Remove empty-string terminfo capabilities that cause input to be swallowed (#1861)

Programs like less (v691) and bat fail to accept keyboard input (e.g. '/'
for search) when the terminfo database defines string capabilities as
empty strings. The empty strings get loaded into the command parser's
table and incorrectly match any input, silently consuming it.

Remove the six capabilities that were defined with empty values (ka1,
ka3, kc1, kc3, khlp, kund) from the StringCaps array, and add a
defensive filter in terminfo() to skip any remaining empty-valued string
capabilities.

Signed-off-by: Christian Parpart <christian@parpart.family>

Author: christianparpart

metainfo.xml

@@ -107,6 +107,7 @@
     <release version="0.6.3" urgency="medium" type="development">
       <description>
         <ul>
+          <li>Fixes keyboard input being swallowed in programs like less and bat due to empty-string terminfo capabilities (#1861)</li>
           <li>Fixes F3 key not reaching PTY applications when a keybinding matches but its action does not execute, by falling through to the terminal instead of silently consuming the key event</li>
           <li>Fixes F3/Shift+F3 search navigation keybindings firing in any mode instead of only when Search mode is active</li>
           <li>Fixes build failure on Alpine Linux (musl libc) due to missing close_range() function (#1879)</li>

src/vtbackend/Capabilities.cpp

@@ -274,10 +274,6 @@ namespace
         String { Undefined, "kUP5"sv, "\033[1;5A"sv },
         String { Undefined, "kUP6"sv, "\033[1;6A"sv },
         String { Undefined, "kUP7"sv, "\033[1;7A"sv },
-        String { "K1"_tcap, "ka1"sv, ""sv },         // upper left of keypad
-        String { "K3"_tcap, "ka3"sv, ""sv },         // upper right of keypad
-        String { "K4"_tcap, "kc1"sv, ""sv },         // center of keypad
-        String { "K5"_tcap, "kc3"sv, ""sv },         // lower right of keypad
         String { "kl"_tcap, "kcub1"sv, "\033OD"sv }, // app: cursor left
         String { "kd"_tcap, "kcud1"sv, "\033OB"sv }, // app: cursor left
         String { "kr"_tcap, "kcuf1"sv, "\033OC"sv }, // app: cursor right
@@ -347,14 +343,11 @@ namespace
         String { "k7"_tcap, "kf7"sv, "\033[18~"sv },
         String { "k8"_tcap, "kf8"sv, "\033[19~"sv },
         String { "k9"_tcap, "kf9"sv, "\033[20~"sv },
-        String { "%1"_tcap, "khlp"sv, ""sv },
         String { "kh"_tcap, "khome"sv, "\033OH"sv },
         String { "kI"_tcap, "kich1"sv, "\033[2~"sv },
         String { "Km"_tcap, "kmous"sv, "\033[M"sv },
         String { "kN"_tcap, "knp"sv, "\033[6~"sv },
         String { "kP"_tcap, "kpp"sv, "\033[5~"sv },
-        String { "&8"_tcap, "kund"sv, ""sv },
-
         // {{{ Extensions originally introduced by tmux.
         //
         String { "Ss"_tcap, "Ss"sv, "\033[%p1%d q" }, // Set cursor style.
@@ -492,7 +485,7 @@ string StaticDatabase::terminfo() const
             output << "    " << cap.name << "#" << cap.value << ",\n";
 
     for (auto const& cap: strings)
-        if (!cap.name.empty())
+        if (!cap.name.empty() && !cap.value.empty())
             output << "    " << cap.name << "=" << crispy::escape(cap.value, crispy::numeric_escape::Octal)
                    << ",\n";
 

