cargo-apk: Work around missing libgcc on NDK r23 with linker script

Rust still searches for libgcc even though [85806] replaces internal use
with libunwind, especially now that the Android NDK (since r23-beta3)
doesn't ship with any of gcc anymore.  The apparent solution is to build
your application with nightly and compile std locally (`-Zbuild-std`),
but that is not desired for the majority of users.  [7339] suggests to
provide a local `libgcc.a` as linker script, which simply redirects
linking to `libunwind` instead - and that has proven to work fine so
far.

Intead of shipping this file with the crate or writing it to an existing
link-search directory on the system, we write it to a new directory that
can be easily passed or removed to `rustc`, say in the event that a user
switches to an older NDK and builds without cleaning.  For this we need
to switch from `cargo build` to `cargo rustc`, but the existing
arguments and desired workflow remain identical.

Author: MarijnS95

cargo-apk/CHANGELOG.md

@@ -1,6 +1,8 @@
 # Unreleased
 
 - Fixed the library name in case of multiple build artifacts in the Android manifest.
+- Work around missing `libgcc` on NDK r23 beta 3 and above, by providing linker script that "redirects" to `libunwind`.
+  See https://github.com/rust-windowing/android-ndk-rs/issues/149 and https://github.com/rust-lang/rust/pull/85806 for more details.
 
 # 0.8.1 (2021-08-06)
 

cargo-apk/src/apk.rs

@@ -161,11 +161,32 @@ impl<'a> ApkBuilder<'a> {
             let target_sdk_version = config.manifest.sdk.target_sdk_version.unwrap();
 
             let mut cargo = cargo_ndk(&config.ndk, *target, target_sdk_version)?;
-            cargo.arg("build");
+            cargo.arg("rustc");
             if self.cmd.target().is_none() {
                 cargo.arg("--target").arg(triple);
             }
             cargo.args(self.cmd.args());
+
+            // Workaround for https://github.com/rust-windowing/android-ndk-rs/issues/149:
+            // Rust (1.56 as of writing) still requires libgcc during linking, but this does
+            // not ship with the NDK anymore since NDK r23 beta 3.
+            // See https://github.com/rust-lang/rust/pull/85806 for a discussion on why libgcc
+            // is still required even after replacing it with libunwind in the source.
+            // XXX: Add an upper-bound on the Rust version whenever this is not necessary anymore.
+            if self.ndk.build_tag() > 7272597 {
+                if !self.cmd.args().contains(&"--".to_owned()) {
+                    cargo.arg("--");
+                }
+                let cargo_apk_link_dir = self
+                    .cmd
+                    .target_dir()
+                    .join("cargo-apk-temp-extra-link-libraries");
+                std::fs::create_dir_all(&cargo_apk_link_dir)?;
+                std::fs::write(cargo_apk_link_dir.join("libgcc.a"), "INPUT(-lunwind)")
+                    .expect("Failed to write");
+                cargo.arg("-L").arg(cargo_apk_link_dir);
+            }
+
             if !cargo.status()?.success() {
                 return Err(NdkError::CmdFailed(cargo).into());
             }

ndk-build/CHANGELOG.md

@@ -1,5 +1,7 @@
 # Unreleased
 
+- Provide NDK `build_tag` version from `source.properties` in the NDK root.
+
 # 0.4.2 (2021-08-06)
 
 - Pass UNIX path separators to `aapt` on non-UNIX systems, ensuring the resulting separator is compatible with the target device instead of the host platform.

ndk-build/src/ndk.rs

@@ -9,6 +9,7 @@ pub struct Ndk {
     sdk_path: PathBuf,
     ndk_path: PathBuf,
     build_tools_version: String,
+    build_tag: u32,
     platforms: Vec<u32>,
 }
 
@@ -54,6 +55,31 @@ impl Ndk {
             .max()
             .ok_or(NdkError::BuildToolsNotFound)?;
 
+        let build_tag = std::fs::read_to_string(ndk_path.join("source.properties"))
+            .expect("Failed to read source.properties");
+
+        let build_tag = build_tag
+            .split('\n')
+            .find_map(|line| {
+                let (key, value) = line
+                    .split_once('=')
+                    .expect("Failed to parse `key = value` from source.properties");
+                if key.trim() == "Pkg.Revision" {
+                    // AOSP writes a constantly-incrementing build version to the patch field.
+                    // This number is incrementing across NDK releases.
+                    let mut parts = value.trim().split('.');
+                    let _major = parts.next().unwrap();
+                    let _minor = parts.next().unwrap();
+                    let patch = parts.next().unwrap();
+                    // Can have an optional `XXX-beta1`
+                    let patch = patch.split_once('-').unwrap().0;
+                    Some(patch.parse().unwrap())
+                } else {
+                    None
+                }
+            })
+            .expect("No `Pkg.Revision` in source.properties");
+
         let ndk_platforms = std::fs::read_to_string(ndk_path.join("build/core/platforms.mk"))?;
         let ndk_platforms = ndk_platforms
             .split('\n')
@@ -88,6 +114,7 @@ impl Ndk {
             sdk_path,
             ndk_path,
             build_tools_version,
+            build_tag,
             platforms,
         })
     }
@@ -104,6 +131,10 @@ impl Ndk {
         &self.build_tools_version
     }
 
+    pub fn build_tag(&self) -> u32 {
+        self.build_tag
+    }
+
     pub fn platforms(&self) -> &[u32] {
         &self.platforms
     }