Hide expected Git errors from stderr

CHANGES.rst

@@ -12,6 +12,8 @@ Fixed
 -----
 - Ensure a full revision range ``--revision <COMMIT_A>..<COMMIT_B>`` where
   COMMIT_B is *not* ``:WORKTREE:`` works too.
+- Hide fatal error from Git on stderr when ``git show`` doesn't find the file in rev1.
+  This isn't fatal from Darker's point of view since it's a newly created file.
 
 
 1.2.4_ - 2021-06-27

src/darker/git.py

@@ -8,7 +8,7 @@
 from datetime import datetime
 from functools import lru_cache
 from pathlib import Path
-from subprocess import CalledProcessError, check_output
+from subprocess import PIPE, CalledProcessError, check_output
 from typing import Iterable, List, Set
 
 from darker.diff import diff_and_get_opcodes, opcodes_to_edit_linenums
@@ -70,6 +70,8 @@ def git_get_content_at_revision(path: Path, revision: str, cwd: Path) -> TextDoc
         )
     except CalledProcessError as exc_info:
         if exc_info.returncode != 128:
+            for error_line in exc_info.stderr.splitlines():
+                logger.error(error_line)
             raise
         # The file didn't exist at the given revision. Act as if it was an empty
         # file, so all current lines appear as edited.
@@ -147,14 +149,21 @@ def _git_check_output_lines(
     """Log command line, run Git, split stdout to lines, exit with 123 on error"""
     logger.debug("[%s]$ %s", cwd, " ".join(cmd))
     try:
-        return check_output(["git"] + cmd, cwd=str(cwd), encoding="utf-8").splitlines()
+        return check_output(
+            ["git"] + cmd, cwd=str(cwd), encoding="utf-8", stderr=PIPE
+        ).splitlines()
     except CalledProcessError as exc_info:
-        if exc_info.returncode == 128 and exit_on_error:
-            # Bad revision or another Git failure. Follow Black's example and return the
-            # error status 123.
-            sys.exit(123)
-        else:
+        if not exit_on_error:
             raise
+        if exc_info.returncode != 128:
+            sys.stderr.write(exc_info.stderr)
+            raise
+
+        # Bad revision or another Git failure. Follow Black's example and return the
+        # error status 123.
+        for error_line in exc_info.stderr.splitlines():
+            logger.error(error_line)
+        sys.exit(123)
 
 
 def git_get_modified_files(

src/darker/tests/test_git.py

@@ -1,11 +1,12 @@
 """Unit tests for :mod:`darker.git`"""
 
-# pylint: disable=redefined-outer-name,protected-access
+# pylint: disable=redefined-outer-name,protected-access,too-many-arguments
 
 import os
+import re
 from datetime import datetime, timedelta
 from pathlib import Path
-from subprocess import CalledProcessError, check_call
+from subprocess import PIPE, CalledProcessError, check_call
 from typing import List, Union
 from unittest.mock import patch
 
@@ -152,7 +153,7 @@ def test_git_get_content_at_revision_git_calls(revision, expect):
         assert check_output.call_count == len(expect)
         for expect_call in expect:
             check_output.assert_any_call(
-                expect_call.split(), cwd="cwd", encoding="utf-8"
+                expect_call.split(), cwd="cwd", encoding="utf-8", stderr=PIPE
             )
 
 
@@ -239,6 +240,71 @@ def test_git_check_output_lines(branched_repo, cmd, exit_on_error, expect_templa
         check(git._git_check_output_lines(cmd, branched_repo.root, exit_on_error))
 
 
+@pytest.mark.kwparametrize(
+    dict(
+        cmd=["show", "{initial}:/.file2"],
+        exit_on_error=True,
+        expect_exc=SystemExit,
+        expect_log=(
+            r"ERROR    darker\.git:git\.py:\d+ fatal: "
+            r"[pP]ath '/\.file2' does not exist in '{initial}'\n$"
+        ),
+    ),
+    dict(
+        cmd=["show", "{initial}:/.file2"],
+        exit_on_error=False,
+        expect_exc=CalledProcessError,
+    ),
+    dict(
+        cmd=["non-existing", "command"],
+        exit_on_error=True,
+        expect_exc=CalledProcessError,
+        expect_stderr="git: 'non-existing' is not a git command. See 'git --help'.\n",
+    ),
+    dict(
+        cmd=["non-existing", "command"],
+        exit_on_error=False,
+        expect_exc=CalledProcessError,
+    ),
+    expect_stderr="",
+    expect_log=r"$",
+)
+def test_git_check_output_lines_stderr_and_log(
+    git_repo, capfd, caplog, cmd, exit_on_error, expect_exc, expect_stderr, expect_log
+):
+    """Git non-existing file error is logged and suppressed from stderr"""
+    git_repo.add({"file1": "file1"}, commit="Initial commit")
+    initial = git_repo.get_hash()[:7]
+    git_repo.add({"file2": "file2"}, commit="Second commit")
+    capfd.readouterr()  # flush captured stdout and stderr
+    cmdline = [s.format(initial=initial) for s in cmd]
+    with pytest.raises(expect_exc):
+
+        git._git_check_output_lines(cmdline, git_repo.root, exit_on_error)
+
+    outerr = capfd.readouterr()
+    assert outerr.out == ""
+    assert outerr.err == expect_stderr
+    expect_log_re = expect_log.format(initial=initial)
+    assert re.match(expect_log_re, caplog.text), repr(caplog.text)
+
+
+def test_git_get_content_at_revision_stderr(git_repo, capfd, caplog):
+    """No stderr or log output from ``git_get_content_at_revision`` for missing file"""
+    git_repo.add({"file1": "file1"}, commit="Initial commit")
+    initial = git_repo.get_hash()[:7]
+    git_repo.add({"file2": "file2"}, commit="Second commit")
+    capfd.readouterr()  # flush captured stdout and stderr
+
+    result = git.git_get_content_at_revision(Path("file2"), initial, git_repo.root)
+
+    assert result == TextDocument()
+    outerr = capfd.readouterr()
+    assert outerr.out == ""
+    assert outerr.err == ""
+    assert caplog.text == ""
+
+
 @pytest.mark.kwparametrize(
     dict(paths=["a.py"], expect=[]),
     dict(expect=[]),