WIP: RoutingBlobAccess

[minor-fixes]

This change adds a config option for RoutingBlobAccess, which is
intended to hold all possible ways to rewrite instance names.

While it is possible to affect instance name by (ab)using
DemultiplexingBlobAccess, there are some sharp edges (e.g. using it to
add an instance name may not work as expected, as not only must the
instance name be valid on the inner blobstore, but it must be valid once
returned upwards as well). RoutingBlobAccess will attempt to fix that
by offering easier-to-use semantics around instance name rewriting.

Tested: builds only, never ran

---

This PR's current purpose is to:

• give me an opportunity to familiarize myself with the codebase (OK if this is a bad idea and should die here - I learned things)
• validate the proposal of adding a BlobAccess wrapper with these semantics (vs. e.g. bug fixes to DemultiplexingBlobAccess)
  • I still need to go back and collect specific details/minimal repro for issues found using instance renaming with DemultiplexingBlobAccess, to better make the case for this
• shed light on any misguided assumptions I have about how digests/instance names propagate through various backends

[EdSchouten]

using it to add an instance name may not work as expected, as not only must the instance name be valid on the inner blobstore, but it must be valid once returned upwards as well

This I don't understand. Are you saying that this currently does not hold? Namely, that BlobAccess.FindMissing() returns digests that can't be translated back to the original instance name? If that's the case, then we should figure out why that is happening.

[EdSchouten]

I don't think we should gain yet another backend for this. Can't the same be achieved using the existing DemultiplexedBlobAccess? If not, can't it be improved to support this?

From a configuration perspective, I guess all we'd need to change is something like this:

diff --git a/src/bb-storage/pkg/proto/configuration/blobstore/blobstore.proto b/src/bb-storage/pkg/proto/configuration/blobstore/blobstore.proto
index 6a1dfd76..671699c3 100644
--- a/src/bb-storage/pkg/proto/configuration/blobstore/blobstore.proto
+++ b/src/bb-storage/pkg/proto/configuration/blobstore/blobstore.proto
@@ -853,9 +853,15 @@ message DemultiplexedBlobAccessConfiguration {
   // The backend to which requests are forwarded.
   BlobAccessConfiguration backend = 1;
 
-  // Add a prefix to the instance name of all requests forwarded to this
-  // backend.
-  string add_instance_name_prefix = 2;
+  oneof translation {
+    // Add a prefix to the instance name of all requests forwarded to
+    // this backend.
+    string add_instance_name_prefix = 2;
+
+    // Set the instance name to an exact value for all requests
+    // forwarded to this backend.
+    string set_instance_name = 3;
+  }
 }
 
 message ActionResultExpiringBlobAccessConfiguration {

I guess that if we add something like this, we should also add symmetry on the remote execution side?

diff --git a/src/bb-storage/pkg/proto/configuration/builder/builder.proto b/src/bb-storage/pkg/proto/configuration/builder/builder.proto
index a5170bac..88779fe1 100644
--- a/src/bb-storage/pkg/proto/configuration/builder/builder.proto
+++ b/src/bb-storage/pkg/proto/configuration/builder/builder.proto
@@ -15,10 +15,15 @@ message SchedulerConfiguration {
   // The gRPC endpoint at which the scheduler can be reached.
   buildbarn.configuration.grpc.ClientConfiguration endpoint = 1;
 
-  // Add a prefix to the instance name of all requests forwarded to this
-  // scheduler. By default, the prefix that was used to match the
-  // request against a scheduler is stripped from the instance name.
-  // This option can be used to re-add that prefix in case perfect
-  // forwarding is necessary.
-  string add_instance_name_prefix = 2;
+  oneof translation  {
+    // Add a prefix to the instance name of all requests forwarded to
+    // this scheduler. By default, the prefix that was used to match the
+    // request against a scheduler is stripped from the instance name.
+    // This option can be used to re-add that prefix in case perfect
+    // forwarding is necessary.
+    string add_instance_name_prefix = 2;
+
+    // ...
+    string set_instance_name = 3;
+  }
 }

Once we start doing that, maybe smarter to factor this out into a common message?