sets up three paths for backend initialization

codex/codex.nim

@@ -178,7 +178,9 @@ proc start*(s: CodexServer) {.async.} =
   let proofCeremonyUrl = await s.bootstrapInteractions()
 
   if prover =? s.codexNode.prover:
-    prover.start(s.config, proofCeremonyUrl)
+    if err =? (await prover.start(s.config, proofCeremonyUrl)).errorOption:
+      error "Failed to start prover", msg = err.msg
+      return # should we abort start-up this way?
 
   await s.codexNode.start()
   s.restServer.start()
@@ -289,12 +291,10 @@ proc new*(
           $config.circomZkey
         else: ""
 
-      echo "Prover is some!"
       some Prover.new(
         store,
         config.numProofSamples)
     else:
-      echo "Prover is none!"
       none Prover
 
     codexNode = CodexNodeRef.new(

codex/slots/proofs/prover.nim

@@ -44,7 +44,7 @@ type
 
   AnyProofInputs* = ProofInputs[Poseidon2Hash]
   Prover* = ref object of RootObj
-    backend: AnyBackend
+    backend: ?AnyBackend
     store: BlockStore
     nSamples: int
 
@@ -63,24 +63,27 @@ proc prove*(
 
   trace "Received proof challenge"
 
-  without builder =? AnyBuilder.new(self.store, manifest), err:
-    error "Unable to create slots builder", err = err.msg
-    return failure(err)
+  if backend =? self.backend:
+    without builder =? AnyBuilder.new(self.store, manifest), err:
+      error "Unable to create slots builder", err = err.msg
+      return failure(err)
 
-  without sampler =? AnySampler.new(slotIdx, self.store, builder), err:
-    error "Unable to create data sampler", err = err.msg
-    return failure(err)
+    without sampler =? AnySampler.new(slotIdx, self.store, builder), err:
+      error "Unable to create data sampler", err = err.msg
+      return failure(err)
 
-  without proofInput =? await sampler.getProofInput(challenge, self.nSamples), err:
-    error "Unable to get proof input for slot", err = err.msg
-    return failure(err)
+    without proofInput =? await sampler.getProofInput(challenge, self.nSamples), err:
+      error "Unable to get proof input for slot", err = err.msg
+      return failure(err)
 
-  # prove slot
-  without proof =? self.backend.prove(proofInput), err:
-    error "Unable to prove slot", err = err.msg
-    return failure(err)
+    # prove slot
+    without proof =? backend.prove(proofInput), err:
+      error "Unable to prove slot", err = err.msg
+      return failure(err)
 
-  success (proofInput, proof)
+    success (proofInput, proof)
+  else:
+    return failure("Prover was not started")
 
 proc verify*(
   self: Prover,
@@ -89,16 +92,49 @@ proc verify*(
   ## Prove a statement using backend.
   ## Returns a future that resolves to a proof.
 
-  self.backend.verify(proof, inputs)
+  if backend =? self.backend:
+    return backend.verify(proof, inputs)
+  else:
+    return failure("Prover was not started")
+
+proc initializeFromConfig(
+  self: Prover,
+  config: CodexConf): ?!void =
+
+  # check provided files exist
+  # initialize backend with files
+  # or failure
+
+  self.backend = some CircomCompat.init($config.circomR1cs, $config.circomWasm, $config.circomZkey)
+  success()
+
+proc initializeFromCeremonyFiles(
+  self: Prover): ?!void =
+
+  # initialize from previously-downloaded files if they exist
+  echo "todo"
+  success()
+
+proc initializeFromCeremonyUrl(
+  self: Prover,
+  proofCeremonyUrl: ?string): Future[?!void] {.async.} =
+
+  # download the ceremony url
+  # unzip it
+  return self.initializeFromCeremonyFiles()
 
 proc start*(
   self: Prover,
   config: CodexConf,
-  proofCeremonyUrl: ?string
-) =
-  echo "prover start!"
-  echo proofCeremonyUrl
-  self.backend = CircomCompat.init($config.circomR1cs, $config.circomWasm, $config.circomZkey)
+  proofCeremonyUrl: ?string): Future[?!void] {.async.} =
+  if cliErr =? self.initializeFromConfig(config).errorOption:
+    info "Could not initialize prover backend from CLI options...", msg = cliErr.msg
+    if localErr =? self.initializeFromCeremonyFiles().errorOption:
+      info "Could not initialize prover backend from local files...", msg = localErr.msg
+      if urlErr =? (await self.initializeFromCeremonyUrl(proofCeremonyUrl)).errorOption:
+        warn "Could not initialize prover backend from ceremony url...", msg = urlErr.msg
+        return failure(urlErr)
+  return success()
 
 proc new*(
   _: type Prover,
@@ -107,4 +143,5 @@ proc new*(
 
   Prover(
     store: store,
+    backend: none AnyBackend,
     nSamples: nSamples)