Default `try_load_options` to true when DB is specified (#9937)

Summary:
If the DB path is specified, the user would expect ldb loads the
options from the path, but it's not:
```
$ ldb list_live_files_metadata --db=`pwd`
```
Default `try_load_options` to true in that case. The user can still
disable that by:
```
$ ldb list_live_files_metadata --db=`pwd` --try_load_options=false
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9937

Test Plan:
`ldb list_live_files_metadata --db=`pwd`` is able to work for
a db generated with different options.num_levels.

Reviewed By: ajkr

Differential Revision: D36106708

Pulled By: jay-zhuang

fbshipit-source-id: 2732fdc027a4d172436b2c9b6a9787b56b10c710

HISTORY.md

@@ -21,6 +21,7 @@
 
 ### Behavior changes
 * Enforce the existing contract of SingleDelete so that SingleDelete cannot be mixed with Delete because it leads to undefined behavior. Fix a number of unit tests that violate the contract but happen to pass.
+* ldb `--try_load_options` default to true if `--db` is specified and not creating a new DB, the user can still explicitly disable that by `--try_load_options=false` (or explicitly enable that by `--try_load_options`).
 
 ## 7.2.0 (04/15/2022)
 ### Bug Fixes

include/rocksdb/utilities/ldb_cmd.h

@@ -288,6 +288,9 @@ class LDBCommand {
   bool IsValueHex(const std::map<std::string, std::string>& options,
                   const std::vector<std::string>& flags);
 
+  bool IsTryLoadOptions(const std::map<std::string, std::string>& options,
+                        const std::vector<std::string>& flags);
+
   /**
    * Converts val to a boolean.
    * val must be either true or false (case insensitive).

tools/ldb_cmd.cc

@@ -408,7 +408,7 @@ LDBCommand::LDBCommand(const std::map<std::string, std::string>& options,
   is_value_hex_ = IsValueHex(options, flags);
   is_db_ttl_ = IsFlagPresent(flags, ARG_TTL);
   timestamp_ = IsFlagPresent(flags, ARG_TIMESTAMP);
-  try_load_options_ = IsFlagPresent(flags, ARG_TRY_LOAD_OPTIONS);
+  try_load_options_ = IsTryLoadOptions(options, flags);
   force_consistency_checks_ =
       !IsFlagPresent(flags, ARG_DISABLE_CONSISTENCY_CHECKS);
   enable_blob_files_ = IsFlagPresent(flags, ARG_ENABLE_BLOB_FILES);
@@ -1064,6 +1064,24 @@ bool LDBCommand::IsValueHex(const std::map<std::string, std::string>& options,
           ParseBooleanOption(options, ARG_VALUE_HEX, false));
 }
 
+bool LDBCommand::IsTryLoadOptions(
+    const std::map<std::string, std::string>& options,
+    const std::vector<std::string>& flags) {
+  if (IsFlagPresent(flags, ARG_TRY_LOAD_OPTIONS)) {
+    return true;
+  }
+  // if `DB` is specified and not explicitly to create a new db, default
+  // `try_load_options` to true. The user could still disable that by set
+  // `try_load_options=false`.
+  // Note: Opening as TTL DB doesn't support `try_load_options`, so it's default
+  // to false. TODO: TTL_DB may need to fix that, otherwise it's unable to open
+  // DB which has incompatible setting with default options.
+  bool default_val = (options.find(ARG_DB) != options.end()) &&
+                     !IsFlagPresent(flags, ARG_CREATE_IF_MISSING) &&
+                     !IsFlagPresent(flags, ARG_TTL);
+  return ParseBooleanOption(options, ARG_TRY_LOAD_OPTIONS, default_val);
+}
+
 bool LDBCommand::ParseBooleanOption(
     const std::map<std::string, std::string>& options,
     const std::string& option, bool default_val) {

tools/ldb_tool.cc

@@ -50,7 +50,10 @@ void LDBCommandRunner::PrintHelp(const LDBOptions& ldb_options,
              " with 'put','get','scan','dump','query','batchput'"
              " : DB supports ttl and value is internally timestamp-suffixed\n");
   ret.append("  --" + LDBCommand::ARG_TRY_LOAD_OPTIONS +
-             " : Try to load option file from DB.\n");
+             " : Try to load option file from DB. Default to true if " +
+             LDBCommand::ARG_DB +
+             " is specified and not creating a new DB and not open as TTL DB. "
+             "Can be set to false explicitly.\n");
   ret.append("  --" + LDBCommand::ARG_DISABLE_CONSISTENCY_CHECKS +
              " : Set options.force_consistency_checks = false.\n");
   ret.append("  --" + LDBCommand::ARG_IGNORE_UNKNOWN_OPTIONS +