MM-43733: Set concurrency limits via configuration (#20413)

```release-note
We create a new config option MaxImageDecoderConcurrency which
indicates how many images can be decoded concurrently at once.

The default is -1 which means number of CPUs present.

This affects the total memory consumption of the server. The
maximum memory of a single image is dictated by MaxImageResolution * 24 bytes.
Therefore, a good rule of thumb to follow is that MaxImageResolution
* MaxImageDecoderConcurrency * 24 should be less then the allocated memory for
image decoding.
```

https://mattermost.atlassian.net/browse/MM-43733

app/channels.go

@@ -197,8 +197,12 @@ func NewChannels(s *Server, services map[ServiceKey]interface{}) (*Channels, err
 	}
 
 	var imgErr error
+	decoderConcurrency := int(*ch.cfgSvc.Config().FileSettings.MaxImageDecoderConcurrency)
+	if decoderConcurrency == -1 {
+		decoderConcurrency = runtime.NumCPU()
+	}
 	ch.imgDecoder, imgErr = imaging.NewDecoder(imaging.DecoderOptions{
-		ConcurrencyLevel: runtime.NumCPU(),
+		ConcurrencyLevel: decoderConcurrency,
 	})
 	if imgErr != nil {
 		return nil, errors.Wrap(imgErr, "failed to create image decoder")

i18n/en.json

@@ -8095,6 +8095,10 @@
     "id": "model.config.is_valid.group_unread_channels.app_error",
     "translation": "Invalid group unread channels for service settings. Must be 'disabled', 'default_on', or 'default_off'."
   },
+  {
+    "id": "model.config.is_valid.image_decoder_concurrency.app_error",
+    "translation": "Invalid decoder concurrency {{.Value}}. Should be a positive number or -1."
+  },
   {
     "id": "model.config.is_valid.image_proxy_type.app_error",
     "translation": "Invalid image proxy type. Must be 'local' or 'atmos/camo'."

model/config.go

@@ -1406,6 +1406,7 @@ type FileSettings struct {
 	EnableMobileDownload       *bool   `access:"site_file_sharing_and_downloads,cloud_restrictable"`
 	MaxFileSize                *int64  `access:"environment_file_storage,cloud_restrictable"`
 	MaxImageResolution         *int64  `access:"environment_file_storage,cloud_restrictable"`
+	MaxImageDecoderConcurrency *int64  `access:"environment_file_storage,cloud_restrictable"`
 	DriverName                 *string `access:"environment_file_storage,write_restrictable,cloud_restrictable"`
 	Directory                  *string `access:"environment_file_storage,write_restrictable,cloud_restrictable"`
 	EnablePublicLink           *bool   `access:"site_public_links,cloud_restrictable"`
@@ -1446,6 +1447,10 @@ func (s *FileSettings) SetDefaults(isUpdate bool) {
 		s.MaxImageResolution = NewInt64(7680 * 4320) // 8K, ~33MPX
 	}
 
+	if s.MaxImageDecoderConcurrency == nil {
+		s.MaxImageDecoderConcurrency = NewInt64(-1) // Default to NumCPU
+	}
+
 	if s.DriverName == nil {
 		s.DriverName = NewString(ImageDriverLocal)
 	}
@@ -3395,6 +3400,10 @@ func (s *FileSettings) isValid() *AppError {
 		return NewAppError("Config.IsValid", "model.config.is_valid.directory.app_error", nil, "", http.StatusBadRequest)
 	}
 
+	if *s.MaxImageDecoderConcurrency < -1 || *s.MaxImageDecoderConcurrency == 0 {
+		return NewAppError("Config.IsValid", "model.config.is_valid.image_decoder_concurrency.app_error", map[string]interface{}{"Value": *s.MaxImageDecoderConcurrency}, "", http.StatusBadRequest)
+	}
+
 	return nil
 }
 

services/telemetry/telemetry.go

@@ -537,6 +537,7 @@ func (ts *TelemetryService) trackConfig() {
 		"amazon_s3_trace":               *cfg.FileSettings.AmazonS3Trace,
 		"max_file_size":                 *cfg.FileSettings.MaxFileSize,
 		"max_image_resolution":          *cfg.FileSettings.MaxImageResolution,
+		"max_image_decoder_concurrency": *cfg.FileSettings.MaxImageDecoderConcurrency,
 		"enable_file_attachments":       *cfg.FileSettings.EnableFileAttachments,
 		"enable_mobile_upload":          *cfg.FileSettings.EnableMobileUpload,
 		"enable_mobile_download":        *cfg.FileSettings.EnableMobileDownload,