With cooldown on, the RubyGems compact index and versions API, the Hex
package and Hex API entry, and conda's repodata.json were fetched with
plain HTTP requests on every client request: no TTL, no revalidation,
no coalescing and no stale fallback, even with cache_metadata.
Gem and Hex now read both documents through the metadata cache and
filter the cached copies. The filtered result goes through the rewrite
cache and is served with an ETag. When the times document is older than
the index and misses a version, it is fetched again; if that fails the
version is left out. Conda filters a fresh stored repodata.json as it
reads it from storage, uses the metadata cache otherwise, and remembers
the ETag of the filtered copy until a withheld entry leaves cooldown.
A Go module's .mod and .info for a canonical version, and RPM repodata
files named after their checksum, never change once published. They
went through the metadata TTL, so each cost a revalidation every TTL with
cache_metadata on and a full fetch on every request with it off.
They are now kept in the metadata cache with or without cache_metadata
and served whatever their age. A stored copy is checked against its
digest and fetched again if it doesn't match. Go queries, @v/list,
@latest and repomd.xml stay on the TTL.
A streamed cache hit whose bytes don't match the row's digest now
records an integrity failure and deletes the row, queueing its blob for
deletion, as a corrupt cached artifact is handled. The next request
fetches the document from upstream instead of aborting until the TTL
runs out.
With cache_metadata on, every metadata cache hit read the whole stored
document into its own buffer before any coalescing, since
coalesceMetadata only covers upstream misses. metadata_max_size bounds
one request, not the sum: 30 concurrent hits on a 20 MB document held
about 820 MB.
The handlers that go through ProxyCached serve the stored document
unchanged, so a hit there no longer needs the bytes in memory.
serveCachedMetadata streams the blob from storage to the response
instead, with the row's Content-Type, Content-Encoding, size, ETag and
Last-Modified, and answers If-None-Match and If-Modified-Since with 304
as before. The bytes are checked against the row's content digest as
they are sent, and a mismatch aborts the response like a corrupt cached
artifact. A row without a digest or size, a document over the size
limit, or a blob that can't be opened takes the buffered path as before.
Handlers that rewrite or filter metadata keep reading it into memory,
and the rewrite cache stays in front of them. writeMetadataValidators
is split out of writeMetadataCachedResponseWithEncoding so both paths
share the conditional request handling.
