Fix broken fish completion and multiline help string

Fix regression introduced in 8.4.0 by #3126 and properly fix #3043 and #3502

Author: sitiyou

src/click/shell_completion.py

@@ -181,18 +181,14 @@ def __getattr__(self, name: str) -> t.Any:
 COMP_CWORD=(commandline -t) %(prog_name)s);
 
     for completion in $response;
-        set -l metadata (string split \n $completion);
+        set -l metadata (string split "," $completion);
 
         if test $metadata[1] = "dir";
             __fish_complete_directories $metadata[2];
         else if test $metadata[1] = "file";
             __fish_complete_path $metadata[2];
         else if test $metadata[1] = "plain";
-            if test $metadata[3] != "_";
-                echo $metadata[2]\t$metadata[3];
-            else;
-                echo $metadata[2];
-            end;
+            echo $metadata[2];
         end;
     end;
 end;
@@ -427,15 +423,15 @@ def format_completion(self, item: CompletionItem) -> str:
             Escape newlines in value and help to fix completion errors with
             multi-line help strings.
         """
-        # The fish completion script splits each response line on literal
-        # newlines, so any newline in the value or help would corrupt the
-        # frame. Replace them with the two-character escape "\n" so the text
-        # round-trips through fish without breaking the format. The "_"
-        # sentinel for missing help mirrors :class:`ZshComplete`.
-        help_ = item.help or "_"
-        value = item.value.replace("\n", r"\n")
-        help_escaped = help_.replace("\n", r"\n")
-        return f"{item.type}\n{value}\n{help_escaped}"
+        # According to https://fishshell.com/docs/current/cmds/complete.html
+        # Command substitutions found in ARGUMENTS should return a newline-
+        # separated list of arguments, and each argument may optionally have a tab
+        # character followed by the argument description.
+        if item.help:
+            help_ = item.help.replace("\n", "\\n").replace("\t", " ")
+            return f"{item.type},{item.value}\t{help_}"
+
+        return f"{item.type},{item.value}"
 
 
 ShellCompleteType = t.TypeVar("ShellCompleteType", bound="type[ShellComplete]")

tests/test_shell_completion.py

@@ -359,9 +359,9 @@ def test_full_source(runner, shell):
         ("bash", {"COMP_WORDS": "a b", "COMP_CWORD": "1"}, "plain,b\n"),
         ("zsh", {"COMP_WORDS": "", "COMP_CWORD": "0"}, "plain\na\n_\nplain\nb\nbee\n"),
         ("zsh", {"COMP_WORDS": "a b", "COMP_CWORD": "1"}, "plain\nb\nbee\n"),
-        ("fish", {"COMP_WORDS": "", "COMP_CWORD": ""}, "plain\na\n_\nplain\nb\nbee\n"),
-        ("fish", {"COMP_WORDS": "a b", "COMP_CWORD": "b"}, "plain\nb\nbee\n"),
-        ("fish", {"COMP_WORDS": 'a "b', "COMP_CWORD": '"b'}, "plain\nb\nbee\n"),
+        ("fish", {"COMP_WORDS": "", "COMP_CWORD": ""}, "plain,a\nplain,b\tbee\n"),
+        ("fish", {"COMP_WORDS": "a b", "COMP_CWORD": "b"}, "plain,b\tbee\n"),
+        ("fish", {"COMP_WORDS": 'a "b', "COMP_CWORD": '"b'}, "plain,b\tbee\n"),
     ],
 )
 @pytest.mark.usefixtures("_patch_for_completion")
@@ -576,50 +576,3 @@ def cli(ctx, config_file):
             assert not current_warnings, "There should be no warnings to start"
             _get_completions(cli, args=[], incomplete="")
             assert not current_warnings, "There should be no warnings after either"
-
-
-@pytest.mark.usefixtures("_patch_for_completion")
-def test_fish_multiline_help_complete(runner):
-    """Test Fish completion with multi-line help text doesn't cause errors."""
-    cli = Command(
-        "cli",
-        params=[
-            Option(
-                ["--at", "--attachment-type"],
-                type=(str, str),
-                multiple=True,
-                help=(
-                    "\b\nAttachment with explicit mimetype,\n--at image.jpg image/jpeg"
-                ),
-            ),
-            Option(["--other"], help="Normal help"),
-        ],
-    )
-
-    result = runner.invoke(
-        cli,
-        env={
-            "COMP_WORDS": "cli --",
-            "COMP_CWORD": "--",
-            "_CLI_COMPLETE": "fish_complete",
-        },
-    )
-
-    # Should not fail
-    assert result.exit_code == 0
-
-    # Output should contain escaped newlines, not literal newlines
-    # Fish expects: plain\n--at\n{help_with_\\n}
-    lines = result.output.split("\n")
-
-    # Find the --at completion block (3 lines: type, value, help)
-    for i in range(0, len(lines) - 2, 3):
-        if lines[i] == "plain" and lines[i + 1] in ("--at", "--attachment-type"):
-            help_line = lines[i + 2]
-            # Help should have escaped newlines (\\n), not actual newlines
-            assert "\\n" in help_line
-            # Should contain the example text
-            assert "image.jpg" in help_line.replace("\\n", " ")
-            break
-    else:
-        pytest.fail("--at completion not found in output")