src/vtbackend/Capabilities_test.cpp

@@ -5,7 +5,8 @@
 
 #include <catch2/catch_test_macros.hpp>
 
-#include <format>
+#include <regex>
+#include <string>
 
 using namespace std::string_view_literals;
 using crispy::fromHexString;
@@ -32,3 +33,84 @@ TEST_CASE("Capabilities.get")
     auto const bce = tcap.numericCapability("bce");
     REQUIRE(bce);
 }
+
+// Issue #1861: Empty string terminfo capabilities cause input to be swallowed in programs
+// like less and bat. When capabilities are defined as empty strings (e.g., "ka1=,"),
+// buggy parsers match any input against them.
+TEST_CASE("Capabilities.terminfo_no_empty_string_values", "[issue-1861]")
+{
+    vtbackend::capabilities::StaticDatabase const tcap;
+    auto const terminfo = tcap.terminfo();
+
+    // The regex matches lines like "    name=," where the value after '=' is empty.
+    // In terminfo format, "name=value," defines a string capability.
+    // "name=," means the value is an empty string — this must never appear.
+    auto const emptyValuePattern = std::regex(R"(^\s+(\w+)=,$)", std::regex::multiline);
+
+    auto begin = std::sregex_iterator(terminfo.begin(), terminfo.end(), emptyValuePattern);
+    auto end = std::sregex_iterator();
+
+    std::string emptyCapNames;
+    for (auto it = begin; it != end; ++it)
+    {
+        if (!emptyCapNames.empty())
+            emptyCapNames += ", ";
+        emptyCapNames += (*it)[1].str();
+    }
+
+    INFO("Capabilities with empty string values: " << emptyCapNames);
+    CHECK(emptyCapNames.empty());
+}
+
+// Verify that previously-empty keypad capabilities (ka1, ka3, kc1, kc3) are no longer
+// present in the terminfo output at all — they should be omitted, not set to empty.
+TEST_CASE("Capabilities.keypad_caps_not_in_terminfo", "[issue-1861]")
+{
+    vtbackend::capabilities::StaticDatabase const tcap;
+    auto const terminfo = tcap.terminfo();
+
+    CHECK(terminfo.find("ka1=") == std::string::npos);
+    CHECK(terminfo.find("ka3=") == std::string::npos);
+    CHECK(terminfo.find("kc1=") == std::string::npos);
+    CHECK(terminfo.find("kc3=") == std::string::npos);
+}
+
+// Verify that khlp and kund (which were also empty) are omitted.
+TEST_CASE("Capabilities.help_undo_caps_not_in_terminfo", "[issue-1861]")
+{
+    vtbackend::capabilities::StaticDatabase const tcap;
+    auto const terminfo = tcap.terminfo();
+
+    CHECK(terminfo.find("khlp=") == std::string::npos);
+    CHECK(terminfo.find("kund=") == std::string::npos);
+}
+
+// Verify that non-empty string capabilities are still present in terminfo output.
+TEST_CASE("Capabilities.non_empty_caps_still_present", "[issue-1861]")
+{
+    vtbackend::capabilities::StaticDatabase const tcap;
+    auto const terminfo = tcap.terminfo();
+
+    // These are well-known capabilities that must still be present.
+    CHECK(terminfo.find("bold=") != std::string::npos);
+    CHECK(terminfo.find("clear=") != std::string::npos);
+    CHECK(terminfo.find("kcub1=") != std::string::npos);
+    CHECK(terminfo.find("kcud1=") != std::string::npos);
+    CHECK(terminfo.find("kf1=") != std::string::npos);
+    CHECK(terminfo.find("smkx=") != std::string::npos);
+    CHECK(terminfo.find("rmkx=") != std::string::npos);
+}
+
+// Verify that the stringCapability() API returns an empty view for removed capabilities,
+// confirming they are no longer part of the static database.
+TEST_CASE("Capabilities.removed_caps_return_empty_from_api", "[issue-1861]")
+{
+    vtbackend::capabilities::StaticDatabase const tcap;
+
+    CHECK(tcap.stringCapability("ka1").empty());
+    CHECK(tcap.stringCapability("ka3").empty());
+    CHECK(tcap.stringCapability("kc1").empty());
+    CHECK(tcap.stringCapability("kc3").empty());
+    CHECK(tcap.stringCapability("khlp").empty());
+    CHECK(tcap.stringCapability("kund").empty());
+}