Fix: reload and merge easy-install pth file before save

changelog.d/3128.change.rst

@@ -0,0 +1 @@
+In deprecated easy_install, reload and merge the pth file before saving.

setuptools/command/easy_install.py

@@ -1567,50 +1567,79 @@ def get_exe_prefixes(exe_filename):
 class PthDistributions(Environment):
     """A .pth file with Distribution paths in it"""
 
-    dirty = False
-
     def __init__(self, filename, sitedirs=()):
         self.filename = filename
         self.sitedirs = list(map(normalize_path, sitedirs))
         self.basedir = normalize_path(os.path.dirname(self.filename))
-        self._load()
+        self.paths, self.dirty = self._load()
+        # keep a copy if someone manually updates the paths attribute on the instance
+        self._init_paths = self.paths[:]
         super().__init__([], None, None)
         for path in yield_lines(self.paths):
             list(map(self.add, find_distributions(path, True)))
 
-    def _load(self):
-        self.paths = []
-        saw_import = False
+    def _load_raw(self):
+        paths = []
+        dirty = saw_import = False
         seen = dict.fromkeys(self.sitedirs)
-        if os.path.isfile(self.filename):
-            f = open(self.filename, 'rt')
-            for line in f:
-                if line.startswith('import'):
-                    saw_import = True
-                    continue
-                path = line.rstrip()
-                self.paths.append(path)
-                if not path.strip() or path.strip().startswith('#'):
-                    continue
-                # skip non-existent paths, in case somebody deleted a package
-                # manually, and duplicate paths as well
-                path = self.paths[-1] = normalize_path(
-                    os.path.join(self.basedir, path)
-                )
-                if not os.path.exists(path) or path in seen:
-                    self.paths.pop()  # skip it
-                    self.dirty = True  # we cleaned up, so we're dirty now :)
-                    continue
-                seen[path] = 1
-            f.close()
+        f = open(self.filename, 'rt')
+        for line in f:
+            path = line.rstrip()
+            # still keep imports and empty/commented lines for formatting
+            paths.append(path)
+            if line.startswith(('import ', 'from ')):
+                saw_import = True
+                continue
+            stripped_path = path.strip()
+            if not stripped_path or stripped_path.startswith('#'):
+                continue
+            # skip non-existent paths, in case somebody deleted a package
+            # manually, and duplicate paths as well
+            normalized_path = normalize_path(os.path.join(self.basedir, path))
+            if normalized_path in seen or not os.path.exists(normalized_path):
+                log.debug("cleaned up dirty or duplicated %r", path)
+                dirty = True
+                paths.pop()
+                continue
+            seen[normalized_path] = 1
+        f.close()
+        # remove any trailing empty/blank line
+        while paths and not paths[-1].strip():
+            paths.pop()
+            dirty = True
+        return paths, dirty or (paths and saw_import)
 
-        if self.paths and not saw_import:
-            self.dirty = True  # ensure anything we touch has import wrappers
-        while self.paths and not self.paths[-1].strip():
-            self.paths.pop()
+    def _load(self):
+        if os.path.isfile(self.filename):
+            return self._load_raw()
+        return [], False
 
     def save(self):
         """Write changed .pth file back to disk"""
+        # first reload the file
+        last_paths, last_dirty = self._load()
+        # and check that there are no difference with what we have.
+        # there can be difference if someone else has written to the file
+        # since we first loaded it.
+        # we don't want to lose the eventual new paths added since then.
+        for path in last_paths[:]:
+            if path not in self.paths:
+                self.paths.append(path)
+                log.info("detected new path %r", path)
+                last_dirty = True
+            else:
+                last_paths.remove(path)
+        # also, re-check that all paths are still valid before saving them
+        for path in self.paths[:]:
+            if path not in last_paths \
+                    and not path.startswith(('import ', 'from ', '#')):
+                absolute_path = os.path.join(self.basedir, path)
+                if not os.path.exists(absolute_path):
+                    self.paths.remove(path)
+                    log.info("removing now non-existent path %r", path)
+                    last_dirty = True
+
+        self.dirty |= last_dirty or self.paths != self._init_paths
         if not self.dirty:
             return
 
@@ -1619,17 +1648,16 @@ def save(self):
             log.debug("Saving %s", self.filename)
             lines = self._wrap_lines(rel_paths)
             data = '\n'.join(lines) + '\n'
-
             if os.path.islink(self.filename):
                 os.unlink(self.filename)
             with open(self.filename, 'wt') as f:
                 f.write(data)
-
         elif os.path.exists(self.filename):
             log.debug("Deleting empty %s", self.filename)
             os.unlink(self.filename)
 
         self.dirty = False
+        self._init_paths[:] = self.paths[:]
 
     @staticmethod
     def _wrap_lines(lines):

setuptools/tests/test_easy_install.py

@@ -337,6 +337,49 @@ def test_add_from_site_is_ignored(self):
         pth.add(PRDistribution(location))
         assert not pth.dirty
 
+    def test_many_pth_distributions_merge_together(self, tmpdir):
+        """
+        If the pth file is modified under the hood, then PthDistribution
+        will refresh its content before saving, merging contents when
+        necessary.
+        """
+        # putting the pth file in a dedicated sub-folder,
+        pth_subdir = tmpdir.join("pth_subdir")
+        pth_subdir.mkdir()
+        pth_path = str(pth_subdir.join("file1.pth"))
+        pth1 = PthDistributions(pth_path)
+        pth2 = PthDistributions(pth_path)
+        assert (
+            pth1.paths == pth2.paths == []
+        ), "unless there would be some default added at some point"
+        # and so putting the src_subdir in folder distinct than the pth one,
+        # so to keep it absolute by PthDistributions
+        new_src_path = tmpdir.join("src_subdir")
+        new_src_path.mkdir()  # must exist to be accounted
+        new_src_path_str = str(new_src_path)
+        pth1.paths.append(new_src_path_str)
+        pth1.save()
+        assert (
+            pth1.paths
+        ), "the new_src_path added must still be present/valid in pth1 after save"
+        # now,
+        assert (
+            new_src_path_str not in pth2.paths
+        ), "right before we save the entry should still not be present"
+        pth2.save()
+        assert (
+            new_src_path_str in pth2.paths
+        ), "the new_src_path entry should have been added by pth2 with its save() call"
+        assert pth2.paths[-1] == new_src_path, (
+            "and it should match exactly on the last entry actually "
+            "given we append to it in save()"
+        )
+        # finally,
+        assert PthDistributions(pth_path).paths == pth2.paths, (
+            "and we should have the exact same list at the end "
+            "with a fresh PthDistributions instance"
+        )
+
 
 @pytest.fixture
 def setup_context(tmpdir):