ci(windows): generate MSVC .lib import libraries for ffmpeg DLLs

CI run 26063971492 build-windows passed the openh264 link-test (gendef +
binutils fix landed in de0e71f) but then JamWide's MSVC link failed with
24 unresolved externals: av_packet_unref, av_freep, avcodec_find_decoder,
avcodec_send_packet, etc. — every ffmpeg API.

Root cause is the same shape as the openh264 issue. ffmpeg's MinGW
autotools install produces .dll.a (MinGW-format import libraries) that
MSVC's link.exe rejects for non-trivial decoration. Apply the same
gendef + dlltool fix pattern to every ffmpeg DLL after `make install`.

Script changes (scripts/build_ffmpeg_lgpl.sh, Windows post-process):

1. Consolidate DLLs from $PREFIX/bin/ (MinGW autotools default) into
   $PREFIX/lib/ so cmake/ffmpeg.cmake finds DLLs + import libs in
   one place.

2. For each *.dll in $PREFIX/lib/: gendef → .def, dlltool --output-lib
   → .lib. dlltool emits COFF that MSVC accepts for ffmpeg's C symbols.

3. Strip MinGW .dll.a files — cmake's file(GLOB) would otherwise pick
   them up and MSVC fails on them anyway. The .lib files we generated
   supersede.

cmake/ffmpeg.cmake WIN32 path:

- _lib_glob now signals MSVC link target (.lib) instead of runtime
  binary (.dll). Two file(GLOB) variants per library — Windows ffmpeg
  DLLs lack the `lib` prefix (avcodec-61.lib) while openh264 keeps it
  (libopenh264.lib). Glob both patterns.

- _win32_no_libprefix flag switches the existing glob block to the
  Windows naming convention; macOS/Linux paths untouched.

Same one-shot bundling pattern applies once this lane goes green:
capture libs/ffmpeg/windows-x86_64/ as artifact, commit, strip MSYS2
setup + Vendor LGPL ffmpeg from Windows CI. macOS already done.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mkschulze

cmake/ffmpeg.cmake

@@ -131,8 +131,15 @@ elseif(UNIX AND NOT APPLE)
     set(_have_symlinks TRUE)
 elseif(WIN32)
     set(_ffmpeg_dir "${CMAKE_CURRENT_LIST_DIR}/../libs/ffmpeg/windows-x86_64")
-    set(_lib_glob "*.dll")
+    # MSVC links against .lib import libraries, not .dll directly.
+    # build_ffmpeg_lgpl.sh's Windows post-process generates these via
+    # gendef + dlltool so $PREFIX/lib has both .dll (runtime) and .lib
+    # (link-time). ffmpeg's MinGW autotools convention drops the `lib`
+    # prefix on Windows (e.g. avcodec-61.dll, avcodec-61.lib) — handle
+    # that below by globbing both `lib${name}.*` and `${name}-*.lib`.
+    set(_lib_glob "lib")
     set(_have_symlinks FALSE)
+    set(_win32_no_libprefix TRUE)
 else()
     message(FATAL_ERROR "ffmpeg::lgpl: unsupported platform")
 endif()
@@ -147,11 +154,22 @@ endif()
 
 # Resolve library paths. The version suffixes track the ffmpeg release vendored
 # by scripts/build_ffmpeg_lgpl.sh (currently ffmpeg 7.1.2 + openh264 2.1.1).
-file(GLOB _avcodec_lib  "${_ffmpeg_dir}/lib/libavcodec.${_lib_glob}")
-file(GLOB _avformat_lib "${_ffmpeg_dir}/lib/libavformat.${_lib_glob}")
-file(GLOB _swscale_lib  "${_ffmpeg_dir}/lib/libswscale.${_lib_glob}")
-file(GLOB _avutil_lib   "${_ffmpeg_dir}/lib/libavutil.${_lib_glob}")
-file(GLOB _openh264_lib "${_ffmpeg_dir}/lib/libopenh264.${_lib_glob}")
+if(_win32_no_libprefix)
+    # Windows: ffmpeg DLLs/libs lack the `lib` prefix (avcodec-61.lib) and
+    # are versioned (-61, -7, -8 etc.). Glob both `lib${name}*.lib`
+    # (openh264 keeps the prefix) and `${name}-*.lib` (ffmpeg drops it).
+    file(GLOB _avcodec_lib  "${_ffmpeg_dir}/lib/avcodec-*.lib"  "${_ffmpeg_dir}/lib/libavcodec.lib")
+    file(GLOB _avformat_lib "${_ffmpeg_dir}/lib/avformat-*.lib" "${_ffmpeg_dir}/lib/libavformat.lib")
+    file(GLOB _swscale_lib  "${_ffmpeg_dir}/lib/swscale-*.lib"  "${_ffmpeg_dir}/lib/libswscale.lib")
+    file(GLOB _avutil_lib   "${_ffmpeg_dir}/lib/avutil-*.lib"   "${_ffmpeg_dir}/lib/libavutil.lib")
+    file(GLOB _openh264_lib "${_ffmpeg_dir}/lib/libopenh264*.lib")
+else()
+    file(GLOB _avcodec_lib  "${_ffmpeg_dir}/lib/libavcodec.${_lib_glob}")
+    file(GLOB _avformat_lib "${_ffmpeg_dir}/lib/libavformat.${_lib_glob}")
+    file(GLOB _swscale_lib  "${_ffmpeg_dir}/lib/libswscale.${_lib_glob}")
+    file(GLOB _avutil_lib   "${_ffmpeg_dir}/lib/libavutil.${_lib_glob}")
+    file(GLOB _openh264_lib "${_ffmpeg_dir}/lib/libopenh264.${_lib_glob}")
+endif()
 
