feat: load context from global variable

[eduardoboucas]

Which problem is this pull request solving?

Currently, the client is loading the context from an environment variable called NETLIFY_BLOBS_CONTEXT, so both serverless functions and edge functions need to set the updated context on each request, by setting process.env.NETLIFY_BLOBS_CONTEXT in Node.js and by calling Deno.env.set() in Deno.

I wanted to see if there was any performance penalty in calling Deno.env.set() versus just setting a global variable, so I ran this benchmark and ran it a few times:

Deno.bench("Environment variable", { group: "timing", baseline: true }, () => {
  for (let i = 0; i < 50_000; i++) {
    Deno.env.set("SOME_VAR", `foo${i}`);
  }
});

Deno.bench("Global variable", { group: "timing" }, () => {
  for (let i = 0; i < 50_000; i++) {
    // @ts-expect-error No types for global variable
    globalThis.someVar = `foo${i}`;
  }
});

These are the results I'm seeing:

cpu: Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
runtime: deno 1.35.1 (x86_64-apple-darwin)

file:///Users/eduardoboucas/Sites/tests/deno-bench/index.ts
benchmark                 time (avg)             (min … max)       p75       p99      p995
------------------------------------------------------------ -----------------------------
Environment variable      43.42 ms/iter     (39.8 ms … 52.1 ms)  43.95 ms   52.1 ms   52.1 ms
Global variable               2 ms/iter     (1.68 ms … 3.34 ms)   2.13 ms      3 ms   3.09 ms

summary
  Environment variable
   21.72x slower than Global variable

I'm consistently seeing the environment variable set being ~20x slower than the global variable equivalent. In an environment where every millisecond counts, I think we should try to optimise this as best as we can.

So in this PR I'm making it possible for the client to read the context from a netlifyBlobsContext environment variable before trying to read it from an environment variable.

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[Skn0tt]

Nice!