prpr-avc 链接 zlib 的去留？

[YuevUwU]

(仅简单提及下此事，有点语无伦次了...)
(这个问题涉及到 phira、prpr-avc-ffmpeg 的实作决策，以及构建者是否应该转过去用 prpr-avc-ffmpeg)
(主要是 cross-build Windows target 会发生的事)

起因

prpr-avc-ffmpeg 上沒有提供 zlib.a，如果把 prpr-avc/build.rs 的 z 連結拔掉，Phira 仍然能編譯能跑，就是不知道 libz 在 prpr-avc 的作用是什麼

背景

Since d2c1631 (and released since v0.4.0)
zlib is required to link statically or dynamically. (line 8)

phira/prpr-avc/build.rs

Lines 3 to 10 in 2ff63b3

	fn main() {
	let libs_dir = std::env::var("PRPR_AVC_LIBS").unwrap_or_else(|_| format!("{}/static-lib", std::env::var("CARGO_MANIFEST_DIR").unwrap()));
	let libs_path = Path::new(&libs_dir).join(std::env::var("TARGET").unwrap());
	let libs_path = libs_path.display();
	println!("cargo:rustc-link-search={libs_path}");
	println!("cargo:rustc-link-lib=z");
	println!("cargo:rerun-if-changed={libs_path}");
	}

一个合理的猜测是 v0.4.0 时期链接的 FFmpeg 启用了 zlib

不过实际上无论是下面的 phira-docs 源还是 prpr-avc-ffmpeg 源，nm -A 下来都不包含 *flate* 的符号
但仅有 MSVC 有点小意外 ("this file requires zlib support")

alan54692@MSI:~/phira$ nm -A -C auto prpr-avc/static-lib.new/*/lib*.a 2>/dev/null | grep -iE "flate|zlib|libz|gzip"
prpr-avc/static-lib.new/x86_64-pc-windows-msvc/libavcodec.a:libavcodec/version.o:0000000000000000 R ??_C@_0COB@LLAHAHAF@?9?9disable?9bzlib?5?9?9disable?9debug@
prpr-avc/static-lib.new/x86_64-pc-windows-msvc/libavcodec.a:libavcodec/codec_desc.o:0000000000000000 R ??_C@_04OLICEBLN@zlib@
prpr-avc/static-lib.new/x86_64-pc-windows-msvc/libavformat.a:libavformat/version.o:0000000000000000 R ??_C@_0COB@LLAHAHAF@?9?9disable?9bzlib?5?9?9disable?9debug@
prpr-avc/static-lib.new/x86_64-pc-windows-msvc/libavformat.a:libavformat/mov.o:0000000000000000 R ??_C@_0CN@CIMOMHAN@this?5file?5requires?5zlib?5support@
prpr-avc/static-lib.new/x86_64-pc-windows-msvc/libavutil.a:libavutil/version.o:0000000000000000 R ??_C@_0COB@LLAHAHAF@?9?9disable?9bzlib?5?9?9disable?9debug@
prpr-avc/static-lib.new/x86_64-pc-windows-msvc/libswresample.a:libswresample/version.o:0000000000000000 R ??_C@_0COB@LLAHAHAF@?9?9disable?9bzlib?5?9?9disable?9debug@
prpr-avc/static-lib.new/x86_64-pc-windows-msvc/libswscale.a:libswscale/version.o:0000000000000000 R ??_C@_0COB@LLAHAHAF@?9?9disable?9bzlib?5?9?9disable?9debug@

