_info=True)
+            try:
+                p.write_bytes(launcher_bytes)
+                LOGGER.debug("Created %s as copy of %s", p.name, launcher.name)
+                launcher_remap[str(launcher)] = p
+            except OSError:
+                LOGGER.error("Failed to create global command %s.", alias["name"])
+                LOGGER.debug(exc_info=True)
+
+    p_target = p.with_name(p.name + ".__target__")
+    try:
+        if target.match(p_target.read_text(encoding="utf-8")):
+            return
+    except FileNotFoundError:
+        pass
+    except (OSError, UnicodeDecodeError):
+        LOGGER.debug("Failed to read existing target path.", exc_info=True)
+
+    p_target.write_text(str(target), encoding="utf-8")
 
 
 def _create_shortcut_pep514(cmd, install, shortcut):

tests/test_install_command.py

@@ -22,6 +22,7 @@ class Cmd:
         default_platform = "-64"
 
         def __init__(self, platform=None):
+            self.scratch = {}
             if platform:
                 self.default_platform = platform
 
@@ -81,19 +82,19 @@ def check_warm64(self, cmd, tag, name):
 
 
 def test_write_alias_tag_with_platform(alias_checker):
-    alias_checker.check_32(alias_checker.Cmd, "1.0-32", "testA")
-    alias_checker.check_w32(alias_checker.Cmd, "1.0-32", "testB")
-    alias_checker.check_64(alias_checker.Cmd, "1.0-64", "testC")
-    alias_checker.check_w64(alias_checker.Cmd, "1.0-64", "testD")
-    alias_checker.check_arm64(alias_checker.Cmd, "1.0-arm64", "testE")
-    alias_checker.check_warm64(alias_checker.Cmd, "1.0-arm64", "testF")
+    alias_checker.check_32(alias_checker.Cmd(), "1.0-32", "testA")
+    alias_checker.check_w32(alias_checker.Cmd(), "1.0-32", "testB")
+    alias_checker.check_64(alias_checker.Cmd(), "1.0-64", "testC")
+    alias_checker.check_w64(alias_checker.Cmd(), "1.0-64", "testD")
+    alias_checker.check_arm64(alias_checker.Cmd(), "1.0-arm64", "testE")
+    alias_checker.check_warm64(alias_checker.Cmd(), "1.0-arm64", "testF")
 
 
 def test_write_alias_default_platform(alias_checker):
     alias_checker.check_32(alias_checker.Cmd("-32"), "1.0", "testA")
     alias_checker.check_w32(alias_checker.Cmd("-32"), "1.0", "testB")
-    alias_checker.check_64(alias_checker.Cmd, "1.0", "testC")
-    alias_checker.check_w64(alias_checker.Cmd, "1.0", "testD")
+    alias_checker.check_64(alias_checker.Cmd(), "1.0", "testC")
+    alias_checker.check_w64(alias_checker.Cmd(), "1.0", "testD")
     alias_checker.check_arm64(alias_checker.Cmd("-arm64"), "1.0", "testE")
     alias_checker.check_warm64(alias_checker.Cmd("-arm64"), "1.0", "testF")
 
@@ -110,6 +111,7 @@ def test_write_alias_default(alias_checker, monkeypatch, tmp_path, default):
     class Cmd:
         global_dir = Path(tmp_path) / "bin"
         launcher_exe = None
+        scratch = {}
         def get_installs(self):
             return [
                 {
@@ -146,6 +148,7 @@ def write_alias(*a):
 
 def test_print_cli_shortcuts(patched_installs, assert_log, monkeypatch, tmp_path):
     class Cmd:
+        scratch = {}
         global_dir = Path(tmp_path)
         def get_installs(self):
             return installs.get_installs(None)
@@ -163,6 +166,7 @@ def get_installs(self):
 
 def test_print_path_warning(patched_installs, assert_log, tmp_path):
     class Cmd:
+        scratch = {}
         global_dir = Path(tmp_path)
         def get_installs(self):
             return installs.get_installs(None)