ledger-tool: Reallow custom accounts path with Secondary access (#30228)

Previously, ledger-tool had a guardrail to disallow a custom accounts
path when access mode to the blockstore was Secondary. This was to avoid
potentially pulling the accounts out from underneath solana-validator.
When ledger-tool switched over to use Secondary blockstore access for
all commands that do not need write access, this removed the ability to
use custom accounts paths with ledger-tool at all for these commands.
Custom accounts paths are desirable, especially if that custom path is
in tmpfs to speed up processing.

With this change, when a custom accounts path is passed for a command
using Secondary access, ledger-tool now checks if Primary access is
being held by another process. If not, allow processing to proceed
with the custom accounts path.

The above check isn't fullproof, but it is about equal to the check that
previously existed when ledger-tool would run in Primary access mode
when it didn't need to.

ledger-tool/src/main.rs

@@ -1149,11 +1149,31 @@ fn load_bank_forks(
     }
 
     let account_paths = if let Some(account_paths) = arg_matches.value_of("account_paths") {
+        // If this blockstore access is Primary, no other process (solana-validator) can hold
+        // Primary access. So, allow a custom accounts path without worry of wiping the accounts
+        // of solana-validator.
         if !blockstore.is_primary_access() {
-            // Be defensive, when default account dir is explicitly specified, it's still possible
-            // to wipe the dir possibly shared by the running validator!
-            eprintln!("Error: custom accounts path is not supported under secondary access");
-            exit(1);
+            // Attempt to open the Blockstore in Primary access; if successful, no other process
+            // was holding Primary so allow things to proceed with custom accounts path. Release
+            // the Primary access instead of holding it to give priority to solana-validator over
+            // solana-ledger-tool should solana-validator start before we've finished.
+            info!(
+                "Checking if another process currently holding Primary access to {:?}",
+                blockstore.ledger_path()
+            );
+            if Blockstore::open_with_options(
+                blockstore.ledger_path(),
+                BlockstoreOptions {
+                    access_type: AccessType::Primary,
+                    ..BlockstoreOptions::default()
+                },
+            )
+            .is_err()
+            {
+                // Couldn't get Primary access, error out to be defensive.
+                eprintln!("Error: custom accounts path is not supported under secondary access");
+                exit(1);
+            }
         }
         account_paths.split(',').map(PathBuf::from).collect()
     } else if blockstore.is_primary_access() {