P5c review: bound concurrent uploads; warnings is a list; sift extract keeps a base-URL path

Owner review of p5c (tasks 01-02 by Ornith):
- extract.go: an upload (up to 100 MiB) is read whole before the PDF pool's limits apply, so
  any number of concurrent uploads could buffer at once in a 1 GiB service. At most 2 bodies
  are read at a time now; a third waits up to 5 s, then 429 busy with Retry-After 10.
- extract.go: `warnings` was null when empty; it is [] as in /v1/fetch.
- extractcmd.go: extractURL replaced the client base URL's path; it now appends /v1/extract
  like fetch and search do.
Regression tests in extract_review_test.go (api and cmd/sift).

End-to-end (local serve -> Dixie; sift extract with SIFT_CLIENT): Llama 2 paper, 13.6 MB /
77 pages: --pages 1 ok, repeat served from the content-hash cache, --grep 2 hits, --question
answered with page citations in 16.8 s. Log lines carry bytes/pages/mode/cached only.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/api.go

@@ -74,6 +74,10 @@ type Server struct {
 	reqN, blockN, scrapeN atomic.Uint64
 
 	pdfPoolOnce sync.Once
+
+	// uploads bounds how many /v1/extract bodies (up to PDFMaxUpload each) are read into
+	// memory at once, ahead of the PDF pool's own limits.
+	uploads chan struct{}
 }
 
 func New(version string, g *guard.Guard, f *fetch.Fetcher, keys *auth.Store, log *slog.Logger) *Server {
@@ -90,6 +94,7 @@ func New(version string, g *guard.Guard, f *fetch.Fetcher, keys *auth.Store, log
 		OutCap:  200_000,
 		global:  make(chan struct{}, 8),
 		per:     map[string]chan struct{}{},
+		uploads: make(chan struct{}, maxUploads),
 	}
 	s.Prom = metrics.New(s)
 	s.Mux.HandleFunc("GET /healthz", s.healthz)

internal/api/extract.go

@@ -27,6 +27,17 @@ func (s *Server) v1extract(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	select {
+	case s.uploads <- struct{}{}:
+		defer func() { <-s.uploads }()
+	case <-time.After(uploadWait):
+		w.Header().Set("Retry-After", "10")
+		writeErr(w, 429, "busy", "")
+		return
+	case <-r.Context().Done():
+		return
+	}
+
 	body, rerr := io.ReadAll(r.Body)
 	if rerr != nil {
 		var me *http.MaxBytesError
@@ -105,7 +116,7 @@ func (s *Server) v1extract(w http.ResponseWriter, r *http.Request) {
 		content = full
 	}
 
-	var warn []string
+	warn := []string{}
 	var digMeta map[string]any
 	var digestMS int64
 	if mode == "digest" {
@@ -192,6 +203,14 @@ func (s *Server) v1extract(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, 200, out)
 }
 
+// maxUploads is how many uploads may be in memory at once, and uploadWait how long a further
+// one waits for a slot before a 429: at 100 MiB each, a few are all a 1 GiB service can hold
+// beside the extraction children.
+const (
+	maxUploads = 2
+	uploadWait = 5 * time.Second
+)
+
 // pdfMaxUpload is the /v1/extract upload cap: the configured value, or the default when unset.
 func (s *Server) pdfMaxUpload() int64 {
 	if s.PDFMaxUpload > 0 {

internal/api/extract_review_test.go

@@ -0,0 +1,42 @@
+package api
+
+// P5c owner review: regression tests for /v1/extract.
+
+import (
+	"net/http"
+	"testing"
+	"time"
+)
+
+// Uploads are buffered whole (up to 100 MiB) before the PDF pool sees them, so the number read
+// at once is bounded: a third concurrent upload waits, then is refused busy.
+func TestExtractUploadsBounded(t *testing.T) {
+	r := newPDFRig(t)
+	for i := 0; i < cap(r.s.uploads); i++ {
+		r.s.uploads <- struct{}{}
+	}
+	start := time.Now()
+	code, out := r.extract(t, r.fleet, "", threePDF(), "application/pdf")
+	if code != http.StatusTooManyRequests {
+		t.Fatalf("upload with every slot taken: %d %v, want 429", code, out)
+	}
+	if d := time.Since(start); d < uploadWait-100*time.Millisecond {
+		t.Fatalf("refused after %v, want a wait of about %v first", d, uploadWait)
+	}
+	<-r.s.uploads
+	if code, out := r.extract(t, r.fleet, "", threePDF(), "application/pdf"); code != 200 {
+		t.Fatalf("upload with a free slot: %d %v", code, out)
+	}
+}
+
+// The envelope's warnings are a list, never null, as in /v1/fetch.
+func TestExtractWarningsIsAList(t *testing.T) {
+	r := newPDFRig(t)
+	code, out := r.extract(t, r.fleet, "", threePDF(), "application/pdf")
+	if code != 200 {
+		t.Fatalf("%d %v", code, out)
+	}
+	if _, ok := out["warnings"].([]any); !ok {
+		t.Fatalf("warnings = %#v, want a list", out["warnings"])
+	}
+}