(更新) GNU 發現了同樣的提示字串

alan54692@MSI:~/phira$ strings ./prpr-avc/static-lib.new/x86_64-pc-windows-gnu/*.a | grep -i "zlib" | sort | uniq
--disable-bzlib --disable-debug --disable-doc --disable-programs --disable-shared --disable-vulkan --disable-vaapi --disable-hwaccels --disable-autodetect --enable-static --disable-everything --enable-decoder='h264,hevc,vp9,av1,aac,aac_latm,flac,mp3,wav,vorbis,opus' --enable-encoder='h264,rawvideo' --enable-demuxer='h264,hevc,aac,flv,mov,mpegts,flac,wav,mp3,matroska,ogg' --enable-muxer='h264,adts,rawvideo,flac,mp3,wav,matroska' --enable-parser='h264,hevc,aac,mpegaudio,vp9,opus' --enable-protocol='file,pipe' --enable-bsf=h264_mp4toannexb --enable-swscale --enable-swscale-alpha --enable-filter=scale --arch=x86_64 --target-os=mingw64 --enable-cross-compile --cross-prefix=x86_64-w64-mingw32- --prefix=/home/runner/work/prpr-avc-ffmpeg/prpr-avc-ffmpeg/build/x86_64-pc-windows-gnu/install
LCL (LossLess Codec Library) ZLIB
ZLIB
this file requires zlib support compiled in
zlib
alan54692@MSI:~/phira$ strings ./prpr-avc/static-lib.new/x86_64-pc-windows-msvc/*.a | grep -i "zlib" | sort | uniq
--disable-bzlib --disable-debug --disable-doc --disable-programs --disable-shared --disable-vulkan --disable-vaapi --disable-hwaccels --disable-autodetect --enable-static --disable-everything --enable-decoder='h264,hevc,vp9,av1,aac,aac_latm,flac,mp3,wav,vorbis,opus' --enable-encoder='h264,rawvideo' --enable-demuxer='h264,hevc,aac,flv,mov,mpegts,flac,wav,mp3,matroska,ogg' --enable-muxer='h264,adts,rawvideo,flac,mp3,wav,matroska' --enable-parser='h264,hevc,aac,mpegaudio,vp9,opus' --enable-protocol='file,pipe' --enable-bsf=h264_mp4toannexb --enable-swscale --enable-swscale-alpha --enable-filter=scale --arch=x86_64 --target-os=win64 --toolchain=msvc --prefix=/d/a/prpr-avc-ffmpeg/prpr-avc-ffmpeg/build/x86_64-pc-windows-msvc/install
??_C@_04OLICEBLN@zlib@
??_C@_0CN@CIMOMHAN@this?5file?5requires?5zlib?5support@
??_C@_0COB@LLAHAHAF@?9?9disable?9bzlib?5?9?9disable?9debug@
LCL (LossLess Codec Library) ZLIB
ZLIB
this file requires zlib support compiled in
zlib
alan54692@MSI:~/phira$ strings ./prpr-avc/static-lib.new/x86_64-unknown-linux-gnu/*.a | grep -i "zlib" | sort | uniq
--disable-bzlib --disable-debug --disable-doc --disable-programs --disable-shared --disable-vulkan --disable-vaapi --disable-hwaccels --disable-autodetect --enable-static --disable-everything --enable-decoder='h264,hevc,vp9,av1,aac,aac_latm,flac,mp3,wav,vorbis,opus' --enable-encoder='h264,rawvideo' --enable-demuxer='h264,hevc,aac,flv,mov,mpegts,flac,wav,mp3,matroska,ogg' --enable-muxer='h264,adts,rawvideo,flac,mp3,wav,matroska' --enable-parser='h264,hevc,aac,mpegaudio,vp9,opus' --enable-protocol='file,pipe' --enable-bsf=h264_mp4toannexb --enable-swscale --enable-swscale-alpha --enable-filter=scale --arch=x86_64 --target-os=linux --cc=gcc --cxx=g++ --ld=gcc --as=as --prefix=/home/runner/work/prpr-avc-ffmpeg/prpr-avc-ffmpeg/build/x86_64-unknown-linux-gnu/install
LCL (LossLess Codec Library) ZLIB
ZLIB
this file requires zlib support compiled in
zlib

phira-docs

static-lib.zip in phira-docs 提供: (old blob in repo) (md with mirror maintained by NuanRMxi)

• prpr-avc/static-lib/x86_64-pc-windows-gnu/libz.a (initially by Mivik in What about detailed compilation guide? #271 (comment))
• prpr-avc/static-lib/x86_64-pc-windows-msvc/z.lib (by 9RayAlkalin)

许多 Linux 系统可能已经包含了 libz.so 运行时共享库​​，因此无需特别提供
Windows 系统默认情况下不包含 zlib，需要它们来额外做静态链接，除非 PATH 环境变量中包含 MSYS2 和 Strawberry (Perl) 之类的程序

prpr-avc-ffmpeg

prpr-avc-ffmpeg 仅提供 ffmpeg 构建，并且未找到 libz.a。
如果您使用 WSL 或 cross-build x86_64-pc-windows-gnu，则会引发 note: /usr/bin/x86_64-w64-mingw32-ld: cannot find -lz: No such file or directory。
接着，如果你注释掉/删除 println!("cargo:rustc-link-lib=z");，它就能成功构建并启动游戏了！？（但可靠吗？）

But who uses zlib?

Details

According to "zlib-rs" in Cargo.lock, we can find flate2.
But since flate2-1.0.12, they use pure-rust miniz_oxide crate by default,
and the commit uses flate2-1.0.26 supplied for other crates, and libz-sys is only required when enabling at least zlib feature (use platform-provided zlib).

So...try to build Phira on d2c1631 ? Let's add these patches to let build work, without add real features, refer to [Compare d2c1631 & v0.4.0] and [Compare v0.6.1 & v0.6.2]

Patch

diff --git a/prpr-avc/build.rs b/prpr-avc/build.rs
index aae5a35..eca55b7 100644
--- a/prpr-avc/build.rs
+++ b/prpr-avc/build.rs
@@ -1,4 +1,6 @@
 fn main() {
+    let lib_path = format!("{}/static-lib/{}", std::env::var("CARGO_MANIFEST_DIR").unwrap(), std::env::var("TARGET").unwrap());
     println!("cargo:rustc-link-search={}/static-lib/{}", std::env::var("CARGO_MANIFEST_DIR").unwrap(), std::env::var("CARGO_CFG_TARGET_OS").unwrap());
-    println!("cargo:rustc-link-lib=z");
+    // println!("cargo:rustc-link-lib=z");
+    println!("cargo:rustc-link-search={lib_path}");
 }
diff --git a/prpr-avc/src/ffi.rs b/prpr-avc/src/ffi.rs
index 6032a4a..32a180e 100644
--- a/prpr-avc/src/ffi.rs
+++ b/prpr-avc/src/ffi.rs
@@ -35,6 +35,31 @@ extern "C" {
     pub fn avcodec_default_get_format(s: *mut AVCodecContext, fmt: *const AVPixelFormat) -> AVPixelFormat;
 }

