rc: add _config parameter to set global config for just this rc call

docs/content/rc.md

@@ -273,6 +273,36 @@ $ rclone rc job/list
 }
 ```
 
+### Setting config flags with _config
+
+If you wish to set config (the equivalent of the global flags) for the
+duration of an rc call only then pass in the `_config` parameter.
+
+This should be in the same format as the `config` key returned by
+[options/get](#options-get).
+
+For example, if you wished to run a sync with the `--checksum`
+parameter, you would pass this parameter in your JSON blob.
+
+    "_config":{"CheckSum": true}
+
+If using `rclone rc` this could be passed as
+
+    rclone rc operations/sync ... _config='{"CheckSum": true}'
+
+Any config parameters you don't set will inherit the global defaults
+which were set with command line flags or environment variables.
+
+Note that it is possible to set some values as strings or integers -
+see [data types](/#data-types) for more info. Here is an example
+setting the equivalent of `--buffer-size` in string or integer format.
+
+    "_config":{"BufferSize": "42M"}
+    "_config":{"BufferSize": 44040192}
+
+If you wish to check the `_config` assignment has worked properly then
+calling `options/local` will show what the value got set to.
+
 ### Assigning operations to groups with _group = value
 
 Each rc call has its own stats group for tracking its metrics. By default
@@ -292,14 +322,14 @@ $ rclone rc --json '{ "group": "job/1" }' core/stats
 }
 ```
 
-## Data types
+## Data types {#data-types}
 
 When the API returns types, these will mostly be straight forward
 integer, string or boolean types.
 
 However some of the types returned by the [options/get](#options-get)
 call and taken by the [options/set](#options-set) calls as well as the
-`vfsOpt` and the `mountOpt` are as follows:
+`vfsOpt`, `mountOpt` and the `_config` parameters.
 
 - `Duration` - these are returned as an integer duration in
   nanoseconds. They may be set as an integer, or they may be set with

fs/rc/jobs/job.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/pkg/errors"
+	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/accounting"
 	"github.com/rclone/rclone/fs/rc"
 )
@@ -201,6 +202,20 @@ func getAsync(ctx context.Context, in rc.Params) (context.Context, bool, error)
 	return ctx, isAsync, nil
 }
 
+// See if _config is set and if so adjust ctx to include it
+func getConfig(ctx context.Context, in rc.Params) (context.Context, error) {
+	if _, ok := in["_config"]; !ok {
+		return ctx, nil
+	}
+	ctx, ci := fs.AddConfig(ctx)
+	err := in.GetStruct("_config", ci)
+	if err != nil {
+		return ctx, err
+	}
+	delete(in, "_config") // remove the parameter
+	return ctx, nil
+}
+
 // NewJob creates a Job and executes it, possibly in the background if _async is set
 func (jobs *Jobs) NewJob(ctx context.Context, fn rc.Func, in rc.Params) (job *Job, out rc.Params, err error) {
 	id := atomic.AddInt64(&jobID, 1)
@@ -211,6 +226,11 @@ func (jobs *Jobs) NewJob(ctx context.Context, fn rc.Func, in rc.Params) (job *Jo
 		return nil, nil, err
 	}
 
+	ctx, err = getConfig(ctx, in)
+	if err != nil {
+		return nil, nil, err
+	}
+
 	ctx, group, err := getGroup(ctx, in, id)
 	if err != nil {
 		return nil, nil, err

fs/rc/jobs/job_test.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/pkg/errors"
+	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/rc"
 	"github.com/rclone/rclone/fs/rc/rcflags"
 	"github.com/rclone/rclone/fstest/testy"
@@ -248,6 +249,29 @@ func TestExecuteJob(t *testing.T) {
 	assert.Equal(t, rc.Params{}, out)
 }
 
+func TestExecuteJobWithConfig(t *testing.T) {
+	ctx := context.Background()
+	jobID = 0
+	jobFn := func(ctx context.Context, in rc.Params) (rc.Params, error) {
+		ci := fs.GetConfig(ctx)
+		assert.Equal(t, 42*fs.MebiByte, ci.BufferSize)
+		return nil, nil
+	}
+	_, _, err := NewJob(context.Background(), jobFn, rc.Params{
+		"_config": rc.Params{
+			"BufferSize": "42M",
+		},
+	})
+	require.NoError(t, err)
+	jobID = 0
+	_, _, err = NewJob(ctx, jobFn, rc.Params{
+		"_config": `{"BufferSize": "42M"}`,
+	})
+	require.NoError(t, err)
+	ci := fs.GetConfig(ctx)
+	assert.NotEqual(t, 42*fs.MebiByte, ci.BufferSize)
+}
+
 func TestExecuteJobErrorPropagation(t *testing.T) {
 	ctx := context.Background()
 	jobID = 0