 # Filter out symlinks — keep only the versioned canonical files. On Windows
 # (.dll) there are no symlinks, so this is a no-op; on macOS/Linux it

scripts/build_ffmpeg_lgpl.sh

@@ -371,9 +371,60 @@ case "$OS" in
     ;;
 
   MINGW64_NT-*|MSYS_NT-*|CYGWIN_NT-*)
-    # Windows: .dll discovery is via consumer-adjacent path / %PATH%.
-    # No install-name rewriting needed. Just verify the .dll files are present.
-    echo "    (Windows: .dll files in $PREFIX/lib — consumers discover via PATH)"
+    # Windows: two-step post-process.
+    #
+    # 1. ffmpeg's MinGW autotools install puts DLLs in $PREFIX/bin/ (no `lib`
+    #    prefix on Windows by convention — e.g. avcodec-61.dll). Consolidate
+    #    into $PREFIX/lib/ so cmake/ffmpeg.cmake finds DLLs + import libs in
+    #    one place.
+    #
+    # 2. JamWide is built with MSVC, but ffmpeg's MinGW build produces
+    #    .dll.a MinGW-format import libraries — MSVC's link.exe rejects
+    #    those for any non-trivial decoration. Generate MSVC-consumable
+    #    .lib import libraries from each DLL via gendef + dlltool. dlltool
+    #    --output-lib emits COFF that MSVC accepts for the C symbols
+    #    ffmpeg exposes.
+    #    Same fix pattern as the openh264 .lib generation we already do
+    #    further up in this script.
+
+    # Step 1: consolidate DLLs from bin/ into lib/.
+    if [ -d "$PREFIX/bin" ]; then
+        find "$PREFIX/bin" -maxdepth 1 -name "*.dll" -exec mv {} "$PREFIX/lib/" \;
+        # Empty bin/ left over — remove if no other content.
+        rmdir "$PREFIX/bin" 2>/dev/null || true
+    fi
+
+    # Step 2: generate MSVC .lib for every ffmpeg DLL.
+    if ! command -v gendef >/dev/null 2>&1 || ! command -v dlltool >/dev/null 2>&1; then
+        echo "ERROR: gendef + dlltool required for Windows MSVC .lib generation" >&2
+        echo "       install via MSYS2 mingw-w64-x86_64-tools-git + mingw-w64-x86_64-binutils" >&2
+        exit 1
+    fi
+
+    ( cd "$PREFIX/lib" && \
+      for dll in *.dll; do
+        [ -f "$dll" ] || continue
+        # Already converted? skip.
+        stem="${dll%.dll}"
+        if [ -f "${stem}.lib" ]; then continue; fi
+        gendef "$dll" >/dev/null 2>&1
+        if [ -f "${stem}.def" ]; then
+            dlltool --dllname "$dll" --def "${stem}.def" \
+                --output-lib "${stem}.lib"
+            echo "    generated ${stem}.lib from $dll"
+            rm -f "${stem}.def"
+        else
+            echo "WARNING: gendef did not produce ${stem}.def — MSVC link may fail" >&2
+        fi
+      done
+    )
+
+    # Strip MinGW-format .dll.a files — they confuse cmake/ffmpeg.cmake's
+    # file(GLOB) (it'd pick the wrong one) and MSVC doesn't consume them
+    # anyway. The .lib files we just generated supersede.
+    find "$PREFIX/lib" -maxdepth 1 -name "*.dll.a" -delete
+
+    echo "    Windows: $PREFIX/lib has DLLs + MSVC .lib import libraries"
     ;;
 esac