Adds experimental stdin support

Allow ansible-lint to perform linting from stdin when filename
argument '/dev/stdin' or '-' are given.

Keep in mind:
- no argument still means auto-detect as I found no reliable way
  to identify that the tool was used with shell pipes
- filename reported is 'stdin', same as flake8 does
- received content is assumed to be a playbook, you cannot lint
  a task file this way. Others are welcomed to propose improvements
  for detecting the file type based on its content, because we do
  not have any `filename` available.

src/ansiblelint/file_utils.py

@@ -3,10 +3,12 @@
 import logging
 import os
 import subprocess
+import sys
 from argparse import Namespace
 from collections import OrderedDict
 from contextlib import contextmanager
 from pathlib import Path
+from tempfile import NamedTemporaryFile
 from typing import TYPE_CHECKING, Any, Dict, Iterator, List, Optional, Set, Union
 
 import wcmatch.pathlib
@@ -83,6 +85,11 @@ def kind_from_path(path: Path):
                 return k
     if path.is_dir():
         return "role"
+
+    if str(path) == '/dev/stdin':
+        # --stdin-display-name=stdin (flake8)
+        return "playbook"
+    # print(path.absolute)
     raise RuntimeError("Unable to determine file type for %s" % pathex)
 
 
@@ -100,6 +107,10 @@ def __init__(
         kind: Optional[FileType] = None,
     ):
         """Create a Lintable instance."""
+        # Filename is effective file on disk, for stdin is a namedtempfile
+        self.filename: str = str(name)
+        self.dir: str = ""
+
         if isinstance(name, str):
             self.name = normpath(name)
             self.path = Path(self.name)
@@ -118,12 +129,25 @@ def __init__(
             if role.exists:
                 self.role = role.name
 
-        self.kind = kind or kind_from_path(self.path)
-        # We store absolute directory in dir
-        if self.kind == "role":
-            self.dir = str(self.path.resolve())
+        if str(self.path) in ['/dev/stdin', '-']:
+            self.file = NamedTemporaryFile(mode="w+", suffix="playbook.yml")
+            self.filename = self.file.name
+            self._content = sys.stdin.read()
+            self.file.write(self._content)
+            self.file.flush()
+            self.path = Path(self.file.name)
+            self.name = 'stdin'
+            self.kind = 'playbook'
+            self.dir = '/'
+            # print(555, self.name, self.path, self.filename)
         else:
-            self.dir = str(self.path.parent.resolve())
+            self.kind = kind or kind_from_path(self.path)
+        # We store absolute directory in dir
+        if not self.dir:
+            if self.kind == "role":
+                self.dir = str(self.path.resolve())
+            else:
+                self.dir = str(self.path.parent.resolve())
 
     def __getitem__(self, item):
         """Provide compatibility subscriptable support."""

src/ansiblelint/runner.py

@@ -41,7 +41,7 @@ def __init__(
         skip_list: List[str] = [],
         exclude_paths: List[str] = [],
         verbosity: int = 0,
-        checked_files: Optional[Set[str]] = None
+        checked_files: Optional[Set[Lintable]] = None
     ) -> None:
         """Initialize a Runner instance."""
         self.rules = rules
@@ -127,7 +127,7 @@ def worker(lintable: Lintable) -> List[MatchError]:
             files = [value for n, value in enumerate(files) if value not in files[:n]]
 
             for file in self.lintables:
-                if str(file.path) in self.checked_files:
+                if file in self.checked_files:
                     continue
                 _logger.debug(
                     "Examining %s of type %s",
@@ -140,7 +140,7 @@ def worker(lintable: Lintable) -> List[MatchError]:
                 )
 
         # update list of checked files
-        self.checked_files.update([str(x.path) for x in self.lintables])
+        self.checked_files.update(self.lintables)
 
         # remove any matches made inside excluded files
         matches = list(
@@ -189,4 +189,12 @@ def _get_matches(rules: "RulesCollection", options: "Namespace") -> LintResult:
     # Assure we do not print duplicates and the order is consistent
     matches = sorted(set(matches))
 
+    # Convert reported filenames into human redable ones, so we hide the
+    # fact we used temporary files when processing input from stdin.
+    for match in matches:
+        for lintable in lintables:
+            if match.filename == lintable.filename:
+                match.filename = lintable.name
+                break
+
     return LintResult(matches=matches, files=checked_files)