From a3a9a5f0a7c794eb2db5b38dc7f749c81887f60b Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Mon, 25 Mar 2024 18:16:08 +0000
Subject: [PATCH] Factor out `onMegolmDecryptionError`

---
 src/rust-crypto/rust-crypto.ts | 88 ++++++++++++++++------------------
 1 file changed, 42 insertions(+), 46 deletions(-)

diff --git a/src/rust-crypto/rust-crypto.ts b/src/rust-crypto/rust-crypto.ts
index 1f1b563cc83..512b0822fb5 100644
--- a/src/rust-crypto/rust-crypto.ts
+++ b/src/rust-crypto/rust-crypto.ts
@@ -1684,53 +1684,49 @@ class EventDecryptor {
             };
         } catch (err) {
             if (err instanceof RustSdkCryptoJs.MegolmDecryptionError) {
-                const content = event.getWireContent();
-
-                // If the error looks like it might be recoverable from backup, queue up a request to try that.
-                if (
-                    err.code === RustSdkCryptoJs.DecryptionErrorCode.MissingRoomKey ||
-                    err.code === RustSdkCryptoJs.DecryptionErrorCode.UnknownMessageIndex
-                ) {
-                    this.perSessionBackupDownloader.onDecryptionKeyMissingError(
-                        event.getRoomId()!,
-                        content.session_id!,
-                    );
-                }
-
-                let jsError;
-                switch (err.code) {
-                    case RustSdkCryptoJs.DecryptionErrorCode.MissingRoomKey: {
-                        jsError = new DecryptionError(
-                            DecryptionFailureCode.MEGOLM_UNKNOWN_INBOUND_SESSION_ID,
-                            "The sender's device has not sent us the keys for this message.",
-                            {
-                                session: content.sender_key + "|" + content.session_id,
-                            },
-                        );
-                        break;
-                    }
-                    case RustSdkCryptoJs.DecryptionErrorCode.UnknownMessageIndex: {
-                        jsError = new DecryptionError(
-                            DecryptionFailureCode.OLM_UNKNOWN_MESSAGE_INDEX,
-                            "The sender's device has not sent us the keys for this message at this index.",
-                            {
-                                session: content.sender_key + "|" + content.session_id,
-                            },
-                        );
-                        break;
-                    }
-                    // We don't map MismatchedIdentityKeys for now, as there is no equivalent in legacy.
-                    // Just put it on the `UNKNOWN_ERROR` bucket.
-                    default: {
-                        jsError = new DecryptionError(DecryptionFailureCode.UNKNOWN_ERROR, err.description, {
-                            session: content.sender_key + "|" + content.session_id,
-                        });
-                        break;
-                    }
-                }
-                throw jsError;
+                this.onMegolmDecryptionError(event, err);
+            } else {
+                throw new DecryptionError(DecryptionFailureCode.UNKNOWN_ERROR, "Unknown error");
             }
-            throw new DecryptionError(DecryptionFailureCode.UNKNOWN_ERROR, "Unknown error");
+        }
+    }
+
+    /**
+     * Handle a `MegolmDecryptionError` returned by the rust SDK.
+     *
+     * Fires off a request to the `perSessionBackupDownloader`, if appropriate, and then throws a `DecryptionError`.
+     */
+    private onMegolmDecryptionError(event: MatrixEvent, err: RustSdkCryptoJs.MegolmDecryptionError): never {
+        const content = event.getWireContent();
+
+        // If the error looks like it might be recoverable from backup, queue up a request to try that.
+        if (
+            err.code === RustSdkCryptoJs.DecryptionErrorCode.MissingRoomKey ||
+            err.code === RustSdkCryptoJs.DecryptionErrorCode.UnknownMessageIndex
+        ) {
+            this.perSessionBackupDownloader.onDecryptionKeyMissingError(event.getRoomId()!, content.session_id!);
+        }
+
+        const errorDetails = { session: content.sender_key + "|" + content.session_id };
+        switch (err.code) {
+            case RustSdkCryptoJs.DecryptionErrorCode.MissingRoomKey:
+                throw new DecryptionError(
+                    DecryptionFailureCode.MEGOLM_UNKNOWN_INBOUND_SESSION_ID,
+                    "The sender's device has not sent us the keys for this message.",
+                    errorDetails,
+                );
+
+            case RustSdkCryptoJs.DecryptionErrorCode.UnknownMessageIndex:
+                throw new DecryptionError(
+                    DecryptionFailureCode.OLM_UNKNOWN_MESSAGE_INDEX,
+                    "The sender's device has not sent us the keys for this message at this index.",
+                    errorDetails,
+                );
+
+            // We don't map MismatchedIdentityKeys for now, as there is no equivalent in legacy.
+            // Just put it on the `UNKNOWN_ERROR` bucket.
+            default:
+                throw new DecryptionError(DecryptionFailureCode.UNKNOWN_ERROR, err.description, errorDetails);
         }
     }