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

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,
-			imageprocessor.DefaultMaxConcurrentProcessing()),
+		MaxConcurrentProcessing: loader.intVal(
+			keyMaxConcurrentProcessing, DefaultMaxConcurrentProcessing()),
 		UpstreamFetchTimeout: loader.durationVal(
 			keyUpstreamFetchTimeout, DefaultUpstreamFetchTimeout),
 		UpstreamMaxResponseSize: loader.int64Val(