Perform FD capturing even if the FD is invalid

The `FDCapture`/`FDCaptureBinary` classes, used by `capfd`/`capfdbinary`
fixtures and the `--capture=fd` option (set by default), redirect FDs
1/2 (stdout/stderr) to a temporary file. To do this, they need to save
the old file by duplicating the FD before redirecting it, to be restored
once finished.

Previously, if this duplicating (`os.dup()`) failed, most likely due to
that FD being invalid, the FD redirection would silently not be done. The
FD capturing also performs python-level redirection (monkeypatching
`sys.stdout`/`sys.stderr`) which would still be done, but direct writes
to the FDs would fail.

This is not great. If pytest is run with `--capture=fd`, or a test is
using `capfd`, it expects writes to the FD to work and be captured,
regardless of external circumstances.

So, instead of disabling FD capturing, keep the redirection to a
temporary file, just don't restore it after closing, because there is
nothing to restore to.

changelog/7091.improvement.rst

@@ -0,0 +1,4 @@
+When ``fd`` capturing is used, through ``--capture=fd`` or the ``capfd`` and
+``capfdbinary`` fixtures, and the file descriptor (0, 1, 2) cannot be
+duplicated, FD capturing is still performed. Previously, direct writes to the
+file descriptors would fail or be lost in this case.

src/_pytest/capture.py

@@ -513,14 +513,27 @@ class FDCaptureBinary:
 
     def __init__(self, targetfd, tmpfile=None):
         self.targetfd = targetfd
+
         try:
-            self.targetfd_save = os.dup(self.targetfd)
+            os.fstat(targetfd)
         except OSError:
-            self.start = lambda: None
+            # FD capturing is conceptually simple -- create a temporary file,
-            self.done = lambda: None
+            # redirect the FD to it, redirect back when done. But when the
+            # target FD is invalid it throws a wrench into this loveley scheme.
+            #
+            # Tests themselves shouldn't care if the FD is valid, FD capturing
+            # should work regardless of external circumstances. So falling back
+            # to just sys capturing is not a good option.
+            #
+            # Further complications are the need to support suspend() and the
+            # possibility of FD reuse (e.g. the tmpfile getting the very same
+            # target FD). The following approach is robust, I believe.
+            self.targetfd_invalid = os.open(os.devnull, os.O_RDWR)
+            os.dup2(self.targetfd_invalid, targetfd)
         else:
-            self.start = self._start
+            self.targetfd_invalid = None
-            self.done = self._done
+        self.targetfd_save = os.dup(targetfd)
+
         if targetfd == 0:
             assert not tmpfile, "cannot set tmpfile with stdin"
             tmpfile = open(os.devnull)
@@ -538,24 +551,19 @@ class FDCaptureBinary:
             else:
                 self.syscapture = NoCapture()
         self.tmpfile = tmpfile
-            self.tmpfile_fd = tmpfile.fileno()
 
     def __repr__(self):
-        return "<{} {} oldfd={} _state={!r} tmpfile={}>".format(
+        return "<{} {} oldfd={} _state={!r} tmpfile={!r}>".format(
             self.__class__.__name__,
             self.targetfd,
-            getattr(self, "targetfd_save", "<UNSET>"),
+            self.targetfd_save,
             self._state,
-            hasattr(self, "tmpfile") and repr(self.tmpfile) or "<UNSET>",
+            self.tmpfile,
         )
 
-    def _start(self):
+    def start(self):
         """ Start capturing on targetfd using memorized tmpfile. """
-        try:
+        os.dup2(self.tmpfile.fileno(), self.targetfd)
-            os.fstat(self.targetfd_save)
-        except (AttributeError, OSError):
-            raise ValueError("saved filedescriptor not valid anymore")
-        os.dup2(self.tmpfile_fd, self.targetfd)
         self.syscapture.start()
         self._state = "started"
 
@@ -566,12 +574,15 @@ class FDCaptureBinary:
         self.tmpfile.truncate()
         return res
 
