Define the default max_concurrent_processing in internal/config

The default is configuration, so it now sits with the other defaults as
config.DefaultMaxConcurrentProcessing, and imageprocessor.New reads it
from there for a value below 1. internal/config no longer imports
internal/imageprocessor, so the packages that import internal/config do
not build against libvips.

Model: opus-5-5

TODO.md

@@ -30,6 +30,15 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 `max_concurrent_processing` defaults to half the CPUs pixa sees,
+  rounded down, and at least 1 (closes #242): it defaulted to one image per CPU,
+  and a container with no CPU limit sees every CPU of the host, so on a 48-CPU
+  host pixa processed 48 images at once, each of which can take about 1.6 GiB.
+  `config.DefaultMaxConcurrentProcessing` gives the default to the config and to
+  `imageprocessor.New`; `PIXA_MAX_CONCURRENT_PROCESSING` still overrides it.
+  "Running under upaas" in `README.md` gives the worst-case memory, measured,
+  and tells the operator to set `PIXA_MAX_CONCURRENT_PROCESSING` and
+  `PIXA_UPSTREAM_CONNECTIONS` to fit the container's memory.
 - 2026-10-15 expired negative cache rows are removed on a schedule (closes
   #244): the row of a failed fetch, which makes pixa answer 502 for that source
   URL for 5 minutes, was deleted only when the same URL was requested again
@@ -62,16 +71,6 @@ P2: security: per-origin rate limiting
   not in `allowlist.IsAllowed`, which also matches `referer_blocklist`, where a
   `Referer` is blocked whatever its port. A URL is signed over its host as it
   writes it, in lower case, with any IPv6 brackets, `:` and port kept.
-- 2026-10-15 `max_concurrent_processing` defaults to half the CPUs pixa sees,
-  rounded down, and at least 1 (closes #242): it defaulted to one image per CPU,
-  and a container with no CPU limit sees every CPU of the host, so on a 48-CPU
-  host pixa processed 48 images at once, each of which can take about 1.6 GiB.
-  `imageprocessor.DefaultMaxConcurrentProcessing` gives the default to the
-  config and to `imageprocessor.New`; `PIXA_MAX_CONCURRENT_PROCESSING` still
-  overrides it. "Running under upaas" in `README.md` gives the worst-case
-  memory, measured, and tells the operator to set
-  `PIXA_MAX_CONCURRENT_PROCESSING` and `PIXA_UPSTREAM_CONNECTIONS` to fit the
-  container's memory.
 - 2026-10-15 a `:` in the source path or query no longer moves where the path
   ends and the query begins (closes #247): the signed data and the cache key
   joined host, path and query with `:` and no escaping, so a URL signed for

internal/config/config.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"path/filepath"
 	"regexp"
+	"runtime"
 	"sort"
 	"strconv"
 	"strings"
@@ -20,7 +21,6 @@ import (
 
 	"git.eeqj.de/sneak/smartconfig"
 	"go.uber.org/fx"
-	"sneak.berlin/go/pixa/internal/imageprocessor"
 	"sneak.berlin/go/pixa/internal/logger"
 )
 
@@ -37,6 +37,16 @@ const (
 	DefaultImageRequestsPerMinute     = 600
 )
 
+// DefaultMaxConcurrentProcessing returns the most images processed at once
+// when max_concurrent_processing is omitted: half the CPUs Go uses
+// (runtime.GOMAXPROCS(0), which follows a container's CPU limit), rounded
+// down, and at least 1.
+func DefaultMaxConcurrentProcessing() int {
+	const cpusPerImage = 2
+
+	return max(runtime.GOMAXPROCS(0)/cpusPerImage, 1)
+}
+
 // Configuration key names.
 const (
 	keyDebug                      = "debug"
@@ -297,8 +307,8 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 			keyUpstreamConnectionsPerHost, DefaultUpstreamConnectionsPerHost),
 		UpstreamConnections: loader.intVal(
 			keyUpstreamConnections, DefaultUpstreamConnections),
-		MaxConcurrentProcessing: loader.intVal(keyMaxConcurrentProcessing,
+		MaxConcurrentProcessing: loader.intVal(
-			imageprocessor.DefaultMaxConcurrentProcessing()),
+			keyMaxConcurrentProcessing, DefaultMaxConcurrentProcessing()),
 		UpstreamFetchTimeout: loader.durationVal(
 			keyUpstreamFetchTimeout, DefaultUpstreamFetchTimeout),
 		UpstreamMaxResponseSize: loader.int64Val(

internal/imageprocessor/imageprocessor.go

@@ -7,11 +7,11 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"runtime"
 	"sync"
 	"time"
 
 	"github.com/davidbyttow/govips/v2/vips"
+	"sneak.berlin/go/pixa/internal/config"
 )
 
 // vipsOnce ensures vips is initialized exactly once.
@@ -179,19 +179,10 @@ type Params struct {
 	// If <= 0, DefaultMaxInputBytes is used.
 	MaxInputBytes int64
 	// MaxConcurrentProcessing is the most images processed at once.
-	// If <= 0, DefaultMaxConcurrentProcessing() is used.
+	// If <= 0, config.DefaultMaxConcurrentProcessing() is used.
 	MaxConcurrentProcessing int
 }
 
-// DefaultMaxConcurrentProcessing returns the most images processed at once
-// when none is set: half the CPUs Go uses (runtime.GOMAXPROCS(0), which
-// follows a container's CPU limit), rounded down, and at least 1.
-func DefaultMaxConcurrentProcessing() int {
-	const cpusPerImage = 2
-
-	return max(runtime.GOMAXPROCS(0)/cpusPerImage, 1)
-}
-
 // New creates a new image processor with the given parameters.
 // A zero-value Params{} uses sensible defaults.
 func New(params Params) *ImageProcessor {
@@ -204,7 +195,7 @@ func New(params Params) *ImageProcessor {
 
 	maxConcurrentProcessing := params.MaxConcurrentProcessing
 	if maxConcurrentProcessing <= 0 {
-		maxConcurrentProcessing = DefaultMaxConcurrentProcessing()
+		maxConcurrentProcessing = config.DefaultMaxConcurrentProcessing()
 	}
 
 	return &ImageProcessor{