From 2d933009039610a7c29483d248c3b11a053acc75 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Thu, 20 Aug 2026 07:39:14 -0500
Subject: [PATCH] Detect knobs in multi-token build_flags entries and drop a
 zero flash budget with a warning

---
 esphome/arduino8266/toolchain.py               |  5 +++++
 esphome/build_gen/arduino8266.py               | 10 +++++++---
 tests/unit_tests/build_gen/test_arduino8266.py | 10 ++++++++++
 tests/unit_tests/test_arduino8266_toolchain.py |  5 +++++
 4 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/esphome/arduino8266/toolchain.py b/esphome/arduino8266/toolchain.py
index 0d47063b37..2b0785ca3c 100644
--- a/esphome/arduino8266/toolchain.py
+++ b/esphome/arduino8266/toolchain.py
@@ -126,6 +126,11 @@ def _parse_app_size(build_dir: Path) -> int | None:
     app_size = segment_length(ld_text, "irom0_0_seg")
     if app_size is None:
         _LOGGER.warning("irom0_0_seg not found in %s; skipping Flash summary", ld_path)
+    elif app_size == 0:
+        _LOGGER.warning(
+            "irom0_0_seg has zero length in %s; skipping Flash summary", ld_path
+        )
+        return None
     return app_size
 
 
diff --git a/esphome/build_gen/arduino8266.py b/esphome/build_gen/arduino8266.py
index 112f90172a..cbb09057f2 100644
--- a/esphome/build_gen/arduino8266.py
+++ b/esphome/build_gen/arduino8266.py
@@ -176,9 +176,13 @@ def _flag_defines() -> dict[str, str]:
     """Map define name -> full ``NAME[=VALUE]`` for every -D build flag."""
     defines: dict[str, str] = {}
     for flag in CORE.build_flags:
-        if flag.startswith("-D"):
-            body = flag[2:]
-            defines[body.split("=", 1)[0]] = body
+        # Shell-lex multi-token entries the way PlatformIO does, so a knob
+        # in "-DKNOB -DOTHER" is still detected; single tokens pass verbatim
+        # to keep any quoting in their bodies intact.
+        for tok in shlex.split(flag) if " " in flag else (flag,):
+            if tok.startswith("-D"):
+                body = tok[2:]
+                defines[body.split("=", 1)[0]] = body
     return defines
 
 
diff --git a/tests/unit_tests/build_gen/test_arduino8266.py b/tests/unit_tests/build_gen/test_arduino8266.py
index 3ffbf7309d..b656264a01 100644
--- a/tests/unit_tests/build_gen/test_arduino8266.py
+++ b/tests/unit_tests/build_gen/test_arduino8266.py
@@ -597,3 +597,13 @@ def test_project_flags_lexed_entry_scatters_non_linker_tokens() -> None:
     assert link_flags == ["-Wl,-Map=m"]
     assert "stray" in compile_flags
     assert not libs
+
+
+def test_flag_defines_lexes_multi_token_entries() -> None:
+    """A knob inside a multi-token entry is detected like PlatformIO does."""
+    _set_flags("-DPIO_FRAMEWORK_ARDUINO_LWIP2_HIGHER_BANDWIDTH_LOW_FLASH -DFOO=1")
+    defines = _flag_defines()
+    assert "PIO_FRAMEWORK_ARDUINO_LWIP2_HIGHER_BANDWIDTH_LOW_FLASH" in defines
+    assert defines["FOO"] == "FOO=1"
+    config = _resolve_build_config(defines)
+    assert config.lwip_lib == "lwip2-1460"
diff --git a/tests/unit_tests/test_arduino8266_toolchain.py b/tests/unit_tests/test_arduino8266_toolchain.py
index 9cf59c56e1..a0405110ce 100644
--- a/tests/unit_tests/test_arduino8266_toolchain.py
+++ b/tests/unit_tests/test_arduino8266_toolchain.py
@@ -139,6 +139,11 @@ def test_parse_app_size(tmp_path: Path) -> None:
     with patch("esphome.build_gen.arduino8266.get_flash_ld_path", return_value=ld):
         assert toolchain._parse_app_size(tmp_path) is None
 
+    # A zero-length segment is bad data, not a budget; warn and drop it
+    ld.write_text("MEMORY\n{\n  irom0_0_seg :  org = 0x40201010, len = 0x0\n}\n")
+    with patch("esphome.build_gen.arduino8266.get_flash_ld_path", return_value=ld):
+        assert toolchain._parse_app_size(tmp_path) is None
+
     with patch(
         "esphome.build_gen.arduino8266.get_flash_ld_path",
         return_value=tmp_path / "missing.ld",