args: force proprietary_codecs + ffmpeg_branding=Chrome via args/common.gn

The chromium_linux-x64x11 config ships proprietary_codecs=false /
ffmpeg_branding="Chromium" (verified via 'gn args --list'), so YouTube's
H.264/AAC fallback fails with DEMUXER_ERROR_NO_SUPPORTED_STREAMS. Add a
shared args/common.gn (layered onto every preset by gen.sh, and by release.sh
for the gold build) that forces the doc-mandated proprietary_codecs=true +
ffmpeg_branding="Chrome". VA-API HW decode (use_vaapi=true) is already on in
the stock config.

Corrects the earlier finding/comment that claimed the platform config sets
proprietary_codecs=true. Updates the CI gn-args guard to allow codec overrides
(reject only true platform/build_type base args).

Author: Properrr

.github/workflows/lint.yml

@@ -40,11 +40,12 @@ jobs:
       - uses: actions/checkout@v4
       - name: gn args sanity
         run: |
-          # args/*.gn are EXTRAS layered on Cobalt's chromium_linux-x64x11 config (which already sets
-          # proprietary_codecs=true; see cobalt/build/configs/common.gn), not standalone arg sets.
-          # Guard against re-introducing conflicting full-config args here.
+          # args/*.gn are EXTRAS layered on Cobalt's chromium_linux-x64x11 config (gen.sh), not
+          # standalone arg sets. Codec overrides (proprietary_codecs/ffmpeg_branding) legitimately
+          # live in args/common.gn. Guard only against re-introducing platform/build_type base args,
+          # which come from gn.py -p/-c and must not be hand-set here.
           for f in args/*.gn; do
-            if grep -qE '^\s*(target_os|target_cpu|is_debug|ffmpeg_branding)\b' "$f"; then
-              echo "$f sets a base-config arg; args/*.gn must contain only overrides"; exit 1
+            if grep -qE '^\s*(target_os|target_cpu|is_debug|is_official_build)\b' "$f"; then
+              echo "$f sets a platform/build_type base arg; args/*.gn must contain only overrides"; exit 1
             fi
           done

.internal/findings/milestones/m114.md

@@ -49,8 +49,13 @@ hermetic clang (llvmorg-17) + Debian bullseye sysroot + Rust. Full checkout ≈
   (unstripped, component build), ~43.9k ninja targets, cold build ~1 h on 32c/64G (no sccache yet).
 - **Blocker RESOLVED:** the in-tree depot_tools python3 bootstrap is now baked into the image
   (`bootstrap_python3` RUN in `docker/Dockerfile`) + an idempotent guard in `bootstrap.sh`.
-- Our `args/{dev,deck,asan}.gn` are now **extras-only** layers (the platform config already sets
-  `proprietary_codecs=true`, target_os/cpu, is_debug); dev/deck are intentionally empty for CI parity.
+- Our `args/{common,dev,deck,asan}.gn` are **extras-only** layers (gen.sh appends common.gn then the
+  preset). **Codec correction (verified via `gn args out/dev --list`):** the platform config ships
+  `proprietary_codecs=false`, `ffmpeg_branding="Chromium"`, `media_use_ffmpeg=true`, `use_vaapi=true`,
+  `use_vaapi_x11=true`, HEVC/Dolby/AC3 off. So YT's H.264/AAC fallback fails
+  (`DEMUXER_ERROR_NO_SUPPORTED_STREAMS`) until we override — `args/common.gn` forces
+  `proprietary_codecs=true` + `ffmpeg_branding="Chrome"` (doc hard constraint). VA-API HW decode is
+  already on in the stock config (good for the Deck).
 