+#[link(name = "swresample", kind = "static")]
+extern "C" {
+    pub fn swr_alloc_set_opts(
+        s: *mut SwrContext,
+        out_ch_layout: i64,
+        out_sample_fmt: AVSampleFormat,
+        out_sample_rate: ::std::os::raw::c_int,
+        in_ch_layout: i64,
+        in_sample_fmt: AVSampleFormat,
+        in_sample_rate: ::std::os::raw::c_int,
+        log_offset: ::std::os::raw::c_int,
+        log_ctx: *mut ::std::os::raw::c_void,
+    ) -> *mut SwrContext;
+    pub fn swr_init(s: *mut SwrContext) -> ::std::os::raw::c_int;
+    pub fn swr_get_delay(s: *const SwrContext, base: ::std::os::raw::c_int) -> i64;
+    pub fn swr_convert(
+        s: *mut SwrContext,
+        out: *mut *mut u8,
+        out_count: ::std::os::raw::c_int,
+        in_: *const *const u8,
+        in_count: ::std::os::raw::c_int,
+    ) -> ::std::os::raw::c_int;
+    pub fn swr_free(s: *mut *mut SwrContext);
+}
+
 #[link(name = "swscale", kind = "static")]
 extern "C" {
     pub fn sws_getContext(
@@ -78,6 +103,7 @@ pub type AVSampleFormat = ::std::os::raw::c_int;

 pub type AVCodec = c_void;
 pub type SwsContext = c_void;
+pub type SwrContext = c_void;

 pub const AV_ERROR_MAX_STRING_SIZE: u32 = 64;
 pub const SWS_BICUBIC: u32 = 4;

~/phira$ cargo +nightly-2023-02-01 build --target x86_64-pc-windows-gnu --bin phira-main
    Finished dev [unoptimized + debuginfo] target(s) in 0.24s

It works! It didn't actively find libz.so or not required or it built for itself (?)

Hmm...maybe someone use it in runtime or hide in phira code..?
or FFmpeg use it?

决策问题

现在麻烦的是，缺少了 zlib 会不会影响核心功能之类的
即，是否应该在 prpr-avc-ffmpeg 预构建一个 zlib

另外 phira-docs 如果要迁移到使用 prpr-avc-ffmpeg 后
在 prpr-avc-ffmpeg 提供 libz.a 之前，是变成注释那行，还是单独补上好的 libz.a，还是仍然使用现有 static-lib.zip 镜像？
（在确实有风险考量时，从 prpr-avc 移除 linking 然后叫能补上的补上好像也不太对）
以及是否有必要在静态库加上 zlib 编译指南，至少对于现在的 Phira 来说找不到 zlib 是个硬错误
~~也算是给用户自己编译的FFmpeg上的保险？~~但这是运行时的错误信息而已，也不会panic什么的，它们自己知道自己加也行(?)

[NuanRMxi]

实验性！文档后期修改！zlib晚点问问mivik！总之我那边托管的静态库不会下掉（兼容工作流），现在prpr-avc里面也留了拉取静态库的脚本，文档不会多难写的！

[9RayAlkalin]

Actually z.lib in msvc static lib is by me

[YuevUwU]

理论上有zlib-rs了就不需要zlib的静态库参与了（？

v0.4.0 的 flate2 依照文檔老早就加入沒有zlib的話就自動編譯了(1.0.26的時候預設也不需要C編譯了)，或預設存在其他替代後端
那 zlib 還能用在哪，沒道理啊，whyyy?

[9RayAlkalin]

检查一圈好像只有 .mov 会动到zlib：

jasonli@Alkalins-MacBook-Air phira % nm -A -C auto prpr-avc/static-lib/*/lib*.a 2>/dev/null | grep -iE "flate|zlib|libz|gzip"
…
prpr-avc/static-lib/aarch64-pc-windows-msvc/libavformat.a:libavformat\mov.o: 00000000 R ??_C@_0CN@CIMOMHAN@this?5file?5requires?5zlib?5support@
…
prpr-avc/static-lib/x86_64-pc-windows-msvc/libavformat.a:libavformat\mov.o: 00000000 R ??_C@_0CN@CIMOMHAN@this?5file?5requires?5zlib?5support@

[YuevUwU]

strings x86_64-pc-windows-gnu/libavformat.a | grep "zlib" 也會出現這個警告字串
所以全部編譯產物都有

[9RayAlkalin]

唯一解释可能就是环境变量（摊手
算了事已至此可能还是保留zlib好一点

[YuevUwU]

在更新之前暂且不知道文档该怎么写了，目前 Windows 教程已经迁到 prpr-avc-ffmpeg 了