-    def _done(self):
+    def done(self):
         """ stop capturing, restore streams, return original capture file,
         seeked to position zero. """
-        targetfd_save = self.__dict__.pop("targetfd_save")
+        os.dup2(self.targetfd_save, self.targetfd)
-        os.dup2(targetfd_save, self.targetfd)
+        os.close(self.targetfd_save)
-        os.close(targetfd_save)
+        if self.targetfd_invalid is not None:
+            if self.targetfd_invalid != self.targetfd:
+                os.close(self.targetfd)
+            os.close(self.targetfd_invalid)
         self.syscapture.done()
         self.tmpfile.close()
         self._state = "done"
@@ -583,7 +594,7 @@ class FDCaptureBinary:
 
     def resume(self):
         self.syscapture.resume()
-        os.dup2(self.tmpfile_fd, self.targetfd)
+        os.dup2(self.tmpfile.fileno(), self.targetfd)
         self._state = "resumed"
 
     def writeorg(self, data):
@@ -609,8 +620,7 @@ class FDCapture(FDCaptureBinary):
 
     def writeorg(self, data):
         """ write to original file descriptor. """
-        data = data.encode("utf-8")  # XXX use encoding of original stream
+        super().writeorg(data.encode("utf-8"))  # XXX use encoding of original stream
-        os.write(self.targetfd_save, data)
 
 
 class SysCaptureBinary:

testing/test_capture.py

@@ -943,8 +943,8 @@ class TestFDCapture:
             pytest.raises(AttributeError, cap.suspend)
 
             assert repr(cap) == (
-                "<FDCapture 1 oldfd=<UNSET> _state='done' tmpfile={!r}>".format(
+                "<FDCapture 1 oldfd={} _state='done' tmpfile={!r}>".format(
-                    cap.tmpfile
+                    cap.targetfd_save, cap.tmpfile
                 )
             )
             # Should not crash with missing "_old".
@@ -1150,6 +1150,7 @@ class TestStdCaptureFDinvalidFD:
         testdir.makepyfile(
             """
             import os
+            from fnmatch import fnmatch
             from _pytest import capture
 
             def StdCaptureFD(out=True, err=True, in_=True):
@@ -1158,19 +1159,25 @@ class TestStdCaptureFDinvalidFD:
             def test_stdout():
                 os.close(1)
                 cap = StdCaptureFD(out=True, err=False, in_=False)
-                assert repr(cap.out) == "<FDCapture 1 oldfd=<UNSET> _state=None tmpfile=<UNSET>>"
+                assert fnmatch(repr(cap.out), "<FDCapture 1 oldfd=* _state=None tmpfile=*>")
+                cap.start_capturing()
+                os.write(1, b"stdout")
+                assert cap.readouterr() == ("stdout", "")
                 cap.stop_capturing()
 
             def test_stderr():
                 os.close(2)
                 cap = StdCaptureFD(out=False, err=True, in_=False)
-                assert repr(cap.err) == "<FDCapture 2 oldfd=<UNSET> _state=None tmpfile=<UNSET>>"
+                assert fnmatch(repr(cap.err), "<FDCapture 2 oldfd=* _state=None tmpfile=*>")
+                cap.start_capturing()
+                os.write(2, b"stderr")
+                assert cap.readouterr() == ("", "stderr")
                 cap.stop_capturing()
 
             def test_stdin():
                 os.close(0)
                 cap = StdCaptureFD(out=False, err=False, in_=True)
-                assert repr(cap.in_) == "<FDCapture 0 oldfd=<UNSET> _state=None tmpfile=<UNSET>>"
+                assert fnmatch(repr(cap.in_), "<FDCapture 0 oldfd=* _state=None tmpfile=*>")
                 cap.stop_capturing()
         """
         )
@@ -1178,6 +1185,37 @@ class TestStdCaptureFDinvalidFD:
         assert result.ret == 0
         assert result.parseoutcomes()["passed"] == 3
 
+    def test_fdcapture_invalid_fd_with_fd_reuse(self, testdir):
+        with saved_fd(1):
+            os.close(1)
+            cap = capture.FDCaptureBinary(1)
+            cap.start()
+            os.write(1, b"started")
+            cap.suspend()
+            os.write(1, b" suspended")
+            cap.resume()
+            os.write(1, b" resumed")
+            assert cap.snap() == b"started resumed"
+            cap.done()
+            with pytest.raises(OSError):
+                os.write(1, b"done")
+
+    def test_fdcapture_invalid_fd_without_fd_reuse(self, testdir):
+        with saved_fd(1), saved_fd(2):
+            os.close(1)
+            os.close(2)
+            cap = capture.FDCaptureBinary(2)
+            cap.start()
+            os.write(2, b"started")
+            cap.suspend()
+            os.write(2, b" suspended")
+            cap.resume()
+            os.write(2, b" resumed")
+            assert cap.snap() == b"started resumed"
+            cap.done()
+            with pytest.raises(OSError):
+                os.write(2, b"done")
+
 
 def test_capture_not_started_but_reset():
     capsys = StdCapture()