core: find protobuf-src protoc binary dynamically

core/build.rs

@@ -1,15 +1,67 @@
-use std::io::Result;
+use std::{io::Result, ops::Not, path::PathBuf};
+
+/// Figure out where the protoc executable from protobuf-src is, if available.
+fn find_protoc() -> Option<PathBuf> {
+    // protobuf-src doesn't support Windows,
+    // so the external build environment must provide protoc.exe.
+    // https://github.com/MaterializeInc/rust-protobuf-native/issues/4
+    //
+    // Our setup approximates the changes here:
+    // https://github.com/CerebusOSS/ella/pull/14/files
+    //
+    // But there are some problems...
+    //
+    // We might want to use `if cfg!(not(target_os = "windows"))`
+    // to include protobuf-src on supported platforms,
+    // but this doesn't actually work correctly in a build.rs script,
+    // because the target is for the build.rs script, not the final program,
+    // which is important during cross compilation.
+    //
+    // So then we might want to use
+    // `if std::env::var_os("CARGO_CFG_WINDOWS").is_none()` and we can,
+    // but we can't conditionally compile code within the block,
+    // such as conditionally reference protobuf-src.
+    //
+    // So instead, we look around the build directory for the protoc file
+    // :evilgrin:
+    //
+    // This requires that protobuf-src is listed in the build-dependencies
+    // and that it gets built before this script, which seems to be true.
+
+    // like: .../lancelot/target/debug/build/lancelot-f70fef48d1b50e7f/out
+    let out_dir = std::env::var("OUT_DIR").expect("OUT_DIR not present");
+    let out_dir = PathBuf::from(out_dir);
+
+    // like: .../lancelot/target/debug/build
+    let build_dir = &out_dir.parent().unwrap().parent().unwrap();
+
+    for entry in std::fs::read_dir(build_dir).ok()? {
+        if let Ok(entry) = entry {
+            if entry.file_name().to_string_lossy().starts_with("protobuf-src-").not() {
+                continue;
+            }
+
+            let mut path = entry.path();
+            path.push("out");
+            path.push("bin");
+            path.push("protoc");
+
+            if matches!(std::fs::exists(&path), Ok(true)) {
+                return Some(path);
+            }
+        }
+    }
+
+    None
+}
 
 fn main() -> Result<()> {
-    if std::env::var_os("CARGO_CFG_WINDOWS").is_none() {
-        // protobuf-src doesn't support Windows,
-        // so the external build environment must provide protoc.exe.
-        // https://github.com/MaterializeInc/rust-protobuf-native/issues/4
-        //
-        // Our setup approximates the changes here:
-        // https://github.com/CerebusOSS/ella/pull/14/files
-        std::env::set_var("PROTOC", protobuf_src::protoc());
-        std::env::set_var("PROTOC_INCLUDE", protobuf_src::include());
+    if let Some(protoc) = find_protoc() {
+        std::env::set_var("PROTOC", &*protoc.to_string_lossy());
+    } else if std::env::var_os("CARGO_CFG_WINDOWS").is_none() {
+        panic!("expected to find protoc for non-Windows target")
+    } else {
+        // on Windows, external environment is expected to provide protoc.
     }
 
     // this may fail on windows if protoc.exe is not present.