 ## Boot / headless smoke (S0.1) — verified 2026-07-08
 `just smoke dev` passes: `content_shell` boots under Xvfb and serves CDP on :9222. Requirements

args/common.gn

@@ -0,0 +1,9 @@
+# GN args layered ON TOP OF Cobalt's `chromium_linux-x64x11` config for EVERY preset (see gen.sh).
+# Only deckback-wide OVERRIDES belong here; per-preset extras live in args/{dev,deck,asan}.gn.
+#
+# Codec steering (doc hard constraint): the chromium_linux-x64x11 config ships proprietary_codecs=false
+# + ffmpeg_branding="Chromium", which makes YouTube's H.264/AAC fallback formats fail with
+# "DEMUXER_ERROR_NO_SUPPORTED_STREAMS". YT Leanback needs H.264/AAC (and we steer AV1 -> VP9/H.264 in a
+# MediaCapabilities patch, not here), so force the Chrome-branded ffmpeg + proprietary codecs on.
+proprietary_codecs = true
+ffmpeg_branding = "Chrome"

args/dev.gn

@@ -1,8 +1,7 @@
-# Extra GN args layered ON TOP OF Cobalt's `chromium_linux-x64x11` + `devel` config (via
-# cobalt/build/gn.py). This is NOT a standalone arg set — the platform config already provides
-# target_os/cpu, is_debug, symbol_level, proprietary_codecs=true, media_use_ffmpeg=false, etc.
-# (see cobalt/build/configs/common.gn). Put only deckback-specific OVERRIDES here.
+# Extra GN args layered ON TOP OF Cobalt's `chromium_linux-x64x11` + `devel` config AND args/common.gn
+# (via gen.sh). NOT a standalone arg set — the platform config provides target_os/cpu, is_debug,
+# symbol_level, media_use_ffmpeg=true, use_vaapi=true, etc. (verified via `gn args --list`); note it
+# ships proprietary_codecs=FALSE / ffmpeg_branding="Chromium", which args/common.gn overrides.
+# Put only dev-preset-specific OVERRIDES here.
 #
-# Kept empty for the first bring-up build so it matches Cobalt CI exactly. Candidates once the stock
-# build is green: cc_wrapper = "sccache" (faster incremental) and any remote-debugging / codec
-# steering flags we find we need in S0.1/S0.5.
+# Kept empty for now. Candidate: cc_wrapper = "sccache" (faster incremental) once proven.

scripts/gen.sh

@@ -11,10 +11,12 @@ bt="$(preset_buildtype "$preset")"
 info "gn.py -p ${COBALT_PLATFORM} -c ${bt} -> out/${preset} ..."
 in_container bash -c "cd ${CTR_TREE} && python3 cobalt/build/gn.py --no-rbe -p ${COBALT_PLATFORM} -c ${bt} out/${preset}"
 
-extra="$(grep -vE '^\s*#|^\s*$' "args/${preset}.gn" || true)"
+# Layer deckback overrides: args/common.gn (all presets) first, then the preset's own args/<preset>.gn.
+# Both hold only OVERRIDES on top of Cobalt's platform+build_type config — not full standalone sets.
+extra="$( { cat args/common.gn args/"${preset}".gn 2>/dev/null; } | grep -vE '^\s*#|^\s*$' || true)"
 if [ -n "$extra" ]; then
-  info "Layering extra args from args/${preset}.gn ..."
-  printf '\n# --- extra args from deckback args/%s.gn ---\n%s\n' "$preset" "$extra" \
+  info "Layering extra args from args/common.gn + args/${preset}.gn ..."
+  printf '\n# --- deckback extra args (common + %s) ---\n%s\n' "$preset" "$extra" \
     >> "${COBALT_TREE}/out/${preset}/args.gn"
   in_container bash -c "cd ${CTR_TREE} && gn gen out/${preset}"
 fi

scripts/release.sh

@@ -4,7 +4,8 @@ tag="${1:?usage: release.sh <tag>}"
 
 info "Release build (gold + ThinLTO) ..."
 in_container bash -c "cd $CTR_TREE && python3 cobalt/build/gn.py --no-rbe -p $COBALT_PLATFORM -c gold out/release"
-{ grep -vE '^\s*#|^\s*$' args/deck.gn 2>/dev/null || true; echo 'use_thin_lto = true'; } \
+# Same layering as gen.sh (common.gn + deck.gn) plus ThinLTO for the tagged build.
+{ cat args/common.gn args/deck.gn 2>/dev/null | grep -vE '^\s*#|^\s*$'; echo 'use_thin_lto = true'; } \
   >> "$COBALT_TREE/out/release/args.gn"
 in_container bash -c "cd $CTR_TREE && gn gen out/release && autoninja -C out/release $COBALT_TARGET"