The one thing that blew my mind about Rust is its approach to data sharing in concurrent situations. I had always thought of mutexes as something that is easy to get wrong and was convinced that the use of a RAII pattern to prevent lock leaks never happen (like with Abseil’s MutexLock) was the panacea. (I’m a fan of RAII in C++ by the way, in case you haven’t noticed.)
Aaaah, the borrow checker: the dreaded enemy lurking within the Rust compiler, ready to make its move to bring pain to your life by preventing your code from compiling. Or that’s what everyone seems to say, which is one of the reasons I put off learning Rust for so long. In reality… the borrow checker is a blessing, but it is true that getting past its gates is difficult at first.
Let’s start the deep dive by looking into a powerful feature of Rust: all variables and references are immutable by default unless qualified with mut. To understand why this is important, let’s cover some context first. One of my pet peeves when reviewing C++ code is to ask authors to sprinkle the const qualifier everywhere: if something ain’t mutated, say so explicitly. This includes marking local variables, function arguments, function return values, class attributes, etc.
I had been meaning to learn Rust since I first toyed with Go a couple of years ago. During this period, I’ve written a non-trivial amount of Go code both inside and outside Google, but never found the chance to sit back and learn Rust. This changed a month ago during my yearly family trip to Korea. This time around, I decided upfront that I would not work on any personal or work projects for the 2-week long vacation.
Blaze—the variant of Bazel used internally at Google—was originally designed to build the Google monorepo. One of the beauties of sticking to a monorepo is code reuse, but this has the unfortunate side-effect of dependency bloat. As a result, Bazel and Blaze have evolved to support ever-increasingly-bigger pieces of software. The growth of the projects built by Bazel and Blaze has had the unsurprising consequence that our engineers all now have high-end workstations with access to massive amounts of distributed resources.
During the summer of last year, I hosted an intern who implemented sandboxfs: a FUSE-based file system that exposes an arbitrary view of the host’s file system under the mount point. At the end of his internship, we had a functional sandboxfs implementation and some draft patches for integration in Bazel. The goal of sandboxfs in the context of Bazel is to improve the performance of builds when action sandboxing is enabled.
This post is a short, generalized summary of the preceeding two. I believe those two posts put readers off due to their massive length and the fact that they were seemingly tied to Bazel and Java, thus failing to communicate the larger point I wanted to make. Let’s try to distill their key points here in a language- and project-agnostic manner.