Feature flag for eventually disabling terminal workarounds

We've removed several terminal-specific workarounds but also added
some recently.  Most of the non-Apple issues have been reported
upstream.  Many of our workarounds were only meant to be temporary.
Some workarounds are unreliable and some can cause introduce other
problems.

Add a feature flag we can use later to let users turn off workarounds.

This doesn't disable anything yet, mostly because because despite being
off-by-default, this might surprise people who use "fish_features=all".
The fix would be "fish_features=all,no-omit-term-workarounds".
So we'd want a high degree of confidence.

For now we'll use this only with the next commit.

Closes #11819

CHANGELOG.rst

@@ -1,4 +1,4 @@
-fish ?.?.? (released ???)
+fish 4.3.0 (released ???)
 =========================
 
 Notable improvements and fixes
@@ -11,6 +11,10 @@ Interactive improvements
 ------------------------
 - When typing immediately after starting fish, the first prompt is now rendered correctly.
 
+Improved terminal support
+-------------------------
+- New :ref:`feature flag <featureflags>` ``omit-term-workarounds`` can be turned on to prevent fish from trying to work around incompatible terminals.
+
 For distributors and developers
 -------------------------------
 - ``fish_key_reader`` and ``fish_indent`` are now hardlinks to ``fish``.

doc_src/language.rst

@@ -2027,6 +2027,7 @@ You can see the current list of features via ``status features``::
     mark-prompt             on  4.0 write OSC 133 prompt markers to the terminal
     ignore-terminfo         on  4.1 do not look up $TERM in terminfo database
     query-term              on  4.1 query the TTY to enable extra functionality
+    omit-term-workarounds   off 4.3 skip workarounds for incompatible terminals
 
 Here is what they mean:
 
@@ -2041,6 +2042,7 @@ Here is what they mean:
 - ``query-term`` allows fish to query the terminal by writing escape sequences and reading the terminal's response.
   This enables features such as :ref:`scrolling <term-compat-cursor-position-report>`.
   If you use an incompatible terminal, you can -- for the time being -- work around it by running (once) ``set -Ua fish_features no-query-term``.
+- ``omit-term-workarounds`` prevents fish from trying to work around incompatible terminals.
 
 
 These changes are introduced off by default. They can be enabled on a per session basis::

src/future_feature_flags.rs

@@ -36,6 +36,9 @@ pub enum FeatureFlag {
 
     /// Whether we are allowed to query the TTY for extra information.
     query_term,
+
+    /// Do not try to work around incompatible terminal.
+    omit_term_workarounds,
 }
 
 struct Features {
@@ -140,6 +143,14 @@ pub struct FeatureMetadata {
         default_value: true,
         read_only: false,
     },
+    FeatureMetadata {
+        flag: FeatureFlag::omit_term_workarounds,
+        name: L!("omit-term-workarounds"),
+        groups: L!("4.3"),
+        description: L!("skip workarounds for incompatible terminals"),
+        default_value: false,
+        read_only: false,
+    },
 ];
 
 thread_local!(
@@ -204,6 +215,7 @@ const fn new() -> Self {
                 AtomicBool::new(METADATA[6].default_value),
                 AtomicBool::new(METADATA[7].default_value),
                 AtomicBool::new(METADATA[8].default_value),
+                AtomicBool::new(METADATA[9].default_value),
             ],
         }
     }

src/reader/reader.rs

@@ -75,8 +75,7 @@
 use crate::fd_readable_set::poll_fd_readable;
 use crate::fds::{AutoCloseFd, make_fd_blocking, wopen_cloexec};
 use crate::flog::{FLOG, FLOGF};
-use crate::future_feature_flags;
-use crate::future_feature_flags::FeatureFlag;
+use crate::future_feature_flags::{self, FeatureFlag};
 use crate::global_safety::RelaxedAtomicBool;
 use crate::highlight::{
     HighlightRole, HighlightSpec, autosuggest_validate_from_history, highlight_shell,

tests/checks/status.fish

@@ -55,16 +55,10 @@ eval test_function
 #CHECK: test_function
 
 # Future Feature Flags
-status features
+status features | head -n3
 #CHECK: stderr-nocaret          on  3.0 ^ no longer redirects stderr (historical, can no longer be changed)
 #CHECK: qmark-noglob            on  3.0 ? no longer globs
 #CHECK: regex-easyesc           on  3.1 string replace -r needs fewer \'s
-#CHECK: ampersand-nobg-in-token on  3.4 & only backgrounds if followed by a separator
-#CHECK: remove-percent-self     off 4.0 %self is no longer expanded (use $fish_pid)
-#CHECK: test-require-arg        off 4.0 builtin test requires an argument
-#CHECK: mark-prompt             on  4.0 write OSC 133 prompt markers to the terminal
-#CHECK: ignore-terminfo         on  4.1 do not look up $TERM in terminfo database
-#CHECK: query-term              on  4.1 query the TTY to enable extra functionality
 status test-feature stderr-nocaret
 echo $status
 #CHECK: 0