add cross_test.sh for the C bindings

This will let us add big endian testing to CI for our C code. (We were
already doing it for our Rust code.)

This is adapted from test_vectors/cross_test.sh. It works around the
limitation that the `cross` tool can't reach parent directories. It's an
unfortunate hack, but at least it's only for testing. It might've been
less hacky to use symlinks for this somehow, but I worry that would
break things on Windows, and I don't want to have to add workarounds for
my workarounds.

.github/workflows/ci.yml

@@ -151,6 +151,10 @@ jobs:
       if: startsWith(matrix.arch, 'armv7-') || startsWith(matrix.arch, 'aarch64-')
     # Test vectors. Note that this uses a hacky script due to path dependency limitations.
     - run: ./test_vectors/cross_test.sh --target ${{ matrix.arch }}
+    # C code. Same issue with the hacky script.
+    - run: ./c/blake3_c_rust_bindings/cross_test.sh --target ${{ matrix.arch }}
+    - run: ./c/blake3_c_rust_bindings/cross_test.sh --target ${{ matrix.arch }} --features=neon
+      if: startsWith(matrix.arch, 'armv7-') || startsWith(matrix.arch, 'aarch64-')
 
   # Currently only on x86.
   c_tests:

c/blake3_c_rust_bindings/build.rs

@@ -48,11 +48,22 @@ fn new_build() -> cc::Build {
     build
 }
 
+fn c_dir_path(filename: &str) -> String {
+    // The `cross` tool doesn't support reading files in parent directories. As a hacky workaround
+    // in `cross_test.sh`, we move the c/ directory around and set BLAKE3_C_DIR_OVERRIDE. Regular
+    // building and testing doesn't require this.
+    if let Ok(c_dir_override) = env::var("BLAKE3_C_DIR_OVERRIDE") {
+        c_dir_override + "/" + filename
+    } else {
+        "../".to_string() + filename
+    }
+}
+
 fn main() -> Result<(), Box<dyn std::error::Error>> {
     let mut base_build = new_build();
-    base_build.file("../blake3.c");
-    base_build.file("../blake3_dispatch.c");
-    base_build.file("../blake3_portable.c");
+    base_build.file(c_dir_path("blake3.c"));
+    base_build.file(c_dir_path("blake3_dispatch.c"));
+    base_build.file(c_dir_path("blake3_portable.c"));
     base_build.compile("blake3_base");
 
     if is_x86_64() && !defined("CARGO_FEATURE_PREFER_INTRINSICS") {
@@ -60,27 +71,27 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         // "prefer_intrinsics" feature is enabled.
         if is_windows_msvc() {
             let mut build = new_build();
-            build.file("../blake3_sse2_x86-64_windows_msvc.asm");
-            build.file("../blake3_sse41_x86-64_windows_msvc.asm");
-            build.file("../blake3_avx2_x86-64_windows_msvc.asm");
-            build.file("../blake3_avx512_x86-64_windows_msvc.asm");
+            build.file(c_dir_path("blake3_sse2_x86-64_windows_msvc.asm"));
+            build.file(c_dir_path("blake3_sse41_x86-64_windows_msvc.asm"));
+            build.file(c_dir_path("blake3_avx2_x86-64_windows_msvc.asm"));
+            build.file(c_dir_path("blake3_avx512_x86-64_windows_msvc.asm"));
             build.compile("blake3_asm");
         } else if is_windows_gnu() {
             let mut build = new_build();
-            build.file("../blake3_sse2_x86-64_windows_gnu.S");
-            build.file("../blake3_sse41_x86-64_windows_gnu.S");
-            build.file("../blake3_avx2_x86-64_windows_gnu.S");
-            build.file("../blake3_avx512_x86-64_windows_gnu.S");
+            build.file(c_dir_path("blake3_sse2_x86-64_windows_gnu.S"));
+            build.file(c_dir_path("blake3_sse41_x86-64_windows_gnu.S"));
+            build.file(c_dir_path("blake3_avx2_x86-64_windows_gnu.S"));
+            build.file(c_dir_path("blake3_avx512_x86-64_windows_gnu.S"));
             build.compile("blake3_asm");
         } else {
             // All non-Windows implementations are assumed to support
             // Linux-style assembly. These files do contain a small
             // explicit workaround for macOS also.
             let mut build = new_build();
-            build.file("../blake3_sse2_x86-64_unix.S");
-            build.file("../blake3_sse41_x86-64_unix.S");
-            build.file("../blake3_avx2_x86-64_unix.S");
-            build.file("../blake3_avx512_x86-64_unix.S");
+            build.file(c_dir_path("blake3_sse2_x86-64_unix.S"));
+            build.file(c_dir_path("blake3_sse41_x86-64_unix.S"));
+            build.file(c_dir_path("blake3_avx2_x86-64_unix.S"));
+            build.file(c_dir_path("blake3_avx512_x86-64_unix.S"));
             build.compile("blake3_asm");
         }
     } else if is_x86_64() || is_x86_32() {
@@ -91,7 +102,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         // extension explicitly enabled in the compiler.
 
