Fix cargo kcov crashing/generating incorrect reports for proc-macros

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

tests/integration_tests/build/test_coverage.py

@@ -81,6 +81,13 @@ def test_coverage(test_fc_session_root_path, test_session_tmp_path):
     exclude_region = "'mod tests {'"
     target = "{}-unknown-linux-musl".format(platform.machine())
 
+    # Temporarily modify PATH so that "rustc" resolved to our wrapper script
+    # This is required as cargo kcov parses cargo's verbose output, and looking for lines starting with "Running `rustc":
+    # https://github.com/kennytm/cargo-kcov/blob/7a6bd499a1f9d4719a20cc9a37b6600fa1c9f68a/src/target_finder.rs#L44
+    # However, if we use the RUSTC_WRAPPER environment variable to make cargo run out script, the output will
+    # contain the lines "Running /firecracker/tests/rustc", and thus cargo kcov will fail to resolve the paths of the
+    # binaries compiled.
+    os.environ["PATH"] = "/firecracker/tests" + os.pathsep + os.environ["PATH"]
     cmd = (
         'CARGO_WRAPPER="kcov" RUSTFLAGS="{}" CARGO_TARGET_DIR={} '
         "cargo kcov --all "

tests/rustc

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+# We have to fully qualify the path to rustc here, as we modified $PATH in a way that this script is founds before
+# the actual rust executable.
+# Then we invoke normal rustc, but force the 'link-dead-code' codegen option. This option is required for gathering
+# coverage reports on proc-macros during cross-compilation, as the 'RUSTFLAGS' environment variable is ignored by cargo
+# for build-scripts, plugins and proc-macros in this case.
+# See also https://doc.rust-lang.org/cargo/reference/config.html#buildrustflags
+/usr/local/rust/bin/rustc $@ -Clink-dead-code