Interceptor: relativate execution based on `cwd`

`ROOT_DIR` - if set, otherwise assume `cwd` - is assumed to be the
directory all execution is done relatively to. Hence, when rewriting
commands, do it with that directory as a base.

The interceptor wrapper is determining the INTERCEPTOR_root_dir based on
either ROOT_DIR or falls back to `cwd`.

Bug: 205577427
Signed-off-by: Matthias Maennich <maennich@google.com>
Change-Id: Ifef91ce7ae446d1a6cea7a744f36d0ff27cb4b3e

3 files changed, 57 insertions, 1 deletions

diff --git a/interceptor/interceptor b/interceptor/interceptor
index e57b3e3..54ab449 100644
--- a/interceptor/interceptor
+++ b/interceptor/interceptor
@@ -43,6 +43,13 @@ if [ -n "$COMMAND_LOG" ]; then
     echo "[" > "$INTERCEPTOR_command_log"
 fi
 
+# If ROOT_DIR is not set, assume the current working directory is it
+if [ -z "$ROOT_DIR" ]; then
+    export INTERCEPTOR_root_dir=$(pwd)
+else
+    export INTERCEPTOR_root_dir=$ROOT_DIR
+fi
+
 # Discover the interceptor preload library
 export LD_PRELOAD=$(realpath $(dirname $(readlink -f $0))/../lib64/libinterceptor.so)
 if [ ! -x "$LD_PRELOAD" ]; then
diff --git a/interceptor/interceptor.cc b/interceptor/interceptor.cc
index 71f67a2..98b8e5e 100644
--- a/interceptor/interceptor.cc
+++ b/interceptor/interceptor.cc
@@ -35,6 +35,7 @@ namespace fs = std::filesystem;
 
 // Options passed via environment variables from the interceptor starter
 constexpr static auto ENV_command_log = "INTERCEPTOR_command_log";
+constexpr static auto ENV_root_dir = "INTERCEPTOR_root_dir";
 
 // UTILITY function declarations
 
@@ -121,6 +122,45 @@ std::string Command::repr() const {
   os << "}";
   return os.str();
 }
+
+void Command::make_relative() {
+  // determine the ROOT_DIR
+  std::string root_dir;
+  if (auto it = env().find(ENV_root_dir); it != env().cend()) {
+    root_dir = it->second;
+    if (root_dir[root_dir.size() - 1] != '/') root_dir += '/';
+  } else {
+    return;
+  }
+
+  // determine the relative path to ROOT_DIR from the current working dir
+  std::string rel_root = fs::relative(root_dir);
+  if (rel_root[rel_root.size() - 1] != '/') rel_root += '/';
+  if (rel_root == "./") rel_root = "";
+
+  // TODO: This is generally bad as this means we can't make anything relative.
+  // This happens if the out dir is outside of the root.
+  if (rel_root.find(root_dir) != std::string::npos) {
+    return;
+  }
+
+  cwd_ = fs::relative(cwd_, root_dir);
+
+  // replacement functor
+  const auto replace_all = [&](auto& str) {
+    auto pos = std::string::npos;
+    while ((pos = str.find(root_dir)) != std::string::npos) {
+      str.replace(pos, root_dir.length(), rel_root);
+    }
+  };
+
+  if (!args_.has_value()) args();
+
+  // now go and replace everything
+  replace_all(program_);
+  std::for_each(args_->begin(), args_->end(), replace_all);
+}
+
 }  // namespace interceptor
 
 /// UTILITY FUNCTIONS
@@ -133,9 +173,15 @@ static void process_command(const char* filename, char* const argv[], char* cons
     return;
   }
 
-  // Ok, we can handle that one, let's log it.
+  // Ok, we can handle that one, let's transform it.
 
   interceptor::Command command(filename, argv, envp);
+
+  // rewrite all command line arguments (including the program itself) to use
+  // paths relative to ROOT_DIR. This is essential for reproducible builds and
+  // furthermore necessary to produce cache hits in RBE.
+  command.make_relative();
+
   log(command, "");
 
   // pass down the transformed command to execve
diff --git a/interceptor/interceptor.h b/interceptor/interceptor.h
index 3944bde..77b137a 100644
--- a/interceptor/interceptor.h
+++ b/interceptor/interceptor.h
@@ -41,6 +41,9 @@ class Command {
 
   std::string repr() const;
 
+  // make command line calls relative to ROOT_DIR
+  void make_relative();
+
  private:
   std::string program_;
   std::string cwd_;