Make build output deterministic by remapping paths

Author: TheBlueMatt

deterministic-build-wrappers/rustc

@@ -0,0 +1,30 @@
+#!/bin/bash
+# rustc calculates a unique metadata tag to mangle symbols which includes the
+# actual path to the crate. This breaks our deterministic builds as we depend
+# on a copy of rust-lightning via a path. We insert this shim between cargo and
+# rustc and edit the metadata tag for rust-lightning.
+# While we could just set RUSTFLAGS="-C metadata=42", this would break if we
+# ever (indirectly) depend on multiple versions of the same crate.
+args=("$@")
+IS_LIGHTNING=false
+for ((i=0; i<"${#args[@]}"; ++i)); do
+    case ${args[i]} in
+        --crate-name)
+			if [ "${args[i+1]}" = "lightning" ]; then
+				IS_LIGHTNING=true
+			fi
+			;;
+    esac
+done
+for ((i=0; i<"${#args[@]}"; ++i)); do
+    case ${args[i]} in
+        metadata*)
+			if [ "$IS_LIGHTNING" = "true" ]; then
+				# Pick any random value for metadata, it doesn't matter
+				args[i]="metadata=42"
+			fi
+			;;
+    esac
+done
+
+/usr/bin/rustc "${args[@]}"

genbindings.sh

@@ -50,9 +50,14 @@ else
 	sed -i 's|lightning = { .*|lightning = { path = "'"$LIGHTNING_PATH"'" }|' lightning-c-bindings/Cargo.toml
 fi
 
+# Set path to include our rustc wrapper as well as cbindgen
+PATH="$(pwd)/deterministic-build-wrappers:$PATH:~/.cargo/bin"
 # Now cd to lightning-c-bindings, build the generated bindings, and call cbindgen to build a C header file
-PATH="$PATH:~/.cargo/bin"
 cd lightning-c-bindings
+# Remap paths so that our builds are deterministic
+export RUSTFLAGS="--remap-path-prefix $LIGHTNING_PATH=rust-lightning --remap-path-prefix $(pwd)=ldk-c-bindings --remap-path-prefix $HOME/.cargo= -C target-cpu=generic"
+export CFLAGS="-ffile-prefix-map=$HOME/.cargo="
+
 cargo build
 cbindgen -v --config cbindgen.toml -o include/lightning.h >/dev/null 2>&1