Get S3 credentials lazily #31
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The write VFS (that produces replication data) sometimes needs a
loader, but usually doesn't. Before this commit, creating a loader
could perform (local) network I/O to get credentials. We now
wait until we actually have to make an S3 API call (to fetch a
chunk) before getting the credentials.
Sample debug printer output:
```
cargo test loader -- --nocapture
Finished `test` profile [unoptimized + debuginfo] target(s) in 0.07s
Running unittests src/lib.rs (target/debug/deps/verneuil-106bf643236b6261)
running 1 test
Resolved Loader: Loader { cache: Cache { write_side: None, auto_sync: true, read_side: ReadOnlyCache { stack: [] } }, remote_sources: [RedactedBucket { name: "test-bucket", region: UsEast1 }], known_chunks: {} }
test loader::tests::test_loader_no_credential ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 46 filtered out; finished in 0.00s
```
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We sometimes create a loader and end up not using it to fetch anything from S3 (e.g., because all the chunks were cached in memory). This happens a lot when the read side wants to decode a manifest, which may refer to a base chunk: that base chunk is usually cached in memory.
Wait until we need to fetch something from S3 to get the credentials and create the bucket objects.