s3: drop implicit reader cache budget that throttled S3 GETs (#11384)

* fix(filer): leave reader cache unbounded without an explicit budget

NewReaderCache silently installed a 256MiB ReaderCacheBudget when the
caller passed none. Only weed mount opts into a budget; every other
caller (S3 gateway, WebDAV, query engine, mq logstore) inherited the
cap. Under ~90 concurrent S3 GETs of medium objects, prefetch wants
far more than 64 chunk buffers, so reserve() serialized chunk fetches,
clients timed out and retried, and the retry re-downloaded chunks the
cancelled request had already fetched.

A nil budget now means unbounded, restoring the pre-4.47 behavior for
callers that never asked for a memory cap; reserve/complete/release
are nil-safe. The mount path is unchanged and still enforces
-readerCacheSizeMB.

Fixes #11380

* feat(s3): expose -s3.readerCacheSizeMB reader buffer budget

Operators who want the S3 gateway read path memory-bounded can now
opt in: -s3.readerCacheSizeMB on weed filer/server/mini and
-readerCacheSizeMB on standalone weed s3, matching the mount flag.
The default 0 keeps the unbounded pre-4.47 behavior; a positive value
installs a shared ReaderCacheBudget across in-flight and retained
chunk buffers for all S3 GETs.

* fix(filer): validate chunk size before consulting the reader budget

A nil budget returned early and skipped the negative chunkSize check,
letting a corrupted size reach mem.Allocate and panic. Also drop the
command-specific flag prefix from the S3 validation error since
standalone weed s3 exposes the option as -readerCacheSizeMB.

* filer: drop chunk buffers once fully consumed

ReaderCache retained every completed chunk buffer in the downloaders
map until the slot limit evicted it, so buffers lingered after all
readers finished with them.

Track attached readers on each SingleChunkCacher and remove the cacher
when the last reader consumes the buffer to its end. In-flight download
deduplication and the prefetch handoff are unchanged: a buffer always
survives until fully read, partial reads keep it available, and an
attached reader pins a consumed buffer until it detaches. Repeat reads
now go through the chunk cache where enabled, or refetch.

* filer: drop consumed buffers on last detach, rechecked under cache lock

Two review findings on the drop-on-consume change:

- Removal only fired when the detaching reader itself reached the chunk
  end. If the end-reaching reader finished first and the last remaining
  reader did a partial read or cancelled, the consumed buffer and its
  budget reservation lingered until eviction. Track a persistent
  consumed flag instead, so any end-reaching read marks the buffer and
  the last detach drops it.
- remove() checked only map identity, so a reader attaching between the
  reader count hitting zero and removal could attach to a cacher that
  was then deleted underneath it. removeConsumed() re-checks identity,
  readers == 0, and consumed under the ReaderCache lock; a raced attach
  keeps the cacher and its own detach retries the removal.

weed/s3api/s3api_server.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"math"
 	"net"
 	"net/http"
 	"os"
@@ -68,6 +69,7 @@ type S3ApiServerOption struct {
 	ExternalUrl               string // external URL clients use, tried first during signature verification behind a reverse proxy
 	DefaultFileMode           uint32 // default file permission mode for S3 uploads (e.g. 0660, 0644)
 	CacheSizeMB               int64  // in-memory chunk cache capacity in MB for the shared ReaderCache; 0 disables
+	ReaderCacheSizeMB         int64  // memory budget in MiB for downloaded and in-flight reader buffers across all S3 GETs; 0 means unlimited
 	MaxMB                     int32  // filer's -maxMB, read from the filer configuration at startup
 	// AllowUntrustedRemoteEndpoints lets a read of a remote-only object dial a
 	// mounted endpoint that resolves to a loopback / private / metadata host.
@@ -282,7 +284,7 @@ func NewS3ApiServerWithStore(router *mux.Router, option *S3ApiServerOption, expl
 	//     assumed chunk size (s3ChunkCacheChunkSizeMB), clamped to a small
 	//     floor so tiny caches still function.
 	//
-	// Downloader slots: each slot holds one in-flight / recently-completed
+	// Downloader slots: each slot holds one in-flight or not-yet-consumed
 	// chunk buffer (~4 MiB by default), so this caps both peak memory for
 	// in-flight chunks (s3ReaderCacheDownloaderLimit × chunkSize) and the
 	// global fetch concurrency across all S3 GET requests. WebDAV uses 32
@@ -312,7 +314,14 @@ func NewS3ApiServerWithStore(router *mux.Router, option *S3ApiServerOption, expl
 	} else {
 		chunkCache = (*chunk_cache.TieredChunkCache)(nil)
 	}
-	readerCache := filer.NewReaderCache(s3ReaderCacheDownloaderLimit, chunkCache, filerClient.GetLookupFileIdFunction(), filerClient)
+	if option.ReaderCacheSizeMB < 0 || option.ReaderCacheSizeMB > math.MaxInt64>>20 {
+		return nil, fmt.Errorf("invalid readerCacheSizeMB %d: must be non-negative and fit in an int64 byte budget", option.ReaderCacheSizeMB)
+	}
+	var readerCacheBudget *filer.ReaderCacheBudget
+	if option.ReaderCacheSizeMB > 0 {
+		readerCacheBudget = filer.NewReaderCacheBudget(option.ReaderCacheSizeMB << 20)
+	}
+	readerCache := filer.NewReaderCache(s3ReaderCacheDownloaderLimit, chunkCache, filerClient.GetLookupFileIdFunction(), filerClient, readerCacheBudget)
 
 	s3ApiServer = &S3ApiServer{
 		option:                option,