- improving handling the case where a child process closes the

  coverage file before writing anything to it (typical of a fork-exec);

src/slipcover/__main__.py

@@ -51,8 +51,11 @@ def get_coverage(sci):
         for f in input_tmpfiles:
             try:
                 fname = f.name
-                f.seek(0)
-                sc.merge_coverage(cov, json.load(f))
+                f.seek(0, os.SEEK_END)
+                # If the file is empty, it was likely closed, possibly upon exec
+                if f.tell() != 0:
+                    f.seek(0)
+                    sc.merge_coverage(cov, json.load(f))
             except json.JSONDecodeError as e:
                 warnings.warn(f"Error reading {fname}: {e}")
             finally:

tests/test_coverage.py

@@ -937,12 +937,12 @@ def test_pytest_forked(tmp_path):
 def test_forked_twice(tmp_path, monkeypatch):
     source = (Path('tests') / 'pyt.py').resolve()
     out = tmp_path / "out.json"
-    script = tmp_path / "foo.py"
 
     monkeypatch.chdir(tmp_path)
     test_file = 't.py'
     Path(test_file).write_text(source.read_text())
 
+    script = tmp_path / "foo.py"
     script.write_text(f"""\
 import os
 import sys
@@ -977,6 +977,48 @@ def test_forked_twice(tmp_path, monkeypatch):
     assert [] == cov['missing_lines']
 
 
+@pytest.mark.skipif(sys.platform == 'win32', reason='fork() and and other functions are Unix-specific')
+def test_fork_close(tmp_path, monkeypatch, capfd):
+    source = (Path('tests') / 'pyt.py').resolve()
+    out = tmp_path / "out.json"
+
+    script = tmp_path / "foo.py"
+    script.write_text("""\
+import os
+import sys
+
+if (pid := os.fork()):
+    pid, status = os.waitpid(pid, 0)
+    if status:
+        if os.WIFSIGNALED(status):
+            exitstatus = os.WTERMSIG(status) + 128
+        else:
+            exitstatus = os.WEXITSTATUS(status)
+    else:
+        exitstatus = 0
+
+    sys.exit(exitstatus)
+else:
+    os.closerange(3, os.sysconf("SC_OPEN_MAX")) #16
+""")
+
+    # don't use capture_output here to let pytest manage/display the output.
+    subprocess.run([sys.executable, '-m', 'slipcover', '--debug', '--json', '--out', str(out), script])
+
+    with out.open() as f:
+        cov = json.load(f)
+
+     # no warnings about not being able to read from subprocess JSON coverage file
+    assert capfd.readouterr().err == ""
+
+    check_summaries(cov)
+
+    script = str(script)
+    assert script in cov['files']
+    cov = cov['files'][script]
+    assert 16 not in  cov['executed_lines']
+
+
 def test_merge_flag(cov_merge_fixture):
     subprocess.run([sys.executable, '-m', 'slipcover', '--branch',
                     '--json', '--out', "a.json", "t.py"], check=True)