troubleshooting-rbe: document Missing Digest (#7994)

Add a doc explaining 'missing digest error' and the recommended path to
troubleshoot it.

---------

Co-authored-by: Fabian Meumertzheim <[email protected]>

docs/troubleshooting-rbe.md

@@ -70,3 +70,49 @@ sudo sysctl -w net.ipv4.tcp_keepalive_probes=5
 This will cause the Linux kernel to send keepalive probes earlier and more frequently, before the proxy/gateway in the middle detects and drops the idle connection.
 
 The optimal values may depend on specific network conditions, but try these values as a starting point. Please [contact us](/contact/) if you have any questions / concerns.
+
+## CacheNotFoundException: Missing digest
+
+During remote build execution, Bazel may encounter a `CacheNotFoundException` error with the message `Missing digest`.
+
+```bash
+com.google.devtools.build.lib.remote.common.BulkTransferException: 3 errors during bulk transfer:
+    com.google.devtools.build.lib.remote.common.CacheNotFoundException: Missing digest: d0387e622e30ab61e39b1b91e54ea50f9915789dde7b950fafb0863db4a32ef8/17096
+    com.google.devtools.build.lib.remote.common.CacheNotFoundException: Missing digest: 9718647251c8d479142d459416079ff5cd9f45031a47aa346d8a6e719e374ffa/28630
+    com.google.devtools.build.lib.remote.common.CacheNotFoundException: Missing digest: 785e0ead607a37bd9a12179051e6efe53d7fb3eb05cc291e49ad6965ee2b613d/11504
+```
+
+This error occurs when Bazel is unable to find file(s) in the BuildBuddy Remote Cache that it expects to exist.
+
+The first step to verify this issue would be to copy the hash of the missing blob.
+Then navigate to the Invocation URL -> Cache -> "Cache requests" and paste the hash into the Filter input.
+This will let you know if Bazel has tried to upload the blob to BuildBuddy Remote Cache or not.
+
+If Bazel attempted to upload the blob and failed, there should be multiple retries attempted for the same blob.
+The retry attempts can be configured with the `--remote_retries` (default 5) and `--remote_retry_max_delay` (default 5s) flags.
+Additionally, `--experimental_collect_system_network_usage` (default true since Bazel 8) can be used to collect network usage data on Bazel's host machine.
+This network data will be displayed as a graph in the "Timing" tab of the Invocation page.
+
+If there was no attempt from Bazel to upload the missing blob, this is caused by a mismatch of expectation between Bazel's local state and the BuildBuddy Remote Cache.
+In a previous invocation (usually with Build without the Bytes turned on), Bazel local state was taught to assume that the blob is already in the Remote Cache.
+However, as time passed, the blob was evicted from BuildBuddy Remote Cache without Bazel's knowledge.
+
+The best solution in this scenario is for Bazel to either re-upload the missing blob, or to re-execute the action that created the missing blob.
+This is also known as "Action Rewinding" in Bazel terminology.
+However due to the complexity of Bazel's code base, this feature is not yet fully implemented.
+
+The existing solution includes 2 halves:
+
+a. With `--experimental_remote_cache_lease_extension` and `--experimental_remote_cache_ttl` flags, Bazel will keep track of all the blobs involved in the latest invocation in a side-car thread.
+This side-car will routinely "ping" BuildBuddy Remote Cache to let the server know that these blobs are still being used by Bazel.
+Our remote cache server will update the last used timestamps of these blobs accordingly.
+
+b. With `--experimental_remote_cache_eviction_retries` (default 5) flag, Bazel will detect this specific error code and attempt to reset the local states and re-try the build.
+This will clear the local state kept by Bazel and re-analyze the repository to determine which blobs are missing and which actions need to be re-executed.
+
+If neither of these flags work, try running `bazel clean --noasync` to clear the local state manually.
+Bazel JVM should be shut down by the time the clean finished. You can check your process monitor to verify this.
+Then re-run the build with the same flags as before.
+
+We also recommend disabling the local Disk Cache with `--disk_cache=''` while troubleshooting this type of issue as well as avoid using any remote cache proxy solutions.
+It will help narrowing down the root cause by not having to deal with multiple sources of remote cache.