patch cargo-xbuild to ensure copied Cargo.lock is writable

https://github.com/rust-osdev/cargo-xbuild/issues/96

default.nix

@@ -8,7 +8,9 @@ let
       src = ./.;
       cargoSha256 = "1f2psa1g41pl2j8n60hhik2s2pqdfjhr5capimvajf81kxrnn2ck";
 
-      nativeBuildInputs = [ pkgs.cargo-xbuild ];
+      nativeBuildInputs = [
+        (pkgs.cargo-xbuild.overrideAttrs(oa: { patches = oa.patches ++ [ ./xbuild_writable_lockfile.diff ]; } ))
+      ];
       buildPhase = ''
         export XARGO_RUST_SRC="${rustPlatform.rust.rustc.src}/library"
         export CARGO_HOME=$(mktemp -d cargo-home.XXX)

shell.nix

@@ -8,7 +8,7 @@ in
       rustPlatform.rust.rustc
       rustPlatform.rust.cargo
       pkgs.cacert
-      pkgs.cargo-xbuild
+      (pkgs.cargo-xbuild.overrideAttrs(oa: { patches = oa.patches ++ [ ./xbuild_writable_lockfile.diff ]; } ))
 
       pkgs.openocd pkgs.gdb
       pkgs.openssh pkgs.rsync

xbuild_writable_lockfile.diff

@@ -0,0 +1,27 @@
+diff --git a/src/sysroot.rs b/src/sysroot.rs
+index 1f3c8d1..422d3d0 100644
+--- a/src/sysroot.rs
++++ b/src/sysroot.rs
+@@ -85,10 +85,20 @@ fn build_crate(
+     }
+ 
+     util::write(&td.join("Cargo.toml"), &stoml)?;
+-    fs::copy(lockfile, &td.join("Cargo.lock")).chain_err(||
++    let dst_file = td.join("Cargo.lock");
++    fs::copy(lockfile, &dst_file).chain_err(||
+         format!("failed to copy Cargo.lock from `{}` to `{}`",
+-            lockfile.display(), &td.join("Cargo.lock").display())
++            lockfile.display(), &dst_file.display())
+     )?;
++    let mut perms = fs::metadata(&dst_file).chain_err(||
++        format!("failed to retrieve permissions for `{}`",
++            dst_file.display())
++    )?.permissions();
++    perms.set_readonly(false);
++    fs::set_permissions(&dst_file, perms).chain_err(||
++        format!("failed to update permissions for `{}`",
++            dst_file.display())
++    );
+     util::mkdir(&td.join("src"))?;
+     util::write(&td.join("src/lib.rs"), "")?;
+