The initial work on supporting multiple concurrent exporters was

unfinished in the sense that exporter responses (which are abstracted as
a map of key/value pairs) are combined into a single map at the end of
the export: https://github.com/moby/buildkit/blob/55a7483b0564a7ad5b2ce5e62512789dce327bca/solver/llbsolver/solver.go#L808-L809

In order to provide the correct exporter response, each response
(currently at least each container image exporter) needs to be dedicated
to the exporter instance.

To achieve this, assign and propagate exporter IDs from the client and
have corresponding exporters annotate their responses with the
respective ID so the client can order outputs per exporter instance.

Fixes moby#5556.

Output descriptors in container image exporters with backwards-compatible fallbacks. Move the exporter set up to a public helper API so that users that custom-initialize the session can benefit from being able to configure exporters correctly.
Remove ids from the exporter implementation further shifting the burden of managing exporter identities to the control/client so it can be unified and kept an implementation detail as much as possible. Implement support for multiple exporter responses in APIs.

Signed-off-by: a-palchikov <[email protected]>

api/services/control/control.proto

@@ -35,7 +35,7 @@ message PruneRequest {
 }
 
 message DiskUsageRequest {
-	repeated string filter = 1; 
+	repeated string filter = 1;
 }
 
 message DiskUsageResponse {
@@ -105,7 +105,12 @@ message CacheOptionsEntry {
 }
 
 message SolveResponse {
-	map<string, string> ExporterResponse = 1;
+        // ExporterResponseDeprecated is a combined exporter response - it aggregates
+        // responses from all exporters running in parallel.
+        // It is deprecated in favor of the structured exporter response but will be
+        // populated as long as it is supported.
+	map<string, string> ExporterResponseDeprecated = 1;
+	repeated ExporterResponse exporterResponses = 2;
 }
 
 message StatusRequest {
@@ -245,4 +250,21 @@ message Exporter {
 	string Type = 1;
 	// Attrs specifies exporter configuration
 	map<string, string> Attrs = 2;
+	// ID identifies this exporter.
+	// ID should be treated by the exporter as opaque.
+	string ID = 3;
+}
+
+// ExporterResponse describes the output of an exporter
+message ExporterResponse {
+    // Metadata describes the exporter
+    ExporterMetadata metadata = 1;
+    // Data is the exporter's output
+    map<string, string> data = 2;
+}
+
+// ExporterMetadata describes the output exporter
+message ExporterMetadata {
+    // ID identifies the exporter
+    string ID = 1;
 }