         let mut sse2_build = new_build();
-        sse2_build.file("../blake3_sse2.c");
+        sse2_build.file(c_dir_path("blake3_sse2.c"));
         if is_windows_msvc() {
             // /arch:SSE2 is the default on x86 and undefined on x86_64:
             // https://docs.microsoft.com/en-us/cpp/build/reference/arch-x86
@@ -103,7 +114,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         sse2_build.compile("blake3_sse2");
 
         let mut sse41_build = new_build();
-        sse41_build.file("../blake3_sse41.c");
+        sse41_build.file(c_dir_path("blake3_sse41.c"));
         if is_windows_msvc() {
             // /arch:SSE2 is the default on x86 and undefined on x86_64:
             // https://docs.microsoft.com/en-us/cpp/build/reference/arch-x86
@@ -115,7 +126,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         sse41_build.compile("blake3_sse41");
 
         let mut avx2_build = new_build();
-        avx2_build.file("../blake3_avx2.c");
+        avx2_build.file(c_dir_path("blake3_avx2.c"));
         if is_windows_msvc() {
             avx2_build.flag("/arch:AVX2");
         } else {
@@ -124,7 +135,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         avx2_build.compile("blake3_avx2");
 
         let mut avx512_build = new_build();
-        avx512_build.file("../blake3_avx512.c");
+        avx512_build.file(c_dir_path("blake3_avx512.c"));
         if is_windows_msvc() {
             // Note that a lot of versions of MSVC don't support /arch:AVX512,
             // and they'll discard it with a warning, hopefully leading to a
@@ -142,7 +153,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     // you build this crate by hand with the "neon" feature for some reason.
     if defined("CARGO_FEATURE_NEON") {
         let mut neon_build = new_build();
-        neon_build.file("../blake3_neon.c");
+        neon_build.file(c_dir_path("blake3_neon.c"));
         // ARMv7 platforms that support NEON generally need the following
         // flags. AArch64 supports NEON by default and does not support -mpfu.
         if is_armv7() {

c/blake3_c_rust_bindings/cross_test.sh

@@ -0,0 +1,31 @@
+#! /usr/bin/env bash
+
+# This hacky script works around the fact that `cross test` does not support
+# path dependencies. (It uses a docker shared folder to let the guest access
+# project files, so parent directories aren't available.) Solve this problem by
+# copying the entire project to a temp dir and rearranging paths to put "c" and
+# "reference_impl" underneath "blake3_c_rust_bindings", so that everything is
+# accessible. Hopefully this will just run on CI forever and no one will ever
+# read this and discover my deep shame.
+
+set -e -u -o pipefail
+
+project_root="$(realpath "$(dirname "$BASH_SOURCE")/../..")"
+tmpdir="$(mktemp -d)"
+echo "Running cross tests in $tmpdir"
+cd "$tmpdir"
+git clone "$project_root" blake3
+mv blake3/c/blake3_c_rust_bindings .
+mv blake3/reference_impl blake3_c_rust_bindings
+mv blake3/c blake3_c_rust_bindings
+cd blake3_c_rust_bindings
+sed -i 's|reference_impl = { path = "../../reference_impl" }|reference_impl = { path = "reference_impl" }|' Cargo.toml
+
+export BLAKE3_C_DIR_OVERRIDE="./c"
+cat > Cross.toml << EOF
+[build.env]
+passthrough = [
+    "BLAKE3_C_DIR_OVERRIDE",
+]
+EOF
+cross test "$@"