Stop trimming whitespace in docstrings

The PEP 257 algorithm used in psf#1053 results in trimming trailing
whitespace in docstrings -- see psf#1415.

Adjust the algorithm to preserve leading and trailing whitespace; this
reverts the changes in psf#1417.  This diverges from PEP 257, but better
retains the contents of the docstring.

Fixes psf#1452 because we no longer can end up with four trailing quotes.

src/black/__init__.py

@@ -6025,14 +6025,13 @@ def _stringify_ast(
 
         else:
             # Constant strings may be indented across newlines, if they are
-            # docstrings; fold spaces after newlines when comparing. Similarly,
-            # trailing and leading space may be removed.
+            # docstrings; fold spaces after newlines when comparing
             if (
                 isinstance(node, ast.Constant)
                 and field == "value"
                 and isinstance(value, str)
             ):
-                normalized = re.sub(r" *\n[ \t]+", "\n ", value).strip()
+                normalized = re.sub(r"\n([ \t]+|(?=\n))", "\n ", value)
             else:
                 normalized = value
             yield f"{'  ' * (depth+2)}{normalized!r},  # {value.__class__.__name__}"
@@ -6440,13 +6439,22 @@ def fix_docstring(docstring: str, prefix: str) -> str:
         if stripped:
             indent = min(indent, len(line) - len(stripped))
     # Remove indentation (first line is special):
-    trimmed = [lines[0].strip()]
-    if indent < sys.maxsize:
+    trimmed = [lines[0]]
+    if indent == sys.maxsize:
+        # We got no indent information from the later lines; they were
+        # all just whitespace.  Preserve the number of newlines.
+        for _ in lines[1:-1]:
+            trimmed.append("")
+        if len(lines) > 1:
+            trimmed.append(prefix)
+    else:
         last_line_idx = len(lines) - 2
         for i, line in enumerate(lines[1:]):
             stripped_line = line[indent:].rstrip()
-            if stripped_line or i == last_line_idx:
-                trimmed.append(prefix + stripped_line)
+            if stripped_line:
+                trimmed.append(prefix + line[indent:])
+            elif i == last_line_idx:
+                trimmed.append(prefix)
             else:
                 trimmed.append("")
     # Return a single string:

tests/data/docstring.py

@@ -73,10 +73,42 @@ def single_line():
     """
     pass
 
+
+def containing_quotes():
+    """No quotes here
+
+    "quotes here" """
+    pass
+
+
+def containing_unbalanced_quotes():
+    """No quotes here
+
+    quote here" """
+    pass
+
+
+def entirely_space():
+    """
+      
+    """
+    pass
+
+
+def just_quote():
+    """ " """
+    pass
+
+
+def escaped_already():
+    """
+    foo\""""
+    pass
+
 # output
 
 class MyClass:
-    """Multiline
+    """ Multiline
     class docstring
     """
 
@@ -88,7 +120,7 @@ def method(self):
 
 
 def foo():
-    """This is a docstring with
+    """This is a docstring with             
     some lines of text here
     """
     return
@@ -146,5 +178,39 @@ def over_indent():
 
 
 def single_line():
-    """But with a newline after it!"""
+    """But with a newline after it!
+
+    """
+    pass
+
+
+def containing_quotes():
+    """No quotes here
+
+    "quotes here" """
+    pass
+
+
+def containing_unbalanced_quotes():
+    """No quotes here
+
+    quote here" """
+    pass
+
+
+def entirely_space():
+    """
+
+    """
+    pass
+
+
+def just_quote():
+    """ " """
+    pass
+
+
+def escaped_already():
+    """
+    foo\